Aramco Engineering - Evaluating Motor Specifications

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Electrical For additional information on this subject, contactFile Reference: EEX20303 W.A. Roussel on 874-1320

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Evaluating Motor Specifications

Saudi Aramco DeskTop Standards

CONTENTS PAGE

Specifying Motor Design Requirements 1

Specifying Motor Enclosure Requirements 28

Specifying Motor Starting Methods 43

Specifying Motor Protection Requirements 61

WORK AID

Work Aid 1: Motor Design Requirements for Saudi Aramco 111Installations Compiled from SADP-P-113, NEMAMG-1 and Established Engineering Practices

Work Aid 2: Motor Enclosure Requirements for Saudi Aramco 119Installations Compiled from SADP-P-113, NEMAMG-1 and Established Engineering Practices

Work Aid 3: Conditions Under Which the Various Types 121of Motor Starters Should be Specified for Use atSaudi Aramco Installations, Compiled fromSADP-P-113, NEMA MG-1, and Established EngineeringPractices

Work Aid 4: Conditions Under Which the Various Types of Motor 126Protection Should be Specified for Use at Saudi AramcoInstallations, Compiled from SADP-P-113, NEMA MG-1,and Established Engineering Practices

GLOSSARY 136

Engineering Encyclopedia Electrical


Saudi Aramco DeskTop Standards 1

SPECIFYING MOTOR DESIGN REQUIREMENTS

This section will provide information on the following topics that are pertinent to specifyingmotor design requirements:

_Stator_Rotor_Bearings_Vibration Monitoring_Mechanical Noise_Shaft Circulating Currents_Stator Windings and RTD's_Rotor Windings_Mounting Details_Cooling System_Control and Supply Leads_Nameplates_Space Heaters_Testing Requirements_Painting and Coating_Packing




Stator

A stator is defined as the stationary part of a machine that houses the windings. The stator isthe integral unit that consists of the outerhousing and the baseplate. The only real designrequirement for the stator frame and baseplate is that they be constructed of fabricated steelthat is strong enough to withstand all of the stresses to which the stator will be exposed duringshipping and operation.

For motors that are above 150 kW (200 Hp), mechanical alignment devices must be installedin the baseplate to provide for accurate horizontal alignment. Examples of motor alignmentdevices are placement pins or foot pegs. The length of these devices can be varied to raise orlower one area of the motor. Stator mechanical alignment devices should not be the solemeans of support for the stator. Shim material also should be provided with the motor toallow for accurate motor alignment and support prior to initial motor operation.

Rotor

A rotor is defined as the rotating component of a machine that has a shaft. The rotor of amotor must support the field winding. The following types of rotors are for use in SaudiAramco motors:

_Cylindrical Rotors_Salient Pole Rotors

Cylindrical Rotors

Squirrel-cage induction cylindrical rotors are the only approved rotors that are used ininduction motors for Saudi Aramco installations. The squirrel-cage induction rotor is asimple, sturdy design that allows the rotor to withstand arduous conditions. The constructionof the squirrel-cage induction rotor begins with a simple shaft. Laminated supports areconnected to the shaft, and, together with the shaft, they form the iron core of the rotor. Theiron core increases the permeability of the rotor. The laminated supports are insulated fromthe rotor and from each other. Rotor bars, which are the material into which a voltage isinduced, are attached to the outside of the laminated supports. An ending or shorting ring isattached at each end of the squirrel-cage induction rotor to electrically connect all of the rotorbars to complete the electrical circuit.

Synchronous motors also can be designed with a cylindrical rotor, which is sometimes calleda turbo rotor. The cylindrical rotor for the synchronous motor is constructed through theembedding of the windings in slots that are machined into the iron core. The embedding ofthe windings limits centrifugal force on the rotor and allows the cylindrical rotor to beoperated at higher speeds without damage. The synchronous cylindrical rotor is used on




motors that run at 3600 rpm or faster. Synchronous motors that have cylindrical rotors arevery rarely applied in Saudi Aramco applications.




Salient Pole Rotors

Salient pole rotors, which are used in synchronous motors, are available in the followingtypes:

_Laminated, salient pole rotor with a cage damper winding in each pole forstarting.

_Solid pole rotor with solid, bolted pole pieces.

Either form of salient pole rotor is acceptable for use in Saudi Aramco applications, but mostSaudi Aramco synchronous motor applications use the solid pole design. The solid poledesign is preferred because of the very simple heavy duty construction and the rotor's highthermal capacity. The basic advantage that the solid pole rotor has over the laminated salientpole rotor is the absence of damper bars and end rings, and this absence ensures that there arefewer failure points on the solid pole rotor.

Solid pole salient rotors for Saudi Aramco applications can be constructed through use of thefollowing designs:

_Solid, forged rotor shaft and pole body with solid pole shoes._Cast steel body and hub with forged steel stub shafts and solid bolted shoes.

When a rotor is specified for use in Saudi Aramco applications, the critical speed of the rotormust be examined. A rotor that operates at or near the critical speed of the motor will causeexcessive vibration of the motor. The running speed of any motor must be different than thecritical speed of the motor in order to prevent vibrational damage to the motor. The followingtwo types of salient pole rotor shafts are acceptable in the analysis of critical speeds:

_Rigid shaft rotors in which the first critical speed for vibration exceeds therunning speed of the motor.

_Flexible shaft rotors in which the first critical speed for vibration is less thanthe running speed of the motor.

The first critical speed for ridged shaft rotors will be at least 115% of rated rotor speed. Thefirst critical speed for flexible shaft rotors will be between 65% and 85% of rated motorspeed.

The second critical speed for both rigid and flexible shaft rotors must not be withinplus/minus 10% of the second harmonic, which occurs at two times the rotor speed.




Bearings

Bearings are involved in a majority of motor failures. Because many motor failures arerelated to bearings, much attention should be paid to them as both the possible cause of aproblem and a symptom of a problem. This discussion of bearings will include the followingtopics:

_Bearing Types and Applications_Bearing Lubrication_Bearing Housing and Protection_Bearing Life

Bearing Types and Applications

The following types of bearings are used in Saudi Aramco motors:

_Antifriction_Sleeve

Antifriction bearings are classified according to the type of rolling mechanism in the bearing.The rolling mechanism of an antifriction bearing can be a ball-type or a roller-typemechanism. The ball-type bearing that is shown in Figure 1A contains small balls, and theroller-type bearing that is shown in Figure 1B contains small rollers.

The following is a comparison of the load capacity and the misalignment capabilities of ball-type and roller-type antifriction bearings:




Radial Load Thrust Load MisalignmentBearing Type Capacity Capacity Capability

Ball Type Good Fair FairRoller Type Excellent Poor Fair

Each antifriction bearing application will have an equivalent ball-type and roller-type bearingthat can be used. The type (ball or roller) of antifriction bearing that is selected should bebased on the speed and load characteristics of the installation. Ball-type antifriction bearingshave a small area of contact between the ball and the race. The small area of contact allowsthe ball-type bearing to operate at higher speeds, but the ability to carry load is reduced.Roller-type antifriction bearings have a much larger area of contact between the roller and therace. The larger area of contact allows the roller-type antifriction bearing to carry a higherload, but the speed capability of this bearing is reduced.

Antifriction BearingsFigure 1




The sleeve bearing will be of the journal type. Sleeve bearings on horizontal machines aresplit to facilitate installation and maintenance. The two halves of horizontal motor sleevebearings must be mechanically interchangeable.

The applications for each type of bearing are based on the speed factor (Dn) of the bearing.The speed factor (Dn) is the product of the internal diameter of the bearing in millimeters(mm) and the motor speed. The application of bearing types for any given speed factor isbased on Saudi Aramco experience, and it is presented in Saudi Aramco Design PracticeSADP-P-113. The following list shows the type of bearing that should be applied fordifferent speed factors:

Speed Factor Bearing Type

up to 250,000 Antifriction, grease or oil lubricatedup to 300,000 Antifriction, oil lubricatedabove 300,000 Sleeve

For an example, the speed factor (Dn) for a motor that has a shaft diameter of 127mm and thatoperates at 3600 RPM can be calculated as follows:

Dn = (Internal Bearing Diameter) (Motor Speed)Dn = 127mm x 3600 RPMDn = 547,200

This calculation shows that the motor should have sleeve bearings because the speed factorexceeds 300,000.

Bearing Lubrication

The lubrication of bearings can be accomplished with a variety of oils and greases that areapplied through use of several methods. The type of lubricant and the method of lubricantapplication that best suits the installation will be determined by the lubricant's characteristics.This section will cover the following topics that are pertinent to bearing lubrication:

_Bearing Lubricants: Types and Applications_Methods of Bearing Lubrication

Bearing Lubricants: Types and Applications - Bearings can be lubricated through the useof oil or grease. The selection of the correct type of lubricant, either oil or grease,depends on the properties of the lubricant and the specifications of the installation.




The first type of bearing lubrication for use in Saudi Aramco applications is oil.Lubricating oils can be manufactured from mineral oil or from synthetic oil.Lubricating oil for use in Saudi Aramco installations must be manufactured fromhighly-refined turbine oil stocks, and it must be blended with additives to producebalanced oil stocks. Two of the main properties to consider in the selection of oil as alubricant are the oil's viscosity and the oil's viscosity index.

The viscosity of an oil is the oil's resistance to flow. An oil with a viscosity that is toohigh or too low can lead to the early failure of the motor. Saudi Aramco applicationsrequire that lubricating oils have a viscosity of 61.2 - 74.8 centistokes (cSt) at 40oC.Oils with a viscosity in this range are designated as ISO viscosity grade 68. Theequivalent U.S. viscosity range is 317 - 389 saybolt universal seconds (SUS).The viscosity index is an empirical measurement of how the viscosity of an oilchanges with temperature. An oil must be utilized that will meet the viscosity needs ofthe installation over the entire range of operating temperatures. The temperatures atwhich an oil can successfully perform its function will vary with the oil that isselected. Oils that have large viscosity indexes have the least change in viscosity for agiven change in temperature.

Greases, which are semisolid lubricants, are the other type of bearing lubrication thatcan be selected. Greases are used when the lubricant must stay in one place or muststick to a part. Most greases are made from mineral oil, but other materials such aswaxes can be utilized. The lubricating properties of greases are determined by thefollowing components from which greases are made:

_Fluid base_Thickener_Additives_Fillers

Mineral oil will be the fluid base for most greases. The fluid base will determine theviscosity of the grease. Greases that are designed for high temperature, low speedservice are produced through the use of high viscosity oils, and greases that aredesigned for low temperature, high speed service are produced through the use of lowviscosity oils. The minimum viscosity of the oil that is used as the fluid base for thegrease must be 100 cSt at 40oC.

The thickener is added to the fluid base to stiffen the grease. The most common typeof thickener is soap. Soap is made from the combination of a fatty material and analkali. Greases are generally named for the type of thickener that is used.




Additives are the chemical compounds that are added to grease to change or add to theproperties of grease. The additives in grease will increase the temperature range atwhich the grease can be utilized or will change the breakdown temperature of thegrease. Additives can also increase the life span of the grease.

Fillers are added to the grease to make the grease more solid and stable. Graphite isthe most commonly used type of filler. Lubricating greases will have a variety ofdifferent properties that change dependent on the materials that are used during theproduction of the grease. To select the appropriate type of grease for an installation,the properties of the grease must be matched to the requirements of the installation.Lubricating greases for use in Saudi Aramco installations are required to performunder continuous temperatures of up to 120oC.

The type of lubricant that should be specified for antifriction and sleeve bearings thatare used in Saudi Aramco applications are as follows:

Antifriction bearings oil or greaseSleeve bearings oil

Methods of Bearing Lubrication - The method of bearing lubrication for use in a motormust account for startup and rundown lubrication of the bearings. The method ofbearing lubrication should be designed so that the bearing will be lubricated duringstartups that follow periods of extended shutdown, and it should permit the uncoupledmotor to run down to standstill without damage to the bearings.

The lubrication of antifriction bearings should be accomplished through the use oftapped holes in the bearing housing. Relief holes or drain plugs shall be located 180ofrom the grease point to provide for removal of old or excess lubrication.

The lubrication of sleeve bearings can be accomplished in two ways. The method thatis used depends on the velocity of the shaft journal as follows:

Shaft Journal Velocity (Meters/Seconds) Lubrication Method

Below 11 Uncooled ring or disc oillubrication

Above 11 Circulated feed oil lubrication

The following formula is used to determine shaft journal velocity in meters per secondfrom RPM:




For example, the shaft journal velocity of a motor that has a shaft diameter of 100mmand that operates at 1800 RPM can be calculated as follows:

The calculation shows that the bearings should be lubricated through use of theuncooled ring or disc method.

The lubrication of sleeve bearings by an uncooled ring or disc involves the use of aloose ring that hangs on the motor shaft or a fixed disc that dips into an oil reservoirthat is below the shaft. The ring or the disk also rotates as the shaft rotates, and thisrotation transfers oil from the reservoir to the bearing surface. Heat is removed fromthe oil through use of natural heat transfer through the bearing housing to the ambient.

The lubrication of sleeve bearings through the use of the circulation of feed oilrequires an entire external system to support the bearing. When a circulating feed oillubrication system is used, two separate pump units must be provided. Each of the twooil pumps must be able to supply 100% of the total operating oil requirements of thebearing. The circulated feed oil lubrication option for lubricating sleeve bearings isonly chosen when it is required by the manufacturer.

Bearing Housing and Protection

The bearing housings will contain the bearing and the lubrication that are necessary for theproper operation of the motor. The bearing housing should be designed to prevent physicaldamage to the bearing from external sources. All horizontal motors that are 3730 kW orabove (5000 Hp and larger) must be equipped with pedestal bearings that are supported fromthe motor's baseplate.

Bearing housings also must be designed to protect the bearing and the lubricant fromcontamination by external foreign matter. This contamination protection will also protect thebearing against the transfer of lubricant out of the bearing housing and into the surroundingatmosphere.

Bearing Life

Because of the dispersion in life of identical bearings that are operated under identicalconditions, a statistical result must be obtained for bearing life. Bearing life is expressed asthe number of operational hours that 90% of a group of identical bearings will achieve orexceed under a given set of conditions, and it is referred to as the L10 life.

There are multiple variables that are taken into account for a bearing life calculation. Becauseof the numerous variables, this section only discusses the basic bearing life equation.




The following is the basic bearing life equation for an antifriction bearing:

where:L10 = Fatigue life for a 90% reliabilityN = Operating speedC = Dynamic load ratingP = Equivalent radial load in newtons or poundsk = Constant that is equal to 3 for ball bearings and 10/3 for roller

bearings

The dynamic load rating (C) is determined by the type of bearing that is used and by thenumber of active bearings that are mounted adjacent to one another.

The equivalent radial load (P) is determined by the following factors:

_Applied thrust load_Thrust load factors_Number of adjacent bearings_Basic static load

There also are three life adjustment factors that could be placed into the basic bearing lifeequation. In most instances, the life adjustment factors can be assumed to be one, which willcancel out of the equation. The life adjustment factors that could be included are as follows:

_Reliability_Bearing material_Application conditions

The bearing life calculation will generally only be done by the manufacturer during the designof a new installation. The manufacturer should include the bearing life value with the bearinginformation.

Bearing life calculations, although they are not routinely performed by the field ElectricalEngineers, can be used for performance data. Maintenance of bearing life records can beused to evaluate the actual life of bearings against the calculated life expectancy, and they canbe utilized to identify bearing application problems.




Vibration Monitoring

The vibration of motors to some degree is expected as a result of the motor's rotation.Vibration monitoring is employed to detect the occurrence of excessive vibration and to avoiddamage to the motor or to adjacent equipment. There following types of vibration monitoringequipment are available for use in Saudi Aramco applications:

_Seismic_Proximity

Seismic-type vibration monitoring equipment is physically mounted so that the detector isconnected to the bearing housing and moves with the motor. The movement of the motorcauses a slug within the seismic detector to move back and forth, which changes the electricalcoupling in the detector. The motor's vibration is proportional to the vibration in the electricalcoupling of the detector. The advantages and disadvantages of the seismic probe resultfrom the method of probe mounting. The advantages of the seismic probe are its ruggeddesign and its ease of mounting. The seismic probe directly mounts to the bearing housing.The disadvantage of the seismic probe is that the failure rate of the seismic probes increasesdue to the extra moving parts that are used to physically mount seismic probes.

Proximity probes are not connected to the bearing housing, and they will not move with themotor. The proximity probe measures the distance between the probe tip and the bearingcasing. The proximity probe establishes a small magnetic field of the tip of the probe and, asthe bearing casing vibrates in the magnetic field, the magnetic field will be distorted. Theamount of distortion in the magnetic field is proportional to the amount of motor vibration.The advantages and disadvantages of the proximity probe also result from the method ofmounting the probe. The advantages of the proximity probe are a much more accurateindication and a much lower failure rate. The accurate indication and lower failure rate areresult from to the fact that the probe does not directly connect to the motor; therefore, theprobe is not susceptible to damage and faults that result from the vibration of the motor. Thedisadvantage of the proximity probe is the elaborate mounting assembly that must beconstructed. Because the proximity probe does not connect directly to the bearing housing,the extra mounting is necessary. Another disadvantage that results from the extra mountingassembly is the need to accurately align the probe with the motor bearing housing. Anymisalignment between the probe and the motor bearing housing will result in an erroneousindication.

Proximity probes can be used for frequency ranges of 1 to 1500 Hz, and seismic probes canbe used for frequency ranges of 1 to 20,000 Hz. The actual requirements for determiningwhen each type of probe should be used are contained in Work Aid 1.




In general, any motor that operates at greater than 185 kW (250 Hp) will be supplied withvibration monitoring. The method and amount of vibration monitoring depends on the size ofthe motor and how the motor is mounted. Horizontal motors that operate from 750 kW (1000Hp) to 3000 kW (4600 Hp) will have one seismic detector mounted on each bearing.Horizontal motors that operate above 3000 kW (4000 Hp) will have two proximity typedetectors that are mounted 90o apart on each bearing. For vertical motors that operate atgreater than 185 kW (250 Hp), two seismic detectors that are mounted 90o apart around thecircumference of the top bearing housing are required. Proximity probes are never used withvertical motors.

Mechanical Noise

Mechanical noise will always be generated in a motor during operation. Different motordesigns and motor mounting techniques will increase or decrease the mechanical noise that isproduced by an operating motor. The following are the terms that are used to discussmechanical noise:

_Sound Power Level_Sound Intensity_Sound Pressure Level_Sound Level

The Saudi Aramco noise limits are based on the sound level of the motor installation, and anunderstanding of the previously mentioned terms is necessary to facilitate this discussion.Sound Power Level

Sound power level (SWL) is a machine-related property that is independent of environmentalconditions or distance from the machine. SWL is defined through use of the followingequation:

SWL = 10 log10 (P/Po) in decibels

where: P = Measured soundPo = Reference level of 10-12 watt (1 picowatt)

Because of environmental conditions, SWL cannot be directly measured. Surroundingequipment would add to any measured sound power level; therefore, another means ofdirectly measuring SWL is necessary.

Sound Intensity

Sound intensity is the density of sound power at a point away from the source, and it isexpressed in watts per square meter. The sound power that is indicated by a source can be




derived through integration of the sound intensity over an enclosed, hypothetical surface ofmeasurement.




Sound Pressure Level (SPL)

Sound Pressure Level (SPL) is the level of pressure in the sound conducting medium thatresults from the sound intensity at the concerned point. SPL can be expressed as follows:

SPL = 20 log10 (P1/P2) dB

where: P2 = Reference pressure that is equal to 20micropascals(2 x 10-5n/sqm)P1 = The sound pressure

Sound Level

Sound level is a weighted measure of the amount of noise that is produced by a machine at agiven point. Note that sound intensity and SPL at a point are a function of both the combinedsurroundings and the source of the noise.

The following equation is for use in the direct calculation of SWL from measured free fieldsound:

SWL = SPL + 20 log10r + 8 dB

where: SWL = Sound power level referred to 10-12 wattsSPL = Average sound pressure level that is referenced to20 micropascalsr = Radius of hemisphere in meters

Saudi Aramco limits the sound level to a maximum of 90 dB when the sound level isreferenced to a base of 20 micropascals for an eight-hour exposure period per day. Areas inwhich the SWL exceeds the 90 dB maximum must have the exposure time shortened toprevent injury to the personnel.

Typical sound power levels from a motor will depend not only on the motor, but also on thetype of enclosure of the motor. Certain types of enclosures such as dripoff, total-enclosedfan-cooled (TEFC), and weather protected type II (WPII) will tend to shield a portion of thesound. Figure 2 shows typical sound power levels for various motor horsepower and kilowattratings at various speeds and with different enclosure types. The sound levels are given indecibels.




Typical Sound Power LevelsFigure 2




Shaft Circulating Currents

Shaft circulating currents are caused by stray voltages that are induced in the rotor duringoperation. The stray voltages that are induced in the rotor can form a closed loop for currentflow. To complete the closed loop for current flow, circulating currents must bridge the oilfilm on the bearing surfaces. Because of the high resistance of the bearing supports toground, the induced voltages cannot be shunted away. When the oil film on the bearingsurfaces is bridged, a closed loop for current flow will exist from the rotor through the bearinghousing, through the stator, through the other bearing housing, and back to the rotor. Thecirculating currents, if allowed to exist, will cause a problem in the form of damage to thebearing and shaft surfaces. The damage will occur in the form of pitting at the point wherethe current passes through the shaft/bearing connection.

To prevent damage that results from shaft circulating current, a method of prevention must beobtained. On horizontal motors that are rated 375 kW (500 Hp) and above, both of thebearings must be electrically insulated from the motor frame. Vertical motors that are ratedabove 185 kW (250 Hp) only require insulation on the top bearing. The insulation resistanceof the bearing must be greater than one megohm.

Stator Windings and RTD's

Stator windings are required to be designed to withstand environmental conditions that arecommon at Saudi Aramco installations. The stator windings must be treated to withstand thetropical conditions and the corrosive effects of industrial sulfurous atmospheres. The varnishimpregnation should be a resin-rich type or a vacuum/pressure impregnation type process forform wound windings. The windings of weather-protected type motors should be providedwith an additional protective coating to inhibit insulation abrasion by sand and salt that isentrained in the cooling air.

Stator windings also need to be braced against excessive vibration to prevent damage to thestator insulation. Stator leads that require bracing within the motor enclosure should beprovided with removable insulated supports to facilitate maintenance.

Stator windings must be supplied with type F insulation systems that are designed so that theinsulation will not exceed the class B temperature rise as measured by an RTD that isimbedded in the stator. The maximum temperature rise is based on a maximum ambienttemperature of 50oC.

To reduce the need for surge suppressors on all motors, the stator windings must be designedto withstand the surges that are caused by normal switching actions or lightning. In oldermotor designs, only coil to frame insulation was tested, and the minimum acceptable coil toframe insulation level was determined from the following equation:




2V + 1 kV

where: V is the phase-to-phase rms voltage of the motor.

Recently, research has shown that in short time surges, most of the voltage will fall across thefirst turn of the stator winding. When existing motors are evaluated, an interturn basicinsulation level (BIL) of only 25% of the coil to frame insulation level can be assumed;therefore, the allowable peak surge voltage can be determined by the following equation:

New motors are specified with stator winding interturn insulation requirements that exceed25% of the coil to frame insulation level to minimize the need for surge suppressors; however,Saudi Aramco still requires both high BIL level and surge suppression for 13.8 kV motors,regardless of the stator winding interturn insulation level.

Rotor Windings

The rotor windings of induction machines should be of the cage-type, they should be formedof copper, copper alloy, or aluminum bar, and they should be treated to withstand tropicalconditions. End-ring connections on cage-type rotors should be of high mechanical strength.Filler metals that are part of the cage-type rotor should be resistant to attack by corrosivesulfurous gases. Copper alloy rotor construction should conform to American WeldingSociety (AWS) A5.8, and it should contain a minimum 40% silver. Copper-phosphorous,bronze-type fillers are unacceptable.

The rotor body of synchronous machines should be of the salient pole type with windings ofinsulated copper wire or strip that also are treated to withstand tropical conditions.

The insulation of rotor windings for both NEMA frame integral motors and form-woundmotors will be class F. The temperature rise above 50oC must not exceed those values thatare acceptable for class B insulation. Form-wound motor insulation systems should consist oflow-hygroscopic materials.

Mounting Details

Motors that are manufactured to IEC and NEMA standards use "dimension letter" codes todefine machine dimensions. To facilitate the replacement of IEC motors by NEMA motors,and vice versa, a comparison of dimensional code details is needed. Figures 3A and 3B showthe dimensional measurements that are necessary for motor replacement and the IEC andNEMA dimension code letters that correspond to the measurements. The NEMA code lettersare shown in parentheses.







Motor Dimensions with NEMA and IEC DimensionsFigure 3A




Motor Dimensions with NEMA and IEC DimensionsFigure 3B




Vertical

Figure 4 is a list of the most common vertical motor measurements and their associatedNEMA and IEC dimension code letters. This list can be used to determine the name of actualvertical motor measurements that were shown in Figures 3A and 3B.

NEMA and IEC Dimension Code Letters for Vertical Motors




Figure 4




Horizontal

Figure 5 is a list of the most common horizontal motor measurements and their associatedNEMA and IEC dimension code letters. This list can be used to determine the name of theactual horizontal motor measurements that were shown in Figures 3A and 3B.

NEMA and IEC Dimension Code Letters for Horizontal Motors




Figure 5




Cooling System

The IEC defines cooling as the means by which the heat that results from losses that occur ina machine is given up first to a primary coolant by means of an increase in coolanttemperature. The heated primary coolant can be replaced by a new coolant at a lowertemperature, or can be cooled by a secondary coolant in some form of heat exchanger.

Because of the increased heat that is produced in larger motors, the importance of motorcooling increases as the size of the motor increases . The environmental conditions to whicha motor is exposed will also dictate the amount of cooling that is required. The maximumtemperature rise of Saudi Aramco motors cannot exceed the temperature rise that is approvedfor class B insulation. The maximum temperature rise that is allowed for Class B insulation is80oC.

Where totally-enclosed machines utilize heat exchangers, closed, air-circuit, air-cooled(CACA) heat exchangers should be mounted on the motor. Top-mounted heat exchangerassemblies should have flanges that extend downward to overlap the motor enclosure on allsides by a minimum of 10 mm (0.4 in).

High-voltage motors with integral air-to-air heat exchangers should be provided withremovable sections or doors to allow easy access to the motor and the cooling fan balanceplanes without dismantling the motor or rotor assembly.

When air-to-air heat exchangers require auxiliary fan cooling, a shaft-mounted cooling fan orfans should be provided. Auxiliary motor-driven fans should not be specified. Internal andexternal cooling fans should be constructed of steel, bronze, or copper-free aluminum that issuitably treated to resist corrosion. Synthetic materials such as plastic are acceptable only forfractional kilowatt motors.

Internal and external fans that are designed for dual rotation are preferred. When uni-directional fans are necessary to meet the motor performance specifications, preference willbe given to fans of a reversible design that will facilitate future reversal of motor rotation.




Control and Supply Leads

The control and supply leads for Saudi Aramco motors must be designed to be moisture- andheat-resistant. The conductors of the control and supply leads should be made of copper, andthey should be designed for operation at a maximum ambient temperature of 50oC. Controland supply leads must have a minimum conductor size of stranded 2.5 mm sq (14 AWG), andeach lead should be clearly and permanently marked with a PVC sleeve wire marker.

Resistance temperature detectors (RTD) are used for temperature monitoring. The RTDsshould be of the platinum, three lead type, that are calibrated to a resistance of 100 ohms at0oC (32oF). The RTDs should be located in the slot portion of stator winding coils asfollows:

_Motors that are rated above 150 kW (200 Hp) and below 1300 kW (1750 Hp)should have one RTD per phase. Motors that are rated 1300 kW (1750 Hp)through 7500 kW (10,000 Hp) should have two RTDs per phase. Motors thatare rated above 7500 kW (10,000 Hp) should have three RTDs per phase. Thehottest reading RTD should be identified by the vendor during factory testing.

_Motors that are rated up to 1 kW (1.34 Hp) should be provided with a built-inthermal protective device that will open the motor supply circuit. RTDs shouldnot be used for these motor ratings.

Nameplates

The nameplates of Saudi Aramco motors should include all the information that is required byNEMA MG1 and IEC 34-1 and the additional information that is required by SAES-P-113.The following is a list of the information that NEMA MG-1 requires on motor nameplates.

_Manufacturer's name, serial number or date code, and suitable identification._Horsepower output or kilowatt._Time rating._Temperature rise._RPM at rated load._Frequency._Number of phases._Voltage._Rated-load amperes._Code letter for locked rotor KVA.




The following additional data is required by SAES-P-113 and can be supplied on a separatenameplate(s):

_Buyer's Purchase Order number._Year of manufacture._Manufacturer's location._Manufacturer's order reference number._Anti-friction bearing number and manufacturer._Class, Group, and Division (explosion-proof motors, only)._Maximum ambient temperature._Insulation system designation._Rotor weight._Total weight of motor.

Where two or more identical motors are supplied on one Purchase Order, the nameplates forall motors must show the temperature data and locked rotor current from the tested motor.

Saudi Aramco also requires that a separate nameplate be supplied to show the direction ofmotor rotation. The direction of rotation should be indicated by an arrow and the nameplateshould be located on the non-drive end of the motor.

The nameplate(s) and rotation arrows must be made from 300 series stainless steel or monel,be securely fastened to the motor by pins of similar material, and be located for easyvisibility. The entries on the nameplates must be marked by etching, engraving, or otherpermanent method of marking.

Space Heaters

The insulation of machines that are out of service for prolonged periods can absorb enoughmoisture to reduce the insulation resistance to a value that is below the allowable limit.Maintenance of the winding temperature 5oC above the surrounding ambient temperature willprevent moisture absorption of the insulation. Space heaters are used to maintain the windingtemperature at 5oC above ambient.

Electric strip heaters are the most common source of heat, and these heaters are convenient,easy to control, and inexpensive. The only inspection that is suggested for space heaters is anoccasional measurement of heater circuit current to detect burned out units or looseconnections. The space heaters should have no exposed elements.




The amount of heat that is required to raise the winding temperature of a given enclosedhorizontal motor approximately 5oC above ambient temperature, where the machine is closedexcept for a small vent at the top and bottom for circulation, is given by the followingformula:

H = 0.28 DL

where: H = Heat in kilowattsD = Machine end-bell diameter in metersL = Machine stator length between end-bell centers inmeters

For example, the heat (kW) that is required to raise the temperature of a horizontal motor withan end-bell diameter of one meter and a length of three meters to the specified 5oC aboveambient can be calculated as follows:

H = .28 DLH = (.28) (1M) (3M)H = .84 kW

The space heaters normally should be specified to operate on 120 Volt power suppliesbecause such supplies are normally available in all locations. In some existing Saudi Aramcoinstallations, it may be necessary to connect heaters in series to supply them from an existing480V system.

To ensure long operating life for a motor space heater, the heater nameplate voltage, asspecified in 17-SAMSS-502, must be twice the supply voltage that is indicated in the dataschedule. The following methods are available to control space heaters:

_Manual Switching.

_Thermostats that automatically energize and deenergize the heatersbased on the temperature inside of the enclosure.

_Auxiliary contacts that automatically energize the heaters when themotor is deenergized and that deenergize the heaters when the motor isenergized.

Saudi Aramco requires that auxiliary contacts in the switchgear be used for heater controlwhenever heaters are installed; therefore, manual switching or thermostats are unacceptablefor use in Saudi Aramco installations.




Heaters should be included with all motors that are supplied with motor operated valves andwith all motors that are rated 2.3 kV or higher. Other motors that are installed outdoors andthat are used only as standby equipment can also require heaters.

The surface temperature of space heaters for motors that are installed in classified areasshould not exceed the listed maximum allowable temperature for the area. The following arethe maximum allowable surface temperatures for classified areas of Saudi Aramcoinstallations:

Area Classification Maximum AllowableSurface Temperature

Class I, Group C 160oC (320oF)Class I, Group D 215oC (419oF)Class II, Group E 200oC (392oF)Class II, Group G 120oC (248oF)

Testing Requirements

The following tests should be made on machines that are completely assembled in the factoryand that are furnished with a shaft and a complete set of bearings:

_Measurement of winding resistance

The motor winding resistance test is performed to ensure that the correctwinding configuration and electrical connections have been made.

_No-load measurements of current, power, and nominal speed at rated voltageand frequency.

These measurements are performed to ensure that the motor operates within theno-load nameplate data.

_High-potential test

The high-potential test is performed to ensure that the motor's insulation systemis adequate.




_Vibration test

The motor vibration test is performed during the no-load run test to ensure thatthe motor is balanced during operation. The vibration test also sets a baselinelevel of vibration for future comparison.

_Measurement of bearing insulation resistance

The bearing insulation resistance must be measured to ensure adequateinsulation for the protection of personnel and equipment. Performance of thebearing insulation resistance test after all of the auxiliaries have been installedto the bearing housing will ensure that no breach in the bearing resistancesystem has been made.

_Bearing/lube oil temperature measurement

The bearing/lube oil temperature measurement is performed during the no-loadrun test to verify that the bearings operate within the established limits for theinstallation and for the lubricant.

The following tests should be performed when specified in the motor installation descriptiondata sheet.

_Performance Determination

The performance determination test are performed to verify, after installation,that the motor is performing its design function within the limits that areestablished by the installation.

_Temperature Tests

The temperature test on the stator is performed to ensure that, under load, theinsulation temperature does not exceed the maximum allowable temperatures.

_Miscellaneous Tests

Miscellaneous tests are any performance tests that the Engineer believes to benecessary for the installation. The exact tests that are performed and theacceptance criteria are established by the Cognizant Design Engineer.




_Surge Tests

The surge test is performed to ensure that the motor winding insulation issufficient to protect the motor windings from harm during any expected surges.

A detailed description of each of these test is included in Module EEX 203.04.

Painting and Coating

All steel surfaces must have the Vendor's standard finish with a minimum of 0.127 mm (5mil) dry thickness. The purpose of painting and coating all steel surfaces is to protect themotor from the environment.

Packing

The packing of equipment should be suitable for shipment by sea and by vehiculartransportation over unpaved, desert roads. Packing should be in accordance with Buyer'sPacking Specification No. 1 and 1.1 of Vendor's standard export packing. Vendor's standardexport packing should be subject to the approval of Saudi Aramco.




SPECIFYING MOTOR ENCLOSURE REQUIREMENTS

The motor enclosure requirements will vary with the type of motor that is installed and thearea in which the motor is installed. There are many different degrees of protection that areafforded by the design of motor enclosures. The discussion here will concentrate only onthose motor enclosures for use in Saudi Aramco applications, and it will cover the followingtopics:

_Motor Enclosure Functions_General Motor Enclosure Requirements_Saudi Aramco Motor Enclosure Requirements_Motor Enclosures for Classified Areas_Enclosures for Motor Auxiliary Equipment_Connection Boxes_Conduit Boxes_Grounding

Motor Enclosure Functions

All motor enclosures must provide the following functions:

_To protect personnel from the motor's energized and rotating parts.

_To protect the motor from the injurious effects of the environment, such assand, dust, rain, and water from cleaning operations (e.g., splashing).

_To afford a reasonable degree of mechanical protection against externaldamage to the motor.

_To protect a hazardous environment from a possible source of ignition.

General Motor Enclosure Requirements

All motor enclosures must protect against environmental and mechanical damage. Protectionfrom the environment is afforded through use of an enclosed air circulation system to preventfumes or gases from damaging the motor. Mechanical damage to the motor is preventedthrough use of screens and tight fittings that do not allow foreign material to enter the motor.




All enclosures for use in Saudi Aramco installations must meet a level of cooling andprotection as defined in International Electrotechnical Commission 34-5 (IEC 34-5) and inInternational Electrotechnical Commission 34-6 (IEC 34-6). IEC 34-5 designates the degreeof protection that the enclosure must provide for the motor. IEC 34-6 designates the degreeand method of cooling a motor.

IEC 34-5 has different protection codes that can be applied to motor enclosures. Figure 6shows the degree of protection that is indicated by the first characteristic numeral of an IECcode. The first column of Figure 6 shows the possible first characteristic numerals (0-5). Thesecond column of Figure 6 gives a brief description of the objects against which thatparticular enclosure will protect. The description of the object is based on the size of theobject. The third column of Figure 6 gives a definition that further describes the degree ofprotection.




Degree of Protection Indicated by the First Characteristic Numeral




Figure 6




Figure 7 shows the degree of protection that is indicated by the second characteristic numeralof the IEC code. The second characteristic numeral indicates the degree of protection theenclosure provides from water ingress. The first column of Figure 7 shows the possiblesecond characteristic numerals (0-8). The second column of Figure 7 gives a brief descriptionof the type of water protection that is indicated through use of the second characteristicnumeral. The third column of Figure 7 gives a definition of the type of protection that isindicated by each second characteristic numeral that further describes thedegree of protection.

Degree of Protection Indicated by the Second Characteristic Numeral




Figure 7




All enclosures for Saudi Aramco installations must be designated as IP44. IEC 34-5 definesthe protection code IP44 as follows:

IP = Ingress protection.

4 = First characteristic numeral indicates the degree of protectionthat is provided by the enclosure with respect to persons and toparts of the machine that are within the enclosure.

4 = Second characteristic numeral indicates the degree of protectionthat is provided by the enclosure with respect to the harmfuleffects of the ingress of water.

Saudi Aramco enclosures also must be designed for cooling in accordance with IEC 34-6.The degree and method of cooling is also designated by an IEC code. The IEC designationcode for the method of cooling of a machine consists of the following:

_The letters IC that indicate an IEC designation.

_A group of one capital letter and two characteristic numerals for each motorcooling circuit (e.g., A01).

The capital letter designates the medium that is used as the coolant, the first characteristicnumeral designates the circuit arrangement for circulating the coolant, and the secondcharacteristic numeral designates the method that is used to supply power for circulating thecoolant.

The following list indicates both the possible mediums that can be used as coolants and theIEC code letters that are associated with those mediums.

For other coolants that are not listed, the nature of the gas or liquid must be stated in full text.When the only coolant is air, the IEC code letter that designates the cooling medium can beomitted. Air is the only cooling medium that is approved for use in Saudi Aramco motors.

Figure 8 is a complete list of the first characteristic numerals for IEC cooling method codes.The first characteristic numeral describes the physical arrangement of the coolant circulatingsystem. The first column is a list of the possible first characteristic numerals (0-9). Thesecond column is a short designation of the coolant system arrangement. The final column isa definition that further describes the short designation of the coolant circuit arrangement foreach first characteristic numeral.




First Characteristic Numeral for Cooling Method Codes




Figure 8




Figure 9 is a complete list of the second characteristic numeral for IEC cooling method codes.The second characteristic numeral describes the method of supplying power for circulatingthe coolant. The first column is a list of the possible first characteristic numerals (0-9). Thesecond column is a short designation of the method of supplying power for circulating thecoolant. The final column is a definition of each second characteristic numeral that furtherdescribes the short designation of each code number.

Second Characteristic Numeral for Cooling Method Codes




Figure 9




The following is an example of an IEC code that designates the degree and the method ofcooling a motor:

ICA01

IC - Indicates that this designation is an IEC designation.

A - The coolant medium is air.

0 - The circuit arrangement is free circulation.

I - The method of supplying power to circulate the coolant isself-circulation.

When more than one cooling circuit is needed to cool a machine, the IEC designation consistsof the following:

_The letters IC.

_A group of one letter and two numerals for the circuit on the user's side that isat the lower temperature (secondary cooling circuit).

_A group of one letter and two numerals for the circuit that is closer to thewinding and that is at the higher temperature (primary cooling unit).

The IEC cooling codes are the same as in the single system. The following is an example ofan IEC code that designates the degree and the method of cooling a motor that requires twocooling circuits:

ICA01A61

IC - Indicates that this designation is an IEC designation.

A01 - Secondary cooling circuit (low temperature).


0 - The circuit arrangement is free circulation.

- The method of supplying power to circulate the coolant isself-circulation.

A61 - Primary Cooling Circuit (high temperature)





6 - The circuit arrangement is a machine-mounted heatexchanger that uses the surrounding medium.

1 - The method of supplying power to circulate the coolant isself-circulation.

Saudi Aramco Motor Enclosure Requirements

NEMA MG-1 allows the use of numerous types of motor enclosures; however, only threetypes of enclosures are approved for use in Saudi Aramco applications. The following aremotor enclosures that are allowed in Saudi Aramco applications:

_Totally-enclosed fan-cooled (TEFC)_Environmental protection totally-enclosed air-to-air cooled (CACA)_Weather protect type II (WP-II).




TEFC

NEMA MG1 defines a TEFC enclosure as a totally-enclosed fan-cooled machine that isequipped for exterior cooling through use of a fan or fans that are integral with the machine,but that are external to the enclosing parts. The level of environmental protection that isprovided by a TEFC enclosure will vary, and stringent environmental protection requirementscause this enclosure to be more complicated and more expensive. TEFC enclosures withoutheat exchangers are not permitted for motors that are rated above 11,000 kW (15,000 Hp).This requirement is due to the heat dissipation requirements of the motor.TEFC enclosures for use in Saudi Aramco applications must have the following IECdesignation codes for environmental protection and cooling:

_IP44 for protection_ICAO1A41 for cooling

CACA

CACA is a variation of the simpler TEFC machine, but it includes an air-to-air heat exchangerto provide more effective cooling on larger machines. CACA is commonly known as a closedair-circuit, air-cooled type of enclosure.

NEMA MG1 defines a CACA as a totally-enclosed air-to-air cooled machine that is cooledthrough circulation of the internal air through a heat exchanger that, in turn, is cooled throughcirculation of external air. The level of environmental protection that is provided by a CACAenclosure will vary, and stringent environmental protection requirements also cause thisenclosure to be more complicated and expensive. A CACA enclosure is provided with an air-to-air heat exchanger for cooling the internal air, a fan or fans that are integral with the rotorshaft or separate for circulating the internal air, and a separate fan for circulating the externalair.

CACA enclosures should be specified for induction motors and for salient pole synchronousmotors that are rated up to 11,000 kW (15,000 Hp).

CACA enclosures for use in Saudi Aramco applications must have the following IECdesignation codes for environmental protection and cooling:

_IP44 for protection_ICA01AG1 for cooling

WP-II

NEMA MG1 defines a WP-II as an open machine with ventilating passages that are soconstructed as to minimize the entrance of rain, snow and air-borne particles to the electric




parts, and with ventilated openings that are so constructed as to prevent the passage of acylindrical rod that is 0.75 inch in diameter.




The WPII ventilating passages at the intake and the discharge are so arranged that high-velocity air and air-borne particles that are blown into the machine by storms or high windscan be discharged without entering the internal ventilating passages that lead directly to theelectric parts of the machine itself. The normal path of the ventilating air that enters theelectric parts of the machine should be arranged through use of baffles or separate housings toprovide at least three abrupt changes in direction, none of which can be less than 90o. Inaddition, an area of low velocity that does not exceed 3 m/s (600 ft/min) should be providedin the intake air path to minimize the possibility of moisture or dirt being carried into theelectric parts of the machine.

The WP-II type of enclosure does not afford the same degree of protection as TEFC types, butit may be acceptable for synchronous motors with rated outputs that are above 11,000 kW(15,000 Hp) where the cost advantage over a TEFC type of enclosure is significant.

WPII enclosures for use in Saudi Aramco applications must have the following IECdesignation codes for environmental protection and cooling:

_IP44 for protection_ICA01 for cooling

Motor Enclosures for Classified Areas

This section only covers the usual Class I, Division 1 and 2 locations with Group D hazardsthat are found in Saudi Aramco installations.

For a Division 1 area, the motor enclosure must be explosion proof (Exd). The totally-enclosed flameproof motor is preferred for motor sizes up to about 500 kW (700 Hp) becauseof the TEFC ruggedness and simplicity. For larger motor sizes, the normal practice is toavoid Division 1 locations because of the cost of the enclosures.




For a Division 2 area, the motor enclosure must be non-sparking (Exn). The type ofprotection "n" is such that during normal operation, the motor is not capable of causingignition, and a fault that is capable of causing ignition is not likely to occur; therefore, anytype of enclosure that prevents sparks can be utilized.

To verify that a motor that is installed in a hazardous area is permitted in that area, additionalinformation must be included on the nameplate. All the information that is required byNEMA MG1 must be on the nameplate, plus the following additional information:

_Class, division, and/or group of hazardous atmosphere type for which themachine is approved.

_Type of protection that is provided.

_Temperature class for which the motor is approved.

_Maximum exposed temperature of the machine.

Enclosures for Motor Auxiliary Equipment

Motor heaters are mounted within the motor enclosure; therefore, no special enclosures arerequired for motor heaters. The surface temperature limitations of the area apply to allsurfaces that are in contact with the air that is inside and outside the motor. The motor heatersmust not exceed the maximum allowable exposed temperature in the area.

Instruments that can be fitted into the motor enclosure also require no special enclosure.Where instruments are external to the motor enclosure, the instrument enclosure should beequal to, or better than, the motor enclosure. Instruments that are fitted to motors in Zone 1and Zone 2 classified areas should be flameproof Exd, unless these instruments are certifiedor approved as part of the motor. Sparking devices must be housed in hermetically-sealed orExd enclosures.

Connection Boxes

The terminal enclosure for termination of main windings, control/measurement circuitry, andauxiliary electrical supplies should meet or exceed the enclosure requirements for the mainmachine. At a minimum, the connection boxes must meet the requirements of NEMA 4.Terminal boxes and connectors must withstand the effect of faults within the enclosure asfollows:




_To accommodate, without detriment, the maximum through fault current thatis available at the terminals with a fault clearance time of 250 milliseconds.The prospective fault MVA's should be as follows:

System Voltage Fault MVA480

2,4004,1606,900

13,800

40250350500750

_To contain, or relieve, the consequence of an internal terminal box faultwithout there being an external detrimental effect to personnel.

All motors that are rated 1 kW (1.5 Hp) and above should be provided with a main connectionbox that is located on the right hand side of the motor frame as viewed from the non-drive endof the motor. When it enters the box, the conduit should be parallel to the shaft, and it shouldenter the box as follows:

_The conduit should enter horizontal motors from the side that is opposite ofthe shaft extension.

_The conduit should enter vertical motors from the side of the shaft extension.

The connection box can be mounted on the end of the motor that is opposite the shaftextension on motors with rated outputs below 1 kW (1.5 Hp).

Connections for auxiliaries are not permitted in the main connection box. Separate boxes,which should normally be mounted on the opposite side of the motor to the main terminalbox, should be used for each type of instrument or auxiliary supply. A single box for allinstruments or transducers that are of one type is preferred. No auxiliary wiring is to be takenthrough the main terminal box. Circuits that have different voltages are not permitted in thesame box unless special precautions are taken and suitable warning labels are provided.




Connection Boxes (Cont'd)

High voltage motors that are rated 2.3 kV and above must be supplied with separableconnectors. The premolded slip-on type cable terminators are suitable for the cable size asspecified in the purchase order. A vented terminal enclosure should be specified to providemechanical protection and to terminate the conduit: it is not necessary to equal the protectionthat is provided to the motor itself. The terminal enclosure does not need to meet theenclosure requirements of the motor because there must be a seal between the two enclosures.

The terminal enclosure must be metallic and meet or exceed the motor enclosurespecification. The terminal enclosure must withstand fault conditions as noted above, and itmay require anti-condensation heaters and a condensate drain. A rotatable, diagonally-splitbox or enclosure is preferred.

Conduit Boxes

All cabling must be enclosed in rigid or flexible steel conduit unless protection is providedthrough the use of armored cables. Flexible connections only can be used where movement isto be expected in service.

The following conduit construction requirements must be met by all conduit installations:

_All conduit connections to motor terminal boxes must be made through use ofthreaded conduit hubs that have tapered pipe threads of which at least fivethreads are fully engaged.

_The conduit assembly must form a weatherproof and dust-tight system that ishighly resistant to mechanical damage.

Connection through the use of couplings can be installed, if necessary, when conduit boxesare installed in Class 1, Division 1, locations, and the conduit box must be sealed within 18inches to complete the explosion-proof enclosure.




Grounding

Saudi Aramco motors must be grounded for the following reasons:

_To safeguard a person from electric shock by ensuring that, under faultconditions, all surfaces with which the person is in contact (including thesurfaces of metallic equipment and the ground) remain at safe relativepotentials.

_To reduce the possibility of static discharge and fire risk in hazardous areas.

The main frames of motors that are rated up to 150 kW (200 Hp) should be provided with acorrosion resistant grounding stud for connection to the ground grid by means of 25 mm sq(No. 4 AWG) grounding cable. A grounding stud should be provided in the motor terminalbox to ground the cable shield. Motors that are rated 150 kW (200 Hp) and above should beprovided with flat corrosion resistant grounding pads that are drilled and tapped for NEMAtwo hole connectors and that are located on diagonally opposite corners of the non-removableportion of the motor main frame.

The ground connection must accommodate the following minimum size of ground cable:Motor Rating Cable Size

kW (Hp) mm sq (AWG/MCM)185 < 370

370 < 33603360 & above

(250 < 500)(400 < 4500)

(4500 & above)

70120185

(2/0)(4/0)(350)

The main connection box and all auxiliary connection boxes should be provided with aninternal grounding clamp or bolt to provide cable grounding facilities. The dimensions of thegrounding clamp or bolt should accommodate the grounding core of main or auxiliary cables,and they should provide a terminal point for the cable ground shield.




SPECIFYING MOTOR STARTING METHODS

When specifying a method of motor starting, the Electrical Engineer must evaluate eachpossible method of starting the motor and the different limitations of each method of starting amotor. This section will cover the following topics that are pertinent to specifying motorstarting methods:

_Methods_Factor Considered in Starting Method Selection

Methods

There are many methods that can be used to start a motor. The designer must compare eachmethod in order to specify the correct method of starting a motor. The following startingmethods are available:

_Full Voltage Starting_Autotransformer Starting_Primary Reactor Starting_Wye-Delta Starting_Primary Resistance Starting_Part Winding Starting_Variable Frequency Starting_Electronically-Controlled Reduced Voltage Starting




Methods (Cont'd)

Full Voltage Starting

Full voltage starting is the preferred method of starting Saudi Aramco motors. Figure 10 is asimplified diagram of a full voltage motor starter. Contacts 1, 2, and 3 are shut through use ofa circuit breaker or a contactor to start a motor that uses a full voltage motor starter. Whencontacts 1, 2, and 3 are shut, power from the line will be applied to the motor stator at fullrated voltage.

Full Voltage Motor StarterFigure 10




Methods (Cont'd)

Figure 11 shows the typical full voltage starting torque, speed, and kVA characteristics. Fullvoltage starting provides the most starting torque of the possible starting methods. Abyproduct of the large starting torque of the full voltage starter is that it draws both the largeststarting current of any of the methods of motor starting and a high initial kVA. The startingcurrent of a motor that uses full voltage starting will remain relatively high until the motor'sspeed reaches about 90% of synchronous speed. In a motor with a long run up time, the largeamount of current becomes a concern because of the extra heating effect of the large kVA andcurrent values. The high torque that is created will reduce the time that the motor requires toreach rated motor speed.

Full voltage starting is the least expensive and the simplest method of starting. Relative costcomparison tables for all types of motor starting are contained in Work Aid 3.

Full Voltage Starting Typical Torque, Speed, kVA CharacteristicsFigure 11




Autotransformer Starting

Figure 12 is a simplified diagram of an autotransformer motor starter. An autotransformer isplaced in each phase (A, B, and C) of the supply voltage that will supply a percentage of fullrated voltage to the motor stator during starting. Contacts 2, 3, 4, 6, and 7 must be shut tostart a motor that uses the autotransformer starter. Shutting contacts 2, 3, 4, 6, 7 will apply aportion of the rated voltage to the motor stator to start the motor. The percent of full ratedvoltage that is applied to the motor is determined by the position of the autotransformer linetaps. When the motor is started and running, an operator or an automatic control circuit willtransfer the motor from start to operate. The transfer causes contacts 2 and 7 to open, contacts1, 5, and 8 to shut, and contacts 3, 4, and 6 to open. This sequence of contact operationapplies full voltage to the motor stator for running the motor.

Autotransformer Motor StarterFigure 12




The line current, motor starting torque, and motor maximum torque are proportional to thesquare of the motor's applied voltage. For this reason, the actual starting torque of a motorthat uses an auto-transformer can be varied through a change in the position of theautotransformer line taps.

Figure 13 shows the typical autotransformer starter torque, speed, and current/kVAcharacteristics. The motor torque for the autotransformer is compared to what the motortorque would be if full voltage starting was employed for comparison. The large spike inmotor kVA, current, and torque at about 75% synchronous speed is the point at which themotor is switched from autotransformer starting to full line voltage.

Autotransformer starting requires the least amount of starting kVA for an equal initial torquerequirement as compared with other starting methods (part-winding excepted). Auto-transformer starting results in a higher initial torque than resistor or reactor starting for anequal supply (line) current.

The current that is drawn by the autotransformer starter is less than the current that is drawn inthe full voltage starter method, but starting torque is proportionally lower.

An autotransformer starter costs approximately two to five times the price of a full voltagestarter. A comparison of cost of the autotransformer starting method with the other startingmethods is included in Work Aid 3.

Autotransformer Starter Torque, Speed, and Current/kVA CurvesFigure 13




Primary Reactor Starting

Figure 14 is a simplified drawing of a primary reactor starting circuit. The addition of thereactors to the motor circuit will lower the voltage that is applied to the motor. Such loweringof voltage will lower the starting current and torque. Contacts 1, 2, and 3 are closed to applyvoltage from the line to the motor stator to start a motor with a primary reactor starter. Thestator will be connected together through reactors that will limit the amount of current flow inthe motor stator during starting. When the primary reactor starter is shifted from start to run,contacts 1, 2, and 3 will remain closed and contacts 4, 5, and 6 will close. Closing contacts 4,5, and 6 will short out the reactors and lower the impedance of the stator. The lowerimpedance of the stator will allow full voltage to be applied to the motor for runningoperations.

Primary Reactor Starting CircuitFigure 14




Methods (Cont'd)

Figure 15 shows typical motor torque, kVA, and speed curves for a motor with a primaryreactor starter. The motor torque of a motor that is started with full-voltage is given for use asa comparison of different methods of starting. Notice the disturbance in KVA and torqueduring the transfer from start to run. A quick transfer from start to run will help provide asmooth run-up of the motor with primary reactor starting. Autotransformer motor startingwill draw less line current for the same amount of initial torque as a primary reactor startedmotor; however, primary reactor or primary resistors give higher accelerating torque over thestarting period for the same initial torque conditions because the voltage across the motorincreases as the motor comes up to speed. With autotransformer starting, the motor's appliedvoltage is constant until the transition is made.

Primary reactor starting provides a smooth run-up speed with only a slight disturbance at thetransition from "start" to "run." The use of a variable reactor can further improve the run-upcharacteristics. The line current at starting is proportional to the motor's applied voltage, andthe starting torque is proportional to the square of the motor's applied voltage.

A primary reactor starter costs approximately 250% of the cost of a full voltage starter. Acomparison of the primary reactor starting method with the other methods is contained inWork Aid 3.

Typical Motor Torque, kVA, Speed Curves of aPrimary Reactor Starting Motor




Figure 15




Wye-Delta Starting

Figure 16 is a simplified diagram of a wye-delta motor starting diagram. The motor is startedin a wye configuration and is switched to a delta configuration at, or just below, full speed.Contacts 1, 2, 3, 4, 5, 6 are shut to start a wye-delta starting motor. These contacts connectthe motor stator winding in a wye configuration. Connection of the stator windings in a wyeconfiguration will reduce the starting current that is necessary to develop the required startingtorque by the. When the motor is just below full speed, the motor is switched to a deltaconfiguration. Contacts 4, 5, and 6 will open and contacts 7, 8, and 9 will shut to switch themotor from a wye to a delta configuration. When contacts 7, 8, and 9 are shut, the statorwindings will be connected in a delta configuration, and this configuration applies full linevoltage to the motor stator windings for normal running operation.

Wye-Delta Motor Starting DiagramFigure 16




Figure 17 shows the torque, speed, and kVA curves for a wye delta starting motor. Motortorque at full voltage is shown for comparison. Through connection of the motor in a wyeconfiguration during starting, the starting kVA and starting torque are reduced toapproximately one-third of their full-voltage values. Wye-delta starting can be used wherelow motor torques are required. The motor's current will follow the kVA requirement whenthe wye-delta motor starting method is used.

When the motor switches from a wye to a delta configuration, a large jump in motor kVA andtorque will result. The jump in the motor kVA and torque can cause disturbances in the motoroperation, and this jump in torque and kVA must be considered in the selection of this methodbecause it can cause the motor speed run up to be rough. Some equipment cannot withstandthe rough run-up when this starting method is used.

A wye-delta starting motor will cost between three and six times the cost of a full voltagestarting motor. A comparison of the relative cost of a wye-delta starting motor and the othermethods of starting is given in Work Aid 3.

Torque, Speed, and kVA Curves for a Wye Delta Starting MotorFigure 17




Primary Resistance Starting

The primary resistance starting method is essentially the same as a primary reactor startingmethod. The only difference between the primary reactors and primary resistance starters isthat, in the primary resistance starting method, the reactors are replaced by resistors in thestarting circuit. Figure 18 is a simplified diagram of a primary resistance motor starter.Notice that the diagram is the same as the diagram for a primary reactor starter, except that thereactors have been replaced by resistors. Contacts 1, 2, and 3 are closed to apply line voltageto the motor stator to start a motor with primary resistance starting. The resistors that are inseries with the stator limit the current flow in the stator. When the motor accelerates to ratedspeed, contacts 4, 5, and 6 close to short out the resistors and to allow full current to flow.

Primary Resistance Motor StarterFigure 18




Figure 19 shows the typical motor torque, speed, and kVA characteristics for a primaryresistance motor starter. Motor torque at full voltage is shown for comparison. Notice thatthe primary resistance motor starter curves are identical to the starter curves of a primaryreactor motor starter. The line current at starting is proportional to the motor's appliedvoltage; starting torque is proportional to the square of the motor's applied voltage. Motorstarting kVA is high for the amount of starting torque that is developed by the motor. Themotor speed run-up will be smooth until the motor is switched from start to run. The rapidjump in motor torque and kVA can cause a disturbance in the motor's speed that is notacceptable for certain loads.

A primary resistance motor starter costs approximately 520% of the cost of a full voltagestarter. For economic reasons, reactors rather than resistors are used with all but the smallersizes of motors. A comparison of the relative cost of a primary resistance motor starter andother motor starting methods is given in Work Aid 3.

Torque, Speed, kVA Curves for Primary Resistance Motor StarterFigure 19




Part Winding Starting

Figure 20 is a simplified diagram of a part winding motor starter. The part winding motorstarter has two sets of contacts to supply the motor. Part-winding motors are similar inconstruction to standard cage motors except that two parallel windings are provided in thestator and six leads must be included. Contacts 1, 2, 3 are closed to start a part windingmotor. This closing will energize one set of windings in the motor. After the motor is almostat full speed, contacts 4, 5, 6 are closed. This closing will energize the other set of windingsto supply full line voltage to the motor stator.

Part Winding Motor StarterFigure 20




Figure 21 shows the typical torque, speed, and kVA curves for a part winding starting motor.Motor torque at full voltage starting is shown for comparison. Starting torque on the firststarting point varies from 48% to 72% of full load torque. The actual percentage depends onmotor design, size, and speed. Notice the spike in motor torque and kVA when the second setof windings is energized. The sudden change in motor torque and KVA can cause a largedisturbance in motor speed and run-up. Starting current varies from 50% to 80% of thelocked-rotor current with both windings. The actual percentage depends on motor design,size, and speed.

Part-winding starting can only be used with limited types of loads because of the smallamount of starting torque that is generated on the first step of acceleration. Part-windingstarters are used with motors that drive low inertia, low-torque starting loads such as air-conditioning compressors, refrigeration compressors, centrifugal pumps, fans and blowers.This method also is used where reduced starting torque is necessary.

A part-winding motor starter will cost approximately two to four times a full voltage starter.A cost comparison of part-winding motor starting with other motor starting methods is givenin Work Aid 3.

Typical Torque, Speed, and kVA Curves of a Part Winding Starting MotorFigure 21




Variable Frequency Starting

Figure 22 is a simplified diagram of a variable frequency motor starter. Variable frequencystarting is essentially the same as full voltage starting except that the full voltage andfrequency input to the motor stator can be converted to different values. The frequency andvoltage converter may be a short-time rated motor-generator set or a solid-state unit.

Both the output voltage and frequency of a variable frequency motor starter must be adjustedproportionally to each other. The output voltage and frequency must vary proportionally toensure that the motor does not draw excessive current as a result of the lower impedance thatis present at low frequencies.

Contacts 1, 2, and 3 are shut to start a motor that uses a variable frequency starter. Whenthese contacts shut, the voltage and frequency at the output of the voltage and frequencyconverter will be applied to the motor stator.

Variable Frequency Motor StarterFigure 22




One frequency and voltage converter may be used for several motors on one bus if only onemotor is to be started at any one time. A smooth motor speed run-up is possible through thesteady increase of the supply frequency to the motor from the starting value to 60 Hz.Because of the rise in the frequency that is applied to the motor, no current surge is imposedon the supply system.

The torque, speed, kVA, and current curves of a variable frequency motor will follow thesame shape as for full voltage starting. The values of torque, speed, kVA, and current willchange proportionally with the applied value of voltage and frequency. Maximum motortorque up to the full voltage, full frequency, breakdown torque is possible throughout the run-up period.

Variable frequency drives are mainly used for speed control. The use of a variable frequencydriver only as a starting device is generally cost prohibitive.

Electronically-Controlled Reduced Voltage Starting

Electronically-controlled reduced voltage starters employ back-to-back, phase-controlledthyristors in two or three of the lines to the motor as shown in Figure 23. The thyristors (G1 -G6) are controlled during the starting period to maintain the starting current at about 300% ofthe full load current through the gradual increase of the motor voltage from the initial valueup to full line voltage.

Electronically Controlled Reduced Voltage StarterFigure 23




The torque, speed, kVA, and current can be easily adjusted through change in the amount oftime that the thyristor will conduct, and this change will change the applied voltage. Themotor run-up will be smooth because the applied voltage will gradually increase as the motorspeeds up and because there is no mechanical switching.

The electronically-controlled reduced voltage starter is applied where the line current iscritical and where repetitive motor starting would limit the life of electromagnetic contactors.The cost of an electronically-controlled reduced voltage starter is prohibitively high in mostinstances.

Factor Considered in Starting Method Selection

Full voltage starting is used for the majority of Saudi Aramco induction and synchronousmotor applications. Reduced voltage starting only should be considered when one of thefollowing conditions exist:

_The calculation of the voltage drop that results from motor starting indicatesthat the applied voltage at the motor terminals will be less than 80% of themotor nameplate voltage.

_The load and/or the connection between the load and the motor may bedamaged by the sudden application of full voltage starting torque.

_The motor will be started several times an hour or the motor draws excessivestarting current.

In certain applications, the motor must be capable of starting under the worst case conditions.The characteristics of a motor that one chooses for a particular application must match anentire range of load torque characteristics; however, one must avoid the unnecessaryexpenditures that can result from over-specification. The worst case conditions are assumedbecause, if a motor will start under these conditions, the motor will start under any conditions.Worst case conditions are hypothetical conditions that assume that all possible detrimentssimultaneously occur. The worst case condition is when there is maximum load on the motor,while there is the lowest possible bus voltage, all other loads are running, and the largestmotor starts. The resulting voltage drop can cause longer acceleration time for the largestmotor and a heating that can result in deterioration and/or failure of the insulation. Also, thevoltage drop can trip breakers and deenergize other loads. Many more possible conditionsthat depend on the conditions that are present at a given installation could exist; therefore,each installation must be individually evaluated.




Factor Considered in Starting Method Selection (Cont'd)

The following factors are considered in the selection of a starting method:

_Permissible system voltage drop_Required starting torque of the load/load connection_Load current (heating) limitations_Comparative cost

Terminal voltage that drops below 80% of full voltage value can still result in successfulstarting of the motor; however, the drop in the terminal voltage will cause the necessarycurrent draw to increase in proportion to the drop in terminal voltage. The problem ofexcessive voltage drops when motors are started is that other loads on the system can beeffected. Checks should be made to ensure that the motor controllers for any running motorsthat are on the same bus or on any other bus that is affected by the voltage depression remainheld-in, and that the running motors do not stall. Figure 37 in Work Aid 3 shows theapproximate voltage drop that results from full voltage starting of a motor.

Motors and their respective loads must be connected by some means. The method ofconnection can be a hard permanent connection, a spider contact, or a simple belt. Themethod of motor/load connection must be considered in the selection of a motor startingmethod. Application of full voltage at starting will cause a large amount of torque on themotor/load connection. Starting torque that exceeds the rating of the motor/load connectionwill cause the motor/load connection to fail.

Motors that are started with full voltage will develop the largest amount of starting currentpossible. The large starting current will add to the heating of the motor. A motor that isstarted several times an hour or that has an excessively high starting current could heat up andfail. The use of a reduced voltage starting method will reduce the starting current and therebyreduce the heating of the motors.

Starting currents will vary with the method of starting the motor. Starting current will beproportional to the applied voltage and the developed starting torque of the motor startingmethod. Starting current will cause the motor to heat, and consideration must be given to howmuch starting current will be drawn and how often the motor will be started.

A comparison of torque and of starting values for various starting methods is given in Figure38 of Work Aid 3.




Factor Considered in Starting Method Selection (Cont'd)

For normal applications, full voltage direct-on-line starting is preferred for economic reasons.When it is necessary to use reduced voltage starting, the selection of appropriate startingmethods will generally be made on economic grounds.

A cost comparison for the different methods of motor starting for various size motors is givenin Work Aid 3.

Because other methods of starting are unusual in Saudi Aramco installations, approval mustbe sought from the Technical Services Department before implementation of a reducedvoltage starter.




SPECIFYING MOTOR PROTECTION REQUIREMENTS

Motor protection requirements will vary with the type, size, duty, and application of motors.Protective devices should be specified with care to allow correct coordination with otherprotective devices and to avoid the possibility of misapplication in the field. Particular care isrequired with the specification of non-adjustable devices such as fuses. The sections thatfollow provide information on the following topics that are pertinent to specifying motorprotection requirements:

_Overload Protection_Short-Circuit Protection_Ground-Fault Protection_Current Unbalance_Vibration Protection_Bearing Failure Protection_Stator Winding High Temperature Protection_Undervoltage Protection_Overvoltage Protection and Surge Protection_Motor Stalling Protection_Differential Protection_Additional Protection for Synchronous Motors_Saudi Aramco Motor Protection Schemes

Overload Protection

Overload protection requirements and methods will vary with the size of the motor that is tobe protected. The reasons for overload protection and range settings of overload protectionwill remain relatively constant, but the type of overload protection that is provided willchange as motor size changes. The following aspects of motor overload protection will bediscussed in this section:

_Reasons for Overload Protection_Settings for Overload Protection_Types of Overload Protection

Reasons for Overload Protection

The effect of an overload is a rise in temperature in the motor windings. Large overloadscause the temperature of the motor to quickly increase to a point where damage to theinsulation and the lubrication of the motor occur; therefore, an inverse relationship existsbetween current and time (e.g., for higher currents, motor damage or "burnout" can occur in ashorter period of time than for lower current).




Overload Protection (Cont'd)

The increase in motor winding temperatures that is caused by overloads shortens motor life bythe deterioration of the insulation. Relatively small overloads of short duration cause littledamage but, if sustained, these small overloads could be just as harmful as overloads ofgreater magnitude. The relationship between the magnitude (percent of full load) and theduration (time in minutes) of an overload is illustrated by the Motor Heating Curve that isshown in Figure 24. At a 300% overload, the particular motor for which this curve ischaracteristic would reach its permissible temperature limit in three minutes. Overheating ormotor damage would occur if the overload persisted beyond this time limit.




Motor Heating Curve DataFigure 24

Settings For Overload Protection

Overload devices should be sized and specified at the design stage with trip points that can beset in the field. Overload devices should be the adjustable type with a minimum range ofadjustment of 90% to 110% of the nominal trip point. The sizing of the overload deviceshould be based on the motor nameplate full load current.


A knowledge of motor starting current, total run-up time, and permissible stall time isrequired to correctly select motor overload protective devices. For most of the run-up period,the motor draws a current that is approximately the same as locked rotor current. Motorcurrent sensing protection can only discriminate between stalling and normal run-up wherethe permissible stall time exceeds the run-up time. Where the run-up time exceeds thepermissible stall time, extra protection for stalling is required. Extra protection for stalling isdiscussed later in this section.

The NEC requirement for running overload protection is that tripping should ultimately occurat motor currents of not more than 115% of nameplate full load current. Excessive damage tothe motor will be prevented if the motor is tripped at less than 115% of full load.

Alternatively, where, because of the high starting current or the long run time, this 115% offull load setting does not allow correct motor starting. A higher trip setting of up to 130% ofnameplate full load current is usually permissible (NEC trip point should be finally set byField Engineers). The field setting should allow limited duration overloads that are essentialto process continuity, and it should be a setting that prevents nuisance tripping during startingor as a result of nominal supply voltage fluctuations.





The key to setting an overload relay is shown in Figure 25. The figure shows a motor heatingcurve and a curve of the time that an overload requires to trip. Setting the overload relays totrip at the value on the time required to trip curve will ensure that the overload device willalways trip at a safe value before the motor overheats.

Motor Heating Curve vs. Time Required to TripFigure 25

Types of Overload Protection

The type of overload protection that is provided for a motor depends on the size of the motor.The two types of overload protection that are available are fuses and relays. Fuses are not




acceptable in Saudi Aramco applications because a fuse cannot be adjusted. The followingare the types of overload relays that are available:

_Magnetic_Thermal





Figure 26 shows a magnetic overload relay. The magnetic overload relay consists of amovable iron core plunger that is inside of a solenoid coil. Current is supplied to the solenoidcoil from the circuit's sensing network. As current passes through the solenoid coil, a flux isestablished in the solenoid coil. The flux that is established in the solenoid coil will pull theiron core plunger upward. The moving contact that is attached to the end of the iron coreplunger also will move upward. Movement of the moving contact will bridge the gapbetween the two fixed contacts that are in line with the circuit trip coil. The circuit to thebreaker trip coil will be completed when the gap is bridged between the fixed contacts, andthe breaker will operate.

Magnetic Overload RelayFigure 26





The movement of the core can be slowed by an oil-filled piston (called a dashpot) that isattached to the core. The rate that the oil passes the piston is adjusted through opening orclosing oil ports on the piston to adjust the tripping point of the overload relay. Closing theoil ports on the piston causes more resistance to the movement of oil past the piston, and therelay will take longer to trip. Conversely, opening the oil ports on the piston will reduce theresistance to movement of the piston, and the relay will trip sooner.

A thermal overload works on the principle of heat buildup, which is caused by current flow.When an overload is placed on a motor, the motor will draw more current. The increase incurrent flow will cause an increase in the heat that is produced by the current flow. Thisincrease in heat is the basis for thermal overload protection. All thermal overload protectionrelays operate on the principle of tripping the motor at a preset heating value of current flow.The thermal overload relay will sense the amount of current that flows to the motor, and thiscurrent flow will cause the thermal overload to heat up and trip the motor. There are twotypes of thermal overloads:

_Melting alloy_Bimetallic

Figure 27 shows a melting alloy type thermal overload relay. In the melting alloy thermaloverload relay, the motor current passes from the equipment connection terminals through aheater winding. The heat that is developed by the heater winding causes a special solder inthe solder pot assembly to melt. The melting of the solder allows a ratchet wheel that wasbeing held by the solder to spin free and open a set of contacts. The motor will trip when thecontacts open.




Melting Alloy Type Thermal Overload RelayFigure 27


Figure 28 shows a bimetallic thermal overload relay. The bimetallic thermal overload relaysenses current through the use of a u-shaped bimetallic strip that is associated with a currentcarrying heater coil. An overload condition will cause an increase in the current that passesthrough the heater coil. The increase in current will cause more heat to be produced in theheater coil. The increased heating of the heater coil will cause the u-shaped bimetal strip toheat up. The u-shaped bimetal strip is constructed of two different metals; as the temperatureof the metals increase, each metal will expand at a different rate. The difference in the rate ofexpansion of the two metals will cause the u-shaped bimetal strip to deflect and close a set ofcontacts on the contact assembly. Opening the contacts will cause the motor to trip.




Bimetallic Thermal Overload RelayFigure 28


Motors that are rated less than 1 kW (1.5 Hp) should be provided with thermal protectivedevices that are built into the starter. Built-in thermal protective devices should be of the self-resetting type except for open motors. The manual reset types are acceptable in open motors.Note that open motors (e.g., motors with drip-proof enclosures) are no longer acceptable inSaudi Aramco applications, but existing units need not be replaced. Automatic-resetprotective devices should not be applied where the sudden restarting of a motor could behazardous to personnel or equipment.

Low voltage integral kW motors up to 600V and above 1.5 Hp require three thermal overloaddevices, of which one device should be in each phase. A trip action from any one device willcause all three phases of the starter to open. Manual reset is required, and the trip settingshould be capable of field adjustment.

Thermal overload devices must be of the ambient-compensated type because, in most SaudiAramco installations, the motor starter/breaker is in an air conditioned environment and themotor is in an outdoor environment. The trip setting of ambient-compensated thermal




overload devices will not vary significantly for ambient temperatures up to 60oC. Ambienttemperatures of up to 60o will cover the majority of Saudi Aramco installations; for higherambient temperatures, the manufacturer's advice on heater selection should be sought.

Where power circuit breakers or molded case circuit breakers are used as motor starters, threecurrent transformer-fed, ambient-compensated, thermal overload relays should be used, ofwhich one relay should be in each phase. The relays should be reset by hand, and they shouldbe arranged to trip the circuit breaker through use of a shunt trip device. Direct-acting tripdevices should not be used to provide motor overload protection.

Motors that operate on 2300 and 4000 V are split into the following two size categories:

_1100 kW (1500 Hp) and below_Above 1100 kW (1500 Hp)

For motors that are 1100 kW (1500 Hp) and below, three current transformer-fed, ambient-compensated, thermal overload relays should be used, of which one relay should be in eachphase. The relays should be reset by hand and arranged to open the supply line to the motor.

For motors that are above 1100 kW (1500 Hp), motor overload protection should be providedby RTDs. The overload protection should be designed to activate a visible alarm in thecontrol room and to trip the motor supply when the stator RTDs reach a preset value.


Overcurrent relays should also be included as a backup for the RTD overload protection. Thetemperature value of the RTDs that will trip the motor should be set so that the motor does notoverheat.

For 6.6 and 13.2 kV motors, only circuit breaker starters should be used. Motor overloadprotection should be by RTD-based protection schemes and induction-disc, overcurrent relaysas backup for the RTD overload protection.

Short-Circuit Protection

The reason for short-circuit protection is to protect the motor branch circuit conductors,control apparatus, and motor windings from damage that is caused by excessive current.Short circuit protection can be provided through use of overcurrent relays or fuses. Thelimitation of fuses is that the trip setpoint cannot be adjusted.




Protection against short circuits in motors or motor circuits will, in general, be through use ofinstantaneous trip devices. When used in combination with overload protective devices, theinstantaneous elements should not be set to trip above 13 times the motor full load current,per NEC requirement.

The most suitable setting for instantaneous trip devices is just above the maximum motorstarting current. Maximum motor starting current should be determined as 1.8 times themotor locked rotor current to allow for DC offset and relay operating tolerances. When thedevices are installed, the setting should be adjustable from 90 to 110% of the nominal tripsetting. The type of motor short circuit protection that is employed will vary with the size ofthe motor and the type of starter that is installed.

Motors 600V and below that use combination controllers with molded case circuit breakers asthe motors fault protecting device should be equipped with adjustable pick-up, instantaneous,magnetic trip units, of which there should be one of these units in each phase.

For motors 600V and below that use power circuit breakers or molded case circuit breakerstarters, short circuit protection will be provided through use of three adjustable pick-up,instantaneous, overcurrent relays (device 50), of which there should be one of these relays ineach phase. The relays should be arranged to trip the circuit breaker and should, in general,be integral with the overload protection employed by the motor.




Short-Circuit Protection (Cont'd)

Motors of 2300V and 4000V are divided as either NEMA Class E2 or circuit breaker starters.In NEMA Class E2 starters, short circuit protection is provided through use of fuses.Manufacturer's application guides allow correct selection of fuse ratings to be made when themotor starting characteristics are known. In combination with the thermal overload relays orinduction disc overcurrent relays that are used for overload protection in NEMA Class E2starters, the fuse operation forms a composite trip-current/time characteristic. The specifiedfuse should not melt or be damaged by motor starting, but it should not be too large to preventsatisfactory coordination with other protection schemes. In Class E2 starters, the fuse mustinterrupt all fault currents that exceed the limited fault interrupting capacity of the contactor toprevent damage to the motor.

For 2300V and 4000V motors that use circuit breaker starters, short circuit detection isprovided through use of three adjustable pick-up, instantaneous, overcurrent relays, of whichthere should be one of these relays in each phase. The relays shall be integral with thethermal overload relays that are specified for overload protection.

For 6.6 kV and 13.2 kV motors, short circuit protection should be provided through use ofthree adjustable pick-up instantaneous, overcurrent relays, one in each phase. The relaysshould be integral with the induction disc over-current relays that are specified for overloadprotection.

Ground-Fault Protection

Ground-fault protection is necessary to prevent excessive overcurrent and burning damage tothe motor that results from a ground-fault. All ground-fault protection is of the instantaneoustype to accomplish the rapid removal of power in a fault condition.

Modern industrial practice favors solid grounding of 480V systems and low resistancegrounding at 4.16 kV, 6.6 kV, and 13.8 kV motor-bus voltages. Saudi Aramco also hasseveral ungrounded installations at 480V and 2400V. This discussion only applies togrounded systems.

For motors that are larger than 30 kW (40 hp), ground fault detection should be providedthrough use of a ground sensor device (device 50GS) that consists of a window-type currenttransformer (C.T.) and an adjustable pick-up, instantaneous current relay. The relay shouldbe arranged to trip the circuit power line portion of the combination controller through use ofa shunt trip device. In a 50/5 window type current transformer, a current relay pick-up settingof 0.5 A will be satisfactory. (After allowance for C.T. inaccuracies, this setting will give aprimary current pick-up of approximately 13 A). The window-type current transformershould be carefully installed in accordance with manufacturer's instructions. Duringinstallation, particular attention must be paid to the termination of cable sheaths and armoring




where applicable. Motors that are less than 30 kW do not require any ground fault protectionin Saudi Aramco installations

Short-Circuit Protection (Cont'd)

Current Unbalance

A current unbalance is caused by a voltage unbalance if the assumption is made that all threephases of the motor will have the same impedance. The current will become unbalanced ifthe voltage in any phase varies from the voltage in another phase. The most common causeof supply current unbalance is the voltage unbalance that follows a blown fuse in one phase ofthe primary supply to a step down transformer. Unequal supply phase impedance fromuntransponded transmission lines can also produce a voltage unbalance and thereby cause asupply current unbalance.

The overall effects of a current unbalance that consisted solely of positive sequence currentswould be minimal unless the value of unbalance became to large. The presence of negativesequence currents will increase the effects of the current unbalance on the motor. There aretwo reasons why the presence of negative sequence currents will adversely effect the motorbecause of the current unbalance: (1) The rapid rise in negative sequence current flow for alow change in voltage unbalance and (2) the increased heating that is caused by the negativesequence current. The frequency of the negative sequence current is higher than the motor'sfrequency. At the higher frequency, the motor resistance is high because of the skin effect,and the motor heating is correspondingly increased.

The overall effect of a current unbalance in a motor is that the higher current in one phase willcause the heating effect on the motor to increase. A current unbalance can cause one phasecurrent to be larger than the current for which the motor is designed. The phase with thehigher current will cause an increase in the heating of the motor. The heat that is producedwill be localized, and it may not be indicated by other means. If the higher temperaturepersists, it can damage the motor.

Operation of a motor in excess of five percent current unbalance is not allowed. Above fivepercent current unbalance, local heating can occur that may not be detected by temperaturesensing methods. This local heating can cause damage to the motor that is not detected until acatastrophic failure occurs.




Current Unbalance (Cont'd)

The following protective devices can be employed to protect against current unbalance:

_Thermal Overload Relays_Current Balance Relays_Negative Sequence Voltage Relays_Thermal Synthesis Relays

Thermal overload relays will tend to either overprotect or underprotect motors that only usethermal overload relays for current unbalance protection. For a motor that is running at fullload, an unbalance current will cause at least one phase to exceed normal full load current.The thermal overload device in that phase will operate. To use thermal overload devices forcurrent unbalance protection, one device must be installed in each phase of the supply.

For motors that are running at near full load, thermal overload relay protection will generallyoverprotect because the magnitude of the largest current gives a pessimistic indication of theactual motor heating that is averaged over the three phases. The degree of protection isreduced for motors that run well below full load and that use thermal overload relays forcurrent unbalance protection because a large proportion of negative sequence current isrequired before the thermal overload trip current is reached.

Current balance relays operate when a preset (typically 25 percent) difference in any twosupply currents is exceeded. Current balance relays are not entirely satisfactory protectionagainst negative sequence currents because the magnitude in difference of the supply currentsis not a direct indication of actual motor heating.

To effectively apply negative sequence voltage relays, a sensitive relay with time delayedoperation is required to prevent operation on momentary supply unbalance. The measurementof negative sequence voltage is not a direct indication of actual motor heating, but one relaycan provide effective whole-bus protection against single phasing of the bus.

Thermal synthesis relays provide the only satisfactory protection against the effects ofunbalanced supply because the thermal synthesis relay correctly simulates the actual motorheating. The thermal synthesis relay is now only available as a static unit, and it generallyincorporates short circuit and running overload protection in one unit. Thermal synthesisrelays sense both positive and negative sequence current components in the motor supply




lines. The actual motor heating effect is simulated through the derivation of a quantity ofheating that is proportional to the following:

Current Unbalance (Cont'd)

where: I1 = positive sequence currentI2 = negative sequence currentK = increased heating effect of negative sequencecurrent, typically 3-6.

Saudi Aramco policy in the application of unbalanced supply current protection is dictated bythe size of installation, the likelihood of single phasing, and the present limited applicabilityof static protective schemes at Saudi Aramco.

For motors 600 V and below, no additional protection is applied for current unbalance. Thethermal overload devices in all three phases are expected to provide adequate protection forcurrent unbalance.

For 2.3 kV motors, no additional protection against current unbalance is necessary in eachindividual motor. The thermal overload devices in all three phases are expected to provideadequate protection for current unbalance. On 2300 volt installations where single phasingmay commonly occur, one negative sequence voltage relay and timer should be used forwhole-bus protection.

4 kV, 6.6 kV, and 13.2 kV installations require unbalanced supply voltage protection if singlephasing can commonly occur. Whole-bus protection should be applied through use of anegative phase sequence voltage relay and auxiliary timer. This arrangement is expected tobe eventually superseded by the application of thermal synthesis relays when such devices arefully approved for Saudi Aramco applications.

Vibration Protection

There are two types of vibration probes available: proximity and seismic. The proximityprobe works on the principle of moving a permeable material through a magnetic field. Thedistortion in the field is representative of the amount of vibration in the motor. The tip of theproximity probe will be placed close to, but not touching, the motor bearing casing. Powerthat is supplied to the proximity probe will establish a small magnetic field at the end of theprobe. The magnetic field will interact with the motor's bearing housing The vibration of the




motor will cause the bearing casing to move in the magnetic field. The movement of thebearing casing in the magnetic field will change the coupling of the magnetic field andproduce an output that is proportional to the amount of motor vibration.

Vibration Protection (Cont'd)

The seismic probe works on a similar principal. In the seismic probe, a core is attached to thebearing housing and is mounted in a coil. The vibration of the motor will cause the core tomove back and forth in the coil. The movement of the core in the coil will change themagnetic coupling in the coil and produce an output that is proportional to the motor'svibration.

The following are the preferred types of vibration detectors for use in Saudi Aramcoapplications:

Equipment DetectorSeismic Type

Proximity Type

Bently Nevada 16699-10-05-02 or equal

Bently Nevada series 21000 or equal

The recommended limits of vibration for all Saudi Aramco motors are those limits that arestated in NEMA MG 1, except when proximity type vibration sensors are fitted. For allmotors, the maximum rotor shaft peak-to-peak amplitude of vibration, as measured in tests oncompletely assembled motors at the vendor's factory at rated no-load conditions, should be asfollows:

_Horizontal Motors with Proximity Probes

Motor SpeedMaximum ShaftVibration Level

Maximum Combined Runout Allowance

rpm Micrometers Mils Micrometers Mils36001800

1200 or less

506376

22.53

8.8612.6612.66

0.350.50.5

When vendors can demonstrate that run-outs due to shaft materialanomalies are present, the value of these run-outs may be added to the aboveacceptable vibration levels up to a maximum of 6.3 micrometers (0.25 mil.).

_Vertical and Horizontal Motors with Seismic Velocity Transducers.




Maximum allowable vibration level should be 4.6 mm/s (0.18 in/s) zero-to-peak, and it should be measured in three planes that are as close as possible tothe bearing housing.




Vibration Protection (Cont'd)

Alarm and shutdown levels for horizontal motors with proximity type shaft probes or casingmounted seismic probes are derived from the maximum allowable vibration for acceptance ofnew machinery. Alarm levels for all horizontal motors are determined through the addition of25 micrometers (1 mil) peak-to-peak to new machinery acceptance vibration levels. In thecase of proximity probes, allowable "runout" is not included in the determination of alarmlevels. The shutdown levels for motors are determined by doubling new machineryacceptance vibration levels.

Vertical motor alarm and shutdown levels are based on acceptance criteria for vertical pumps.Because most, if not all, vertical pumps are actually an integral part of a vertical pump/motorsystem, they must have alarm and shutdown levels set accordingly. Vertical motor/pumpalarm and shutdown levels are determined from the following:

where: n = motor speed in RPM

Tables that show the actual Saudi Aramco alarm and shutdown levels for different speedhorizontal and vertical motors are given in Work Aid 4.

Bearing Failure Protection

Resistance temperature detectors (RTDs) are for use in the provision of bearing temperaturemonitoring. RTD's for bearing temperature monitoring should be supplied and fitted by themotor vendor. The RTD should be the platinum, three-wire type, and it should be calibratedto 100 ohms at 0oC (32oF). When embedded elements are used for bearing temperaturemeasurement, extra elements should be installed in the bearing oil throw-off lines. Theseextra elements should be wired as unconnected spares to the monitor through use of thecommon connection box.

Bearing failure protection monitors should consist of a separate alarm unit for eachtemperature set point and a single, time-shared temperature indicator. The alarm units shouldhave dual set points and outputs, and they should accept the signal directly from the RTDelement. The alarms are displayed on a separate annunciator.




Bearing Failure Protection (Cont'd)

The indicator should have an RTD location selector switch and a nameplate that shows thecorresponding switch positions and RTD locations. The monitor will provide a fault alarm foropen or short circuits in the control wiring between the RTD and the monitor. Temperatureswitches (thermostats) should not be utilized for Saudi Aramco applications.

Maximum bearing temperatures must not exceed 40oC rise above ambient. The oiltemperature rise for pressure lubricated bearings must not exceed 30oC when the inlettemperature is 60oC. The actual trip settings must be in accordance with therecommendations of the rotating machinery manufacturer, and they should be confirmed onthe Buyer's Specification Sheet.

One RTD should be provided in each bearing of horizontal motors that are rated 370 kW (500HP) and larger. Horizontal motors that are rated 185 kW (250 HP) up to but not including370 kW (500 HP) should only include a mounting provision in each bearing for resistancetemperature detection. Vertical motors are not to be provided with RTD's.

Stator Winding High Temperature Protection

The monitoring of motor supply currents does not provide complete protection against statorwinding overheating. In particular, monitoring of the supply current does not guard againstoverheating that is caused by inadequate ventilation. To protect against stator windingoverheating, RTD's are used to detect stator temperature. The RTD's should be of theplatinum, three-wire type, and they should be calibrated to 100 ohm at 0oC (32oF).

Stator mounted RTDs are satisfactory indicators of motor winding temperature under steadystate or slow load change conditions, but they cannot accurately indicate motor temperaturesunder rapid heating conditions such as stalling. RTD-based protection schemes shouldalways by supplemented through use of overload relays that are operated from the motorsupply current.

The hottest reading RTD must be continuously monitored by the temperature relay (device49T). The remaining RTDs should be terminated in the motor auxiliary connection box asfuture spares. This equipment should be used in place of the temperature relay to monitor theremaining stator-mounted RTDs. The actual requirements for the use of stator winding RTD'sare given in Work Aid 4.

The stator winding RTD trip settings for electrical motors must be in accordance with therecommendations of the motor manufacturer. Recommended alarm and trip setpoint forvarious motor insulation classes are given in Work Aid 4.




Undervoltage Protection

Undervoltage protection is necessary to protect the motor from damage that would be causedby higher current levels than the motor would experience if the voltage was below thenameplate design voltage level. A motor will draw enough current to provide the necessarypower to operate the load because power is proportional to voltage and current as shownbelow:

Power _ Voltage X Current

If voltage drops below the designed voltage level, current will rise accordingly to maintain theproper level of power.

The undervoltage relay is a simple magnetic relay that will deenergize when the supplyvoltage drops below a preset value. The deenergized undervoltage relay will open a set ofcontacts that will prevent the motor from restarting. The time delay dropout of the relay is afunction of the relay's design. Certain relay designs will maintain the relay coil field for agiven amount of time after power has been removed from the relay. Saudi Aramco usesinstantaneous or time-delay undervoltage relays to provide motors with undervoltageprotection.

Motors of 2300 V and above should be provided with short time-delay under-voltage relayprotection. The time delay is a function of the drop in voltage, and it typically ranges fromtwo seconds for complete loss of supply to 15 to 20 seconds on voltage drops to 67%. Thecircuit breaker is tripped or the magnetic contactor is prevented from reclosing after theappropriate time delay.

Except for motors that experience excessive starting torques because of out of phase residualrotor fields, the undervoltage time settings for complete loss of voltage should be fourseconds. Thus, for a voltage depression with subsequent restoration within four seconds, themotor would be allowed to be immediately restarted. For voltage restoration after fourseconds, the motor would be permanently locked out until it was manually reset.

In some cases, especially with large motors, reconnection of the supply before the rotormagnetic field has decayed significantly will result in excessive restarting torque and currents.Typically, the field takes a few tenths of a second to decay, and restoration of voltage withinthis time will cause high torques if the residual rotor field has become out-of-phase with the(reapplied) stator field.




These factors must be considered in schemes where the contactor or circuit-breaker is nottripped immediately on undervoltage. Magnetically-held contactors will open onundervoltage. A timer may be needed to prevent reclosure within the critical time. Latchedbreakers or contactors will need to be tripped instantaneously and reclosed after a suitabletime delay. For such applications, an instantaneous undervoltage relay should be used for thisapplication.

Overvoltage Protection and Surge Protection

The operation of an electric motor above the rated voltage will cause a deterioration of themotor insulation. Continuous operation of an electric motor above the motor's rated voltagewill shorten the life the motor because of the slow deterioration of the motor's insulation.

Motors that comply with NEMA MG 1 are capable of satisfactory operation at sustainedsupply voltages up to ten percent above nameplate rated voltage; however, overvoltageprotection should be applied to each motor bus for all voltage and power ratings. Theovervoltage protection should be of the time-delay type and capable of providing a trippingaction at voltages that exceed approximately 110% of motor nameplate rated voltage.

Motors are also susceptible to surge overvoltages. High voltage motors may require surgeovervoltage protection to prevent damage to the motor. Significant voltage surges may occuron the motor terminals because of the following conditions:

_Contactor/circuit breaker switching surges_Lightning induced surges on incoming overhead lines_High frequency voltage spikes from fast switching, solid-state type devices

These surges can cause a failure of the motor's major or ground-wall insulation, and/or afailure of the motor's turn-to-turn insulation. The type of insulation failure that occurs isdependent upon the crest value of the overvoltage surge and the rate-of-rise of the overvoltagesurge.

A failure of a motor's major insulation can occur when the crest value of an overvoltage surgethat gradually rises (e.g., rise time of 5 _s) exceeds the impulse strength of the motor's majorinsulation. The generally accepted impulse strength of a motor's major insulation can becalculated through use of the following equation:

where V is the one minute rms high potential acceptance test voltage for the motor.

For example, the impulse strength of 4000V, 3000 hp induction motor is calculated asfollows:




If the example motor is likely to be subjected to a voltage surge that rises to a crest value thatis _ 15.91 kV in 5 _s, a surge arrestor should be installed to limit the magnitude of theovervoltage surge. Installation of a surge arrestor will protect the motor's major insulationfrom damage.

Overvoltage Protection and Surge Protection (Cont'd)

A failure of a motor's turn-to-turn insulation can occur for lower crest value overvoltagesurges when the rate-of-rise of the overvoltage surge is less than 5 _s. Such failures occurbecause of the large capacitive coupling that exists between the turns of the winding and thegrounded core. When the motor is subjected to an overvoltage surge that has a high rate-of-rise (<5_s), 70% to 90% of the voltage will appear across the first two adjacent turns.Because the insulation that is between the adjacent turns has a much lower impulse strengththan does the major insulation, the turn-to-turn insulation will fail. Such failures can beprevented through installation of a surge capacitor to reduce the rate-of-rise of the overvoltagesurge.

Saudi Aramco requires that surge protection be provided for motors that are rated 3730 kW(5000 hp) and above, and for motors that operate at 13,200 volts. The surge protection is toconsist both of surge capacitors and of surge arrestors.

The surge capacitors must be the last devices that are connected to the motor leads before theleads enter the stator. The connection leads to the surge capacitors must be 120 sq. mm (4/0AWG) conductors. The following are the required surge capacitor ratings:

Motor Rated Voltage (kV) 4.0 6.6 13.2Capacitance (micro-farads) 0.5 0.5 0.25Maximum Operating Voltage (kV) 4.57 7.9 15.2Continuous Temperature Rating (oC) 60 60 60

The only type of surge arrestor that is acceptable for use in the protection of Saudi Aramcomotors is the gapless zinc oxide type. The surge arrestors must be mounted in the terminalbox, and all of the connection leads must be less than 60 cm (24 in) from the surge arrestor tothe motor terminal. The following are the required surge arrestor ratings:




Motor Rated Voltage (kV) 4.0 6.6 13.2Arrestor Voltage Rating (kV rms) 4.5 7.6 15Maximum Discharge Voltage at 1500A (Crest kV) 7.4 12.2 24.2Continuous Temperature Rating (oC) 60 60 60

Motor Stalling Protection

Should a motor stall during operation or be unable to start because of excessive load, themotor will draw from the supply a current that is equivalent to the locked rotor current. Theexcessive current that is drawn during stalling can rapidly damage the motor windings;therefore, quick disconnection of the machine is desirable to avoid damage.

Motor Stalling Protection (Cont'd)

Relays cannot distinguish a stalling condition from a healthy starting condition on the solebasis of current magnitude. The only way to discriminate between a healthy startingcondition and a stalled condition is to arrange the protective device that disconnects the motorto a time delay to sense when the current continues for longer than the normal starting time.

Where the possibility of motor stalling exists, the stalling protection should be applied in viewof the following criteria:

_Where the normal start time of motor, including the effect of reduced terminalvoltage, exceeds the safe stalled (locked rotor) time of the motor or

_Where the normal start time is less than the safe stall (locked rotor) time.

With regard to the second of these two criteria, three methods of protection are acceptable:

_Set the thermal overload to trip after the normal start time has elapsed, but setit so that it trips before the safe stall time of the motor elapses.

_Add a separate relay that will activate upon the starting of a motor but thatwill be inoperative for normal or overload current levels. This relay willprevent tripping of the motor during starting, but it will allow tripping if themotor stalls.

_Use a start sequence timer that is designed to operate if normal full loadcurrent or speed is not achieved in the normal run-up time.




For motors that are rated above 4000 kW (5360 Hp) or where the start time exceeds the safelocked rotor time, the use of one or more of the previously started protective elements shouldbe supplemented by the following stalled rotor protection:

_A start timer with a timer setting that is less than the safe stalled rotor time ofthe motor and that is designed to operate if rotation has not occurred within thetime that is set for the timing relay.

Speed measurement of the rotor should be by direct monitoring of shaft speed through use ofproximity type monitoring equipment and a digital tachometer. The digital tachometer alsoshould provide an alarm (voltage free contact) at approximately ten percent of motor full loadspeed to be used in conjunction with the above mentioned start timer. The tachometer alarmdevice should employ a time range of typically 0.3 to 3 seconds. If the motor does not reachten percent speed within the timer setting, the motor circuit breaker or contactor would betripped.




Differential Protection

Differential protection provides sensitive detection and rapid removal of in-zone ground faultsand phase-to-phase faults while being insensitive to motor starting inrush. The differentialprotection will protect a motor from damage that is caused by internal faults.

The differential relay works on the principle of comparison of the input current of one phasewith the output current of the same phase. The differential relay will receive a signal from acurrent transformer on the input to one phase of the motor and from a current transformer onthe output of the same phase of the motor. The current transformers are wound so that thecurrents that are induced in the transformer secondaries will oppose each other and no currentwill flow through the differential relay. The in-zone protection is between the two currenttransformers. Any fault that occurs to the motor outside of the in-zone protection will notbe sensed by the differential relay. A fault that occurs in-zone will cause the current thatflows out of the protected zone to be different than the current that flows into the protectedzone. The difference in the two currents results because some of the current that flows intothe protected zone flows out through the fault. The difference in the current flows causes adifferential current to flow through the operating winding of the differential relay. Thedifferential relay will operate and send a signal to the trip circuit of a circuit breaker when thedifferential current exceeds the relay setpoint.

Motors that are 3700 kW (5000 Hp) and above should be equipped with motor differentialprotection. The self-balance type of differential protection should be used in preference toother types. Motors that are less than 3700 kW (5000 hp) do not require differentialprotection.

Additional Protection for Synchronous Motors

Synchronous motors require more protection than induction motors because synchronousmotors are more complex. The additional protection that is required by synchronous motorsrelates specifically to the unique nature of how a synchronous motor operates. The followingis a list of the additional protection that is required for synchronous motors:

_Out of step_Sudden restoration of supply power_Overvoltage_Reverse power_Loss of excitation_Over-excitation_Under-excitation_Diode failure




Additional Protection for Synchronous Motors (Cont'd)

Synchronous motors can pull out-of-step because of sudden loading of the shaft or because oftemporary depressions of the supply voltage. The slip cycle time will depend on the initialcause and also the inertia of the drive. The effect of loss of synchronism is induced currentsin rotor circuits and subsequent overheating of the motor.

Out-of-step protection should be applied to detect pull-out or loss of synchronism. A relaywill be provided that detects pull-out conditions through measurement of stator current andvoltage, either in the form of power reversals or by the power factor of stator current. Theprotective relay must allow some time for the motor to attempt to pull into step beforeinitiating a trip. The time delay that is permissible will vary, but it will allow either three halfslip cycles or a total pull-out time of 10 to 15 seconds, as determined by the motor'smanufacturer.

Upon a loss of the supply to a synchronous motor, the motor breaker must be tripped asquickly as possible if there is any possibility of the supply being either automatically restoredor restored without the knowledge of the machine operator. Tripping is necessary in order toprevent the supply from being restored out-of-phase with the motor-generated voltage.Restoration of the supply voltage out of phase with the motor-generated voltage will generatean excessive torque that could damage the motor or the attached equipment. To trip thebreaker, an undervoltage relay is placed on the power supply line. The undervoltage relaywill trip the circuit breaker if the power supply voltage drops below a preset value.

The voltage of the synchronous motor can rise and exceed design limits when no load orminimal load is connected to the busbars that supply the motor and when there is no load onthe motor itself. Under these conditions, the voltage could rise instantaneously from 120% to130% of the nominal values. Busbar overvoltage protection will provide overvoltageprotection, and instantaneously trip the motor. The overvoltage protection will be providedthrough use of a voltage sensing relay on the bus work.

Reverse power protection is only applicable where power reversals do not occur under normaloperating conditions, which is the case for all Saudi Aramco installations. A reverse powerrelay will be provided to prevent the motor from delivering power to other loads on the samebusbar. A slight time delay should be incorporated into the reverse power relay to provide forthe momentary reversals of power that result from system faults. Failure to trip the motor ona power reversal can damage the motor bearing and possibly cause an excessive current toflow in the motor.





Under-excitation or complete loss of excitation in a synchronous motor will cause the motorto run as an induction motor at a slip frequency. The machine's reactive power is thenprovided by means of the system, and it results in a larger stator current that operates at alagging power factor. The larger stator current conditions may not be satisfactory for eitherthe supply system or the motor.

Loss of excitation will not necessarily be detected through use of any of the previouslydescribed protective devices. A separate protective device should be provided that can bearranged either to initiate an alarm or shut down the motor. For large machines that are ratedat above 7500 kW (10000 hp), an additional relay (device 40) should be provided as part ofthe motor switchgear. This relay utilizes an offset mho-type of impedance measuring relaythat continuously monitors motor impedance as seen from the motor terminals. If the motorexcitation fails or falls to a very low value, the machine impedance enters the relay operatingcharacteristic circle, and the relay operates to start a time delay relay. Operation of the timedelay relay should cause tripping of the main circuit breaker and initiation of fieldsuppression. The time delay is provided to prevent false-operation of the protection, whichcan result from transient surges.

Over-excitation of a synchronous motor can occur in the event of failure of the fieldexcitation control circuit. Over-excitation causes the continuous field current to exceed itsrating. Field current protection (over-excitation) can be applied through use of a normalthermal overload relay.

To prevent operation at low field levels and to minimize the problems that are associated withinstability and stator overcurrent, under-excitation protection should be provided. Under-excitation protection will be accomplished through use of an undervoltage relay that is set upto monitor the motor excitation.

Diode failure is detected by monitoring the waveform of the exciter field current andoperating a time delay relay from the harmonic current that is produced by open circuits orshort circuits in the rotating diodes. Operation of the device should trip the main circuitbreaker.





The following list shows the protective equipment that should be provided at the machinecircuit breaker and the excitation control cubicle of a synchronous motor:

Machine Circuit Breaker Excitation Cubicle

Instantaneous Overcurrent Field OvercurrentTime Delayed Overcurrent Diode FailureInstantaneous Ground Fault Field FailureDifferential Current Under-excitationOvervoltageRepeat Start BlockingIncomplete Sequence(including running stall)Stalled RotorOut of StepReverse PowerLoss of Excitation

The following list shows the metering equipment that should be provided at the machinecircuit breaker and the excitation control cubicle of a synchronous motor:

Machine Circuit Breaker Excitation Cubicle

Line Voltage ExcitationCurrent

Line Current Power FactorPower FactorkWvar

The machine vendor should supply, as a minimum, the equipment that is associated with theexcitation control cubicle. The supply of remaining equipment that is located at the machinecircuit breaker should be subject to Project requirements and the capabilities of theprospective vendor.

Saudi Aramco Motor Protection Schemes

Protective schemes should, in general, employ electromechanical or thermal-type relayelements in preference to static type protection. The use of static type protection requires theapproval of the Technical Services Department, except when no alternatives to the static typeare available.




Saudi Aramco Motor Protection Schemes (Cont'd)

Protection against overvoltage, vibration, and bearing failure is not shown and should beadded to the basic protection as appropriate for the given size of the motor. Protectionschemes for the following motor circuits will be discussed in this section:

_Motors 600V or less with combination magnetic starters 1 to 100 hp._Motors 600V or less with circuit breaker starters >100 hp._Motors 4000V or greater with NEMA E2 starters _1500 hp._Motors 4000V or greater with circuit breaker starters <10,000 hp._Motors 4000V or greater with circuit breaker starters _10,000 hp._Synchronous motors <10,000 hp.

Figure 29 shows the protection scheme for motors 600V or less with combination magneticstarters that are 1 to 100 hp. The protective devices are integral units to the combinationmagnetic starter. The combination magnetic starter houses both the circuit breaker and themotor's control contacts. The protective relays that are shown in Figure 29 are identifiedthrough the use of the following standard device function numbers:

_49 is the thermal overload._50 is the instantaneous overcurrent relay._50GS is a ground fault overcurrent with an instantaneous element.

Device 49 (thermal overload) provides overload protection for the motor as follows:

_An increase in load on the motor above the full load rated value will result inan increase in current flow to the motor stator.

_The increase in current flow will cause the heating element in the thermaloverload to produce more heat.

_If the increase in current flow persists for a sufficient length of time, relay 49will operate and cause the motor's control contacts to open.

_When the control contacts open, power to the motor stator will be interruptedand the motor will stop.





Device 50 (instantaneous overcurrent relay) provides overcurrent protection against faults thatoccur downstream of the circuit breaker as follows:

_A fault downstream of the circuit breaker will cause an increase in currentflow through device 50.

_A large increase in current flow will cause the instantaneous element of device50 to operate.

_When the instantaneous element of device 50 operates, the circuit breaker willtrip (open).

_When the circuit breaker opens, the power supply to the motor will bedisconnected and the motor will stop.

Device 50GS (instantaneous ground fault overcurrent relay) provides overcurrent protectionagainst ground faults that occur downstream of the circuit breaker as follows:

_A ground fault downstream of the circuit breaker will cause an increase incurrent flow through the circuit breaker and CT-1.

_The increase in current flow is transmitted to the 50GS device from CT-1.

_A large increase in current will cause the instantaneous element of the 50GSdevice to operate.

_When the instantaneous element of the 50GS device operates, the device willsend a signal to the circuit breaker that causes the circuit breaker to trip open.

_When the circuit breaker opens, the power to the motor stator will beinterrupted and the motor will stop.





Motors 600 V or Less with Combination Magnetic Starters 1 to 100 hpFigure 29





Figure 30 shows the protection scheme for motors 600V or less with circuit breaker startersthat are >100 hp. The protective devices in this protection scheme are identified through theuse of the following standard device function numbers:

_27 is the undervoltage relay._42 is the starting circuit breaker._49 is the thermal overload._50/51 SST is the phase and ground solid state trip device_50GS is the instantaneous ground fault overcurrent relay.

Device 27 (undervoltage relay) provides undervoltage protection for the motor as follows:

_The voltage transformer continuously monitors the voltage that is supplied tothe motor from the power source, and it transmits this voltage signal to theundervoltage relay (device 27).

_If the voltage drops to a level that would cause the motor to draw excessivecurrents (normally about 67% of rated voltage), the undervoltage relay operatesand sends a trip signal to the starting circuit breaker (device 42).

_The starting circuit breaker (device 42) trips and secures power to the motor toprevent motor damage that can result from excessive current flows.


_An increase in load on the motor above the full load rated value will result inan increase in current flow through CT-2.

_The increase in current flow is transmitted to the 49 device from CT-2.

_If the increase in current flow persists for a sufficient length of time, relay 49will operate and send a signal to the starting circuit breaker (42).

_The starting circuit breaker (device 42) will trip and deenergize the motor.





The 50/51 SST device is a solid state trip unit that provides the following types of protectionto the motor:

_Long time adjustable phase overcurrent protection.

_Short time adjustable phase overcurrent protection.

_Instantaneous phase overcurrent protection.

_Instantaneous ground fault overcurrent protection.

_The starting circuit breaker (device 42) will trip (open), which secures powerto the motor and removes the overcurrent.

Device 50GS (instantaneous ground fault overcurrent relay) provides protection againstground faults that occur downstream of the circuit breaker as follows:

_A ground fault that is downstream of the breaker will cause an increase incurrent flow through the circuit breaker and CT-1.

_The increase in current flow is transmitted to device 50GS from CT-1.

_A large increase in current flow will cause the instantaneous element of the50GS device to operate.

_When the instantaneous element of the 50GS device operates, the device willsend a signal to the starting circuit breaker (42).

_The starting circuit breaker (device 42) will trip (open), which secures powerto the motor and removes the ground fault.





Motors 600 V or Less with Circuit Breaker Starters that are >100 hp.Figure 30





Figure 31 shows the protection scheme for motors 4000 volts or greater with NEMA E-2starters that are 1500 hp. Overcurrent protection in this scheme is provided through use ofcurrent limiting fuses that are integral to the NEMA E-2 starter. The protective relays that areshown in Figure 31 are identified through the use of the following standard device functionnumbers:

_27 is the undervoltage relay._42 is the motor contactor._47 is the negative sequence voltage relay._49 is the thermal overload._49T is the RTD actuated thermal/overload relay._50G is the instantaneous ground fault overcurrent relay._51 LR is the locked rotor relay._86 MI is the auxiliary lockout relay.



_If the voltage drops to a level that would cause the motor to draw excessivecurrents (normally about 67% of rated voltage), the undervoltage relay operatesand sends a trip signal to the motor contactor (device 42).

_The motor contactor (device 42) trips and secures power to the motor toprevent motor damage that can result from excessive current flows.

Device 47 (negative sequence voltage relay) provides bus protection against negativesequence voltages as follows:

_When a fault occurs that results in a negative sequence voltage, the negativesequence voltages will be sensed through the use of the voltage transformer.

_The voltage transformers will transmit the negative sequence voltage to thenegative sequence relay (device 47).





_When the negative sequence voltage value exceeds the setpoint of thenegative sequence relay (device 47), the relay operates and sends a signal to themotor contactor (device 42).

_The motor contactor (device 42) will operate and secure power to the motor.




_If the increase in current flow persists for a sufficient length of time, relay 49will operate and send a signal to motor contactor (device 42).

_The motor contactor (device 42) will operate and deenergize the motor, whichremoves the overload.

Device 49T (RTD actuated thermal/overload relay) provides additional overload protection tothe motor as follows:

_An increase in load on the motor above the full load rated value will result inan increase in current flow to the motor.

_The increase in current flow will cause the temperature in the motor stator toincrease.

_When the temperature of the motor stator, as read by RTDs that are embeddedin the stator reaches a preset value, device 49T will operate.

_When device 49T operates, a signal is sent to the motor contactor (device 42).Device 42 will secure power to the motor, and the motor will stop.





Device 50G (ground overcurrent relay) provides protection against ground faults that occurdownstream of the circuit breaker as follows:

_A ground fault downstream of the breaker will cause an increase in currentflow through the circuit breaker and CT-1.

_The increase in current flow is transmitted to device 50G from CT-1.

_A large increase in current flow will cause the instantaneous element of the50G device to operate.

_When the instantaneous element of the 50G device operates, the device willsend a signal to the auxiliary lockout relay (device 86 MI).

_When the auxiliary lockout relay (device 86 MI) operates, the device willsend a signal to the motor contactor (device 42).

_The motor contactor (device 42) will operate to secure power to the motor andto remove the ground fault.

Device 51 LR (locked rotor relay) provides protection against locked rotor conditions asfollows:

_A locked rotor condition will cause an increase in current flow through CT-3.

_The increase in current flow is transmitted to device 51 LR from CT-3.

_If the increased current flow persists for a sufficient length of time, device 51LR will operate and send a signal to the auxiliary lockout relay (device 86 MI).

_The auxiliary lockout relay (device 86 MI) will operate and send a trip signalto the motor contactor (device 42).

_The motor contactor (device 42) trips and secures power to the motor.





Motors 4000 Volts or Greater with NEMA E2 Starters _ 1500 hpFigure 31





While not shown in the protection scheme of Figure 31, a lockout relay (86) and a stallingrelay can be added as the application dictates.

Figure 32 shows the protection scheme for motors 4000 volts or greater with circuit breakerstarters that are < 10,000 hp. The protective relays that are shown in Figure 32 are identifiedthrough use of the following standard device function numbers:

_6T is the motor starting circuit breaker._27 is the undervoltage relay._47 is the negative sequence voltage relay._49 is the thermal overload._49T is the RTD-actuated thermal overload relay._50 are the phase A and phase C instantaneous overcurrent relays._50G is the ground overcurrent relay._50/51 LR is the phase B instantaneous overcurrent/locked rotor relay._86 M1 is the auxiliary lockout relay #1_86 M2 is the auxiliary lockout relay #2._87 are the differential relays.



_If the voltage drops to a level that would cause the motor to draw excessivecurrents (normally about 67% of rated voltage), the undervoltage relay operatesand sends a trip signal to the motor starting circuit breaker (device 6T).

_The motor circuit breaker (device 6T) trips and secures power to the motor toprevent motor damage that can result from excessive current flows.

Device 47 (negative sequence voltage relay) provides bus protection against negativesequence voltages as follows:

_When a fault occurs that results in a negative sequence voltage, the negativesequence voltages will be sensed through use of the voltage transformer.





_The voltage transformer will transmit the negative sequence voltage to thenegative sequence relay (device 47).

_When the negative sequence voltage value exceeds the setpoint of thenegative sequence relay (device 47), the relay operates and sends a trip signalto the motor starting circuit breaker (device 6T).

_The motor starting circuit breaker (device 6T) trips (opens) and secures powerto the motor.




_If the increase in current flow persists for a sufficient length of time, relay 49will operate and send a trip signal to motor starting circuit breaker (device 6T).


Device 49T (RTD-actuated thermal overload relay) provides additional overload protection tothe motor as follows:

_An increase in load on the motor above the full load rated value will result inan increase in current flow to the motor.

_The increase in current flow will cause the temperature in the motor stator toincrease.

_When the temperature of the motor stator, as read by the RTDs that areembedded in the stator, reaches a preset value, device 49T will operate.

_The operation of Device 49T will send a signal to the motor auxiliary lockoutrelay #1 (device 86M1).

_Device 86M1 will operate, which causes the motor starting circuit breaker(device 6T) to trip (open) and secure power to the motor.





Device 50 (instantaneous overcurrent relay) provides overcurrent protection against faults thatoccur downstream of the circuit breaker as follows:

_A fault downstream of the circuit breaker will cause an increase in currentflow through CT-4.



_The operation of device 50 will send a signal to the motor auxiliary lockoutrelay #1 (device 86M1).

_The motor auxiliary lockout relay #1 (device 86M1 will operate and send asignal to the motor starting circuit breaker (device 6T). The motor startingcircuit breaker (device 6T) will trip (open) and secure power to the motor.

Device 50G (ground overcurrent relay) provides protection against ground faults that occurdownstream of the circuit breaker as follows:

_A ground fault that occurs downstream of the breaker will cause an increase incurrent flow through the circuit breaker and CT-1.



_When the instantaneous element of the 50G device operates, the device willsend a signal to the motor auxiliary lockout relay #1 (86M1).

_The motor auxiliary lockout relay #1 (device 86M1) will operate and send atrip signal to the motor starting circuit breaker (device 6T). The motor startingcircuit breaker (device 6T) will trip (open) and secure power to the motor.





Device 51LR (locked color relay) provides protection against locked rotor conditions asfollows:


_The increase in current flow is transmitted to device 51LR from CT-4.

_If the increased current flow persists for a sufficient length of time, device51LR will operate and send a signal to the auxiliary lockout relay (device86M1).

_The auxiliary lockout relay (device 86M1) will operate and send a trip signalto the motor starting circuit breaker (device 6T).

_The motor starting circuit breaker (device 6T) trips and secures power to themotor.

Device 87 (differential relays) provides internal motor fault protection as follows:

_CT-3 will establish the protected zone on the motor.

_A fault in the protected zone will cause a difference in the magnitude of thecurrent that flows into and out of the protected zone.

_The difference in current flows will cause a current to flow through theoperating coil of the differential relay (device 87).

_Device 87 will operate and send a trip signal to the motor auxiliary lockoutrelay #2 (device 86M2).

_Device 86M2 will operate and cause the motor starting circuit breaker (device6T) to trip (open) and secure power to the motor.





Motors 4000 Volts or Greater with Circuit Breaker Starters < 10,000 hpFigure 32





While not shown for the protection scheme in Figure 32, a current balance and reverse phaserelay, a stalling relay, and a repeat start block relay can be added as the application dictates.

Figure 33 shows the protection scheme for motors 4000 volts or greater with circuit breakerstarters that are _ 10,000 hp. The protective relays that are shown in Figure 33 are identifiedthrough use of the following standard device function numbers:

_6T is the motor starting circuit breaker._27 is the undervoltage relay._46 is the current balance and reverse phase relay._47 is the negative sequence voltage relay._50 are the instantaneous phase overcurrent relays._50G is the ground-fault overcurrent relay._50/51G is the residual ground overcurrent relay._51OL are the phase A and phase C overload._51LR is the locked rotor relay._86M1 is the auxiliary lockout relay #1._86M2 is the auxiliary lockout relay #2._87 are the differential relays.



_If the voltage drops to a level that would cause the motor to draw excessivecurrents (normally about 67% of rated voltage), the undervoltage relay operatesand sends a trip signal to the motor starting circuit breaker (device 6T).

_The motor starting circuit breaker (device 6T) trips and secures power to themotor to prevent motor damage that can result from excessive current flows.

Device 46 (current balance and reverse phase relay) provides protection against unbalancedphase currents and reversed phase currents as follows:

_When a fault such as single-phasing occurs, unbalanced phase currents willflow through CT-3.





_The unbalanced phase currents are transmitted to the 46 device from CT-3.

_If a sufficient percentage of unbalance exists between any two phases, the 46device operates.

_The operation of the 46 device will send a signal to the auxiliary lockout relay#2.

_The auxiliary lockout relay #2 will operate and send a trip signal to the motorstarting circuit breaker (device 6T).


Device 47 (negative sequence voltage relay) provides protection against negative sequencevoltages as follows:



_When the negative sequence voltage value exceeds the setpoint of thenegative sequence relay (device 47), the relay operates and sends a signal to themotor starting circuit breaker (device 6T).

_The motor starting circuit breaker (device 6T) will trip (open) and securepower to the motor.

Device 50 (instantaneous phase overcurrent relay) provides overcurrent protection againstfaults that occur downstream of the circuit breaker as follows:

_A fault that occurs downstream of the circuit breaker will cause an increase incurrent flow through CT-1.







_The operation of device 50 will send a signal to the auxiliary lockout relay #1(device 86M1).

_The auxiliary lockout relay #1 (device 86M1) will operate and send a signal tothe motor starting circuit breaker (device 6T). The motor starting circuitbreaker (device 6T) will trip (open) and secure power to the motor.

Device 50G (ground fault overcurrent relay) provides protection against ground faults thatoccur downstream of the circuit breaker as follows:




_When the instantaneous element of the 50G device operates, the device willsend a signal to the auxiliary lockout relay (86M1).

_The auxiliary lockout relay (device 86M1) will operate and send a trip signalto the motor starting circuit breaker (device 6T). The motor starting circuitbreaker (device 6T) will trip (open) and secure power to the motor.

Device 50/51G (residual ground overcurrent relay) provides additional protection againstground-faults as follows:

_When a ground-fault occurs, ground-fault current will flow through CT-5.

_The ground-fault current is transmitted to device 50/51G from CT-5.

_If a sufficient magnitude of ground-fault current flows, the 50/51G deviceoperates and sends a signal to the auxiliary lockout relay #2 (device 86M2).







Device 51OL (overload relay) provides overload protection for the motor as follows:

_An overload on the motor will result in an increased current flow through CT-1.

_The increase in current flow is transmitted to device 51OL from CT-1.

_If the increase in current flow persists for a sufficient length of time, device51OL will operate and send a signal to the auxiliary lockout relay #1 (device86M1).






_The auxiliary lockout relay (device 86M1) will operate and send a trip signalto the motor starting circuit breaker (device 6T).









_Device 87 will operate and send a signal to the auxiliary lockout relay #2(device 86M2).









Motors 4000 Volts or Greater with Circuit Breaker Starters _ 10,000 hpFigure 33





While not shown for the protection scheme in Figure 33, a stalling relay and a repeat startblock relay can be added as the application dictates.

Figure 34 shows the protection scheme for synchronous motors that are < 10,000 hp. Theprotective relays that are shown in Figure 34 are identified through use of the followingstandard device function numbers:

_6T is the motor starting circuit breaker._27 is the undervoltage relay._46 is the current balance and release phase relay._47 is the negative sequence voltage relay._48 is the incomplete sequence relay._49 are the thermal overload relays._50 are the instantaneous phase A and phase B overcurrent relays._50G is the ground-fault overcurrent relay._50/51LR is the instantaneous phase B overcurrent relay and the locked rotorrelay._55 is the power factor (loss of field) relay._86M1 is the auxiliary lockout relay #1._86M2 is the auxiliary lockout relay #2._87 are the differential relays.


_The voltage transformer continuously monitors the voltage that is supplied tothe motor from the power source and transmits this voltage signal to theundervoltage relay (device 27).

_If the voltage drops to a level that would cause the motor to draw excessivecurrents (normally about 67% of rated voltage), the undervoltage relay operatesand sends a signal to the motor starting circuit breaker (device 6T).

_The motor starting circuit breaker (device 6T) trips and secures power to themotor to prevent motor damage that can result from excessive current flows.

Device 46 (current balance and reverse phase relay) provides protection against unbalancedphase currents and reversed phase currents as follows:

_When a fault such as single-phasing occurs, unbalanced phase currents willflow through CT-3._The unbalanced phase currents are transmitted to the 46 device from CT-3.





_If a sufficient percentage of unbalance exists between any two phases, the 46device operates.

_The operation of the 46 device will send a signal to the auxiliary lockout relay#2.



Device 47 (negative sequence voltage relay) provides protection against negative sequencevoltages as follows:



_When the negative sequence voltage value exceeds the setpoint of thenegative sequence relay (device 47), the relay operates and sends a signal to themotor starting circuit breaker (device 6T).


Device 48 (incomplete sequence relay) secures power to the motor in the event that the motorfails to start within its allotted time as follows:

_When the motor starting circuit breaker (device 6T) shifts to apply power tothe motor, a timer that is internal to device 48 starts to time out.

_If the motor fails to reach its normal speed of operation before device 48 timesout, device 48 operates and sends a trip signal to the motor starting circuitbreaker (device 6T).






Device 49 (thermal overload relay) provides overload protection for the motor as follows:

_An increase in load on the motor that is above the full load rated value willresult in an increase in current flow through CT1.

_The increase in current flow is transmitted to the 49 device from CT1.

_If the increase in current flow persists for a sufficient length of time, relay 49will operate and send a signal to the auxiliary lockout relay #1 (device 86M1).



Device 50 (instantaneous phase overcurrent relay) provides overcurrent protection againstfaults that occur downstream of the circuit breaker as follows:

_A fault that occurs downstream of the circuit breaker will cause an increase incurrent flow through CT-1.



_The operation of device 50 will send a signal to the auxiliary lockout relay #1(device 86M1).

_The motor auxiliary lockout relay #1 (device 86M1) will operate and send asignal to the motor starting circuit breaker (device 6T). The motor startingcircuit breaker (device 6T) will trip (open) and secure power to the motor.

Device 50G (ground-fault overcurrent relay) provides protection against ground faults thatoccur downstream of the circuit breaker as follow:








_When the instantaneous element of the 50G device operates, the device willsend a signal to the auxiliary lockout relay (86M1).

_The auxiliary lockout relay (device 86M1) will operate and send a trip signalto the motor starting circuit breaker (device 6T). The motor starting circuitbreaker (device 6T) will trip (open) and secure power to the motor.





_The auxiliary lockout relay (device 86M1) will operate and send a trip signalto motor starting circuit breaker (device 6T).


Device 55 (power factor relay) provides loss of field protection for the motor as follows:

_Synchronous motors normally are designed to operate at power factors thatrange form 0 to 0.8 leading.




_The 55 device continuously monitors the power factor of the motor throughuse of a voltage input from the voltage transformer and a current input fromCT-1.


_Upon a loss of a significant decrease in the motor's field, the motor's powerfactor will become lagging.

_The lagging power factor will cause the 55 device to operate and send a signalto the auxiliary lockout relay #1.







_Device 87 will operate and send a trip signal to the motor starting circuitbreaker (device 6T).





Synchronous Motors < 10,000 hpFigure 34

Saudi Aramco Evaluating Motor Specifications 128

WORK AID 1: MOTOR DESIGN REQUIREMENTS FOR SAUDI ARAMCOINSTALLATIONS COMPILED FROM SADP-P-113, NEMA MG-1AND ESTABLISHED ENGINEERING PRACTICES

Use Work Aid 1 to complete Exercise 1.

Stator

The stator frame should be of fabricated steel construction, with sufficient strength andrigidity to withstand the stresses to which the stator may be subjected in handling, transport,or due to short-circuit or other forces when in service.

Rotor

Induction motors should have a cylindrical rotor of the squirrel-cage type.

Synchronous motors are available in cylindrical rotors or salient pole rotors.

Cylindrical synchronous motors are used in speeds in excess of 1800 rpm.

Salient pole rotors come in two types:

_The laminated salient pole rotor with a cage damper winding in each pole facefor starting.

_The solid pole rotor with solid bolted pole shoes.

The solid pole rotor is the preferred type of synchronous salient pole rotor.

Critical Speed

The critical speeds of motors shall be established by the following chart.

First LateralCritical Speed

Second LateralCritical Speed

Rigid Shaft 115% rated motor rpm Not within _10% of 2 timesrated motor rpm

Flexible Shaft 65 to 85% rated motor speed Not within _10% of 2 timesrated motor rpm

WORK AID 1 (Cont'd)




Bearings

Select bearing type by the chart below.

Motor Size Bearing Type< 15 hp (11 kW)_ 15 hp (11 kW)> 200 hp (150 kW) *

200 or 300 series ball bearings300 series ball bearingsSleeve bearings

* Horizontal Motors

Type of bearing lubricant to use.

Bearing Type LubricationAnti-friction

SleeveOil or Grease

Oil

When grease or oil can be used, use what is most economical. Grease will usually be utilizedbecause it is usually more economical to use.

Method of Bearing Lubrication

Sleeve bearing lubrication methods should be as follows:

Shaft Journal Velocity m/S Type of LubricantBelow 11

11 and above

Uncooled ring or disc oil lubrication

Circulated feed oil lubrication

Ball bearings should be of the regreasable shielded type, furnished without grease fittings, butequipped with plugs in the tapped holes that are normally provided for such fittings. Reliefholes or drain plugs that are located 180o from the grease point should be provided.

Pre-lubricated sealed anti-friction bearings are not acceptable.




WORK AID 1 (Cont'd)

Bearing Housing Protection

Horizontal motors that are rated <3730 kW (5000 hp) should have the bearings housed in theendbells of the motor.

Horizontal motors that are rated 3730 kW (5000 hp) and larger should be provided withpedestal bearings that are supported by the motor baseplate.

The bearing housing for horizontal motors with oil-lubricated bearings should be equippedwith labyrinth type end seals and deflectors where the shaft passes through the housing.Synthetic gaiter (lip) type seals should not be used.

Vertical motors with oil or grease-lubricated bearings should be provided with a positive shaftseal on top and bottom bearings.

Vibration Monitoring

Vibration monitoring should be used as required below:

_Vibration monitoring equipment should not be used for motors below 185 kW(250 hp).

Horizontal Motors

_750 kW (10000 hp) - 3000 kW (4000 hp): one seismic detector should bemounted horizontally at each bearing.

_3001 kW (4001 hp) and above: two proximity detectors should be mounted90o apart at each bearing.

Vertical Motors

_Seismic type vibration detectors should be mounted 90o apart around thecircumference of the top bearing housing.

Mechanical Noise

The sound pressure level at one meter should not exceed 90 dB.




WORK AID 1 (Cont'd)

Shaft Circulating Current

Horizontal motors that are rated 375 kW (500 hp) and larger should have bearings that areelectrically insulated from the motor frame or baseplate. A removable test link should beprovided for the drive end bearing.

Vertical motors that are rated 375 kW (500 hp) and larger should have an electricallyinsulated top bearing.

Stator Winding

The stator insulation system, including the leads, should consist of low-hygroscopic materials.The system should have Class F insulation. The stator windings should be treated towithstand tropical conditions and the corrosive effects of industrial sulfurous atmospheres.Stator temperature should be monitored as follows:

_Motors that are rated 150 kW (200 hp) up to but not including 1300 kW (1750hp) should have one RTD per phase.

_Motors that are rated 1300 kW (1750 hp) through 7500 kW (10,000 hp)should have two RTD's per phase.

_Motors that are rated above 7500 kW (10,000 hp) should have three RTD'sper phase.

_The hottest reading RTD should be identified by the vendor during factorytesting.

Rotor Windings

The rotor windings of induction machines should be of the cage type and formed fromcopper, copper alloy, or aluminum bar.

The rotor body of synchronous machines should be of the salient-pole type with the windingsof insulated copper wire or strip that are treated to withstand tropical conditions.




WORK AID 1 (Cont'd)

In addition to induction motor rotors, salient-pole rotors of synchronous motors mayincorporate a copper starting cage to meet a desired torque characteristic. On squirrel-cagerotors, end-ring connections should be of high mechanical strength. Filler metals that areused should be resistant to attack by corrosive sulfurous gases. Copper alloy rotorconstruction should conform to American Welding Society AWS A5.8 and contain aminimum 40% silver.

Copper-phosphorous bronze type fillers are technically unacceptable.

The motor insulation system, including the leads, should consist of low-hygroscopicmaterials. The system should have Class F insulation.

Mounting Details

Enough measurements must be taken to ensure proper mechanical fit of the new motor. Thenumber of measurements that are taken are determined by the designer according to hisjudgement.

Cooling

If the motor insulation exceeds the limits of class B insulation, a heat exchanger must beinstalled.

Motor InsulationClass

Maximum AllowableInsulation Temperature

in oCBFH

130155180

The cooling of all motor components and lubricating oil (if required) should be by air. Liquidcooling is not acceptable.

Where totally-enclosed machines utilize heat exchangers, closed-air circuit-air cooled(CACA) exchangers should be provided and mounted on the motor. Top-mounted heatexchanger assemblies should have flanges that extend downward to overlap the motorenclosure on all sides by a minimum of 10 mm (0.4 in). When air/air heat exchangers requireauxiliary fan cooling, shaft-mounted cooling fan or fans should be provided. Auxiliarymotor-driven fans should not be specified.




WORK AID 1 (Cont'd)

Control and Supply Leads

Wiring for auxiliary equipment should be moisture and heat resistant and should conform tothe following:

_Conductors should be made of copper and rated in accordance with theNational Electrical Code (NEC) for operation at 50oC ambient temperature.

_Minimum conductor size should be stranded 2.5 mm sq. (No. 14 AWG).

_Conductors should be clearly and permanently identified with sleeve typePVC wire markers.

_All conductors that are passing through an internal baffle or partition that iswithin the motor enclosure should be protected by a bushing.

Nameplates

The following is the minimum information that is required to be on motor nameplates:

_Manufacturer's name, serial number or date code, and suitable identification_Horsepower output or kilowatt rating_Time rating_Temperature rise_RPM at rated load_Frequency_Number of phases_Voltage_Rated-load amperes_Code letter for kVA_Buyer's purchase order number_Year of manufacture_Manufacturer's location_Manufacturer's order reference number_Anti-friction bearing number and manufacturer class, group, and division(explosion-proof motors, only)_Maximum ambient temperature




_Insulation system designation_Rotor weight_Total weight of motor_Direction of rotation arrow

WORK AID 1 (Cont'd)

The nameplate(s) and rotation arrows should be 300 series stainless steel or monel, theyshould be securely fastened by pins of similar material, and they should be located for easyvisibility. Entries should be marked by etching, engraving, or by another permanent methodof marking.

Space Heaters

Heaters should be installed in all motors that are rated 2.3 kV or higher. Other motors that areinstalled outdoors and used only as standby motors may also require heaters. In such cases,the designer should check with the proponent.

Motors should be equipped with space heaters that are completely wired, with leads that arebrought out to a separate terminal box. Heaters with exposed elements are prohibited. Heaternameplate voltage should be twice the supply voltage that is indicated on the Data Schedule.

The heaters will maintain the temperature of the motor windings at approximately 5oC (9oF)above ambient. Surface temperature of the heater elements should not be greater than shownbelow:

Area Classification Surface TemperatureClass I, Group C

Class I, Group D

Class II, Group E

Class II, Group G

160oC (320oF)

215oC (419oF)

200oC (392oF)

120oC (248oF)

The amount of heat that is required to raise the winding temperature of a given enclosedhorizontal motor approximately five degrees C above ambient temperature, where themachine is closed except for a small vent at the top and bottom for circulation, is given by theformula:

H = 0.28 DL




where: H = Heat in kilowattsD = Machine end bell diameter in metersL = Machine stator length between end bell centers in meters

WORK AID 1 (Cont'd)

Heaters should be suitable for supply voltages of 120 VAC, and they should be controlled byauxiliary contacts in the breaker.

Testing Requirement

The following are the minimum tests that are required:

_Measurement of winding resistance.

_No-load motoring readings of current, power, and nominal speed at ratedvoltage and frequency.

_Measurement of open-circuit voltage ratio on wound-rotor machines.

_High-potential test.

_Bearings/lube oil temperature measurement.

When determined by the designer, the following test should be performed:

_Performance determination_Temperature tests_Miscellaneous tests_Surge tests

Painting and Coating

All steel surfaces should have the vendor's standard finish with a minimum of 0.127 mm (5mil) dry thickness.

Packing




The packing of equipment should be suitable for shipment by sea and also for vehiculartransportation over unpaved desert roads. Packing should be in accordance with Buyer'sPacking Specifications No. 1 and No. 1.1 of Vendor's standard export packing. Vendor'sstandard export packing should be subject to the buyer's approval.




WORK AID 2: MOTOR ENCLOSURE REQUIREMENTS FOR SAUDI ARAMCOINSTALLATIONS COMPILED FROM SADP-P-113, NEMA MG-1AND ESTABLISHED ENGINEERING PRACTICES


Technical requirements for specification of a motor enclosure type.

_Totally Enclosed Fan-Cooled (TEFC)

- Allowed on motors that are rated less than 11,000 kW (15,000 hp)- Permitted in zone 1, zone 2, and unclassified locations

_Totally Enclosed Air to Air-Cooled (CACA)

- For use on all induction motors and for salient pole synchronous motorsthat are rated up to 11,000 kW (15,000 hp) when the motor'stemperature rise exceeds the allowable temperature rise for class Binsulation.

- For use on induction motors that are rated greater than 11,000 kW(15,000 hp)

- For use on synchronous motors where the cleaning cost over the life ofthe machine plus the capital cost of an open motor would be more thanthe cost of the CACA enclosure.

- Permitted in zone 1, zone 2, and unclassified areas.

_Weather Protected (WPII)

- Specified for synchronous motors that are rated above 11,000 kW(15,000 hp) where the lifetime motor cleaning costs plus motor capitalcosts are less than the capital cost of a CACA motor.

- Permitted in zone 2 and unclassified locations.

Technical requirements for specification of a motor-enclosed level of protection.

_All Saudi Aramco motor enclosures will be protected to a level that is equal tothe IEC level designation IP44.




WORK AID 2 (Cont'd)

Technical requirements for the specification of a motor enclosure level of cooling.

_TEFC motors will be cooled to an IEC level ICA01A41.

_CACA motors will be cooled to an IEC level ICA01A61.

_WPII motors will be cooled to an IEC level ICA01.

Technical requirements for specification of a motor enclosure level of hazard protection.

_Zone 1 area requires a flameproof EXd motor.

_Zone 2 area requires a non-sparking EXn motor.

_Unclassified area does not require a level of hazard protection for a motor.

Main and auxiliary equipment boxes should meet or exceed the main motor enclosurerequirements.

_Grounding Requirements

- Motors up to 150 kW (200 hp) _ grounding stud- Motors at or above 150 kW (200 hp) _ grounding pad

Ground connections should accommodate the following minimum size of grounding cable:

Motor Rating Cable Size

kW (HP) mm2 (AWG/MCM)185 < 370 (250 < 500) 70 (2/0)370 < 3360 (500 < 4500) 120 (4/0)

3360 & above (4500 & above)185 (350)




WORK AID 3: CONDITIONS UNDER WHICH THE VARIOUS TYPES OF MOTORSTARTERS SHOULD BE SPECIFIED FOR USE AT SAUDIARAMCO INSTALLATIONS, COMPILED FROM SADP-P-113,NEMA MG-1, AND ESTABLISHED ENGINEERING PRACTICES


1. Determine if the voltage drop on the system upon motor start at full voltage is greaterthan the voltage drop that is permissible for the system.

Use the graph that is shown in Figure 35 to approximate the voltage drop on a systemwhen starting a motor.

a. Find on the bottom axis the motor kVA or the motor hp.

b. Find the power supply's rated kVA curve for the power supply that will supplythe motor.

c. Trace the motor rating line up from the bottom and the power supply's ratedkVA curve. Find the point of intersection of these two lines.

d. Trace a line to the left axis (power supply's voltage) and read the percentagevalue of initial voltage.

e. Subtract from 100 the value that is read on the left axis to arrive at the systemvoltage drop upon motor starting.

If the voltage drop is less than 15%, full voltage starting should be used.

If the voltage drop is greater than 15%, the Consulting Services Department should beconsulted to perform a more accurate calculation on a computer. On voltage drops greaterthan 15%, reduced voltage starting should be considered.

Note: For the purpose of performing Exercise 3, the approximate voltage drop should beassumed to be the actual voltage drop. For this Exercise, there is no reason to contactthe Consulting Services Department.

WORK AID 3 (Cont'd)




Approximate Voltage Drop Due to Full Voltage of the MotorFigure 35

WORK AID 3 (Cont'd)

2. Choose the reduced voltage starting to be performed when voltage drop exceeds theallowable value. A table of comparable torque and current values for various startingmethods is shown in Figure 36. Figure 36 shows the method of starting, the motorvoltage as percent of full voltage, the starting torque as percent of full voltage startingtorque, and the line starting current as percent of full voltage starting current.

a. Determine the starting torque that is required by the load and what types ofstarting methods will provide the necessary starting torque.

- If only one method of reduced voltage starting will provide enoughstarting torque, select that starting method.

- If more than one method of reduced voltage starting will provide thenecessary starting torque, proceed to the next selection factor.

b. Determine if motor heating is a concern because of excessive motor starts.

- If heating is a concern, select the reduced voltage starting method withthe lower starting current that meets the motor starting torquerequirements.

- If heating is not a concern, proceed to the next selection factor.




c. If more than one method of reduced voltage starting can meet the startingtorque requirement of the load and the motor heating limitations, choose thelower cost method of starting.




WORK AID 3 (Cont'd)

Comparable Torque and Current Values for Various Starting MotorsFigure 36




2400/4160V NEMA E2 Air Break

Cost comparisons are for floor-mounted starters that are complete with overload andshort circuit protection and all necessary starting contents.

Percent Typical Sizes(2400/4160 V)

Full VoltagePrimary ReactorAuto-transformer

100250285

up to 1100/3000 kWup to 1100/3000 kWup to 1100/3000 kW

WORK AID 3 (Cont'd)

Average Relative Cost of Starting Methods

480V Magnetic Starters (Individual Enclosures)

The starters are costed complete with built-in starting components, overloadprotection, and housed in NEMA general purpose enclosures. Short circuit protectionis not included.

Percent Typical SizesFull VoltagePart WindingPrimary ResistorAuto-transformerWye-Delta

100360520560630

18.5 kW to 75 kW18.5 kW to 75 kW18.5 kW to 75 kW18.5 kW to 75 kW18.5 kW to 75 kW

480V Magnetic Starters (in Motor Control Centers)




Cost comparisons are for starter units that are suitable for mounting in NEMA I motorcontrol centers. The starters are complete with overload and short circuit protection(to 22,000 A) and include all necessary starting components.

Percent Typical SizesFull VoltagePart WindingAuto-transformerWye-Delta

100180285360

37 kW to 185 kW37 kW to 140 kW37 kW to 185 kW37 kW to 185 kW




WORK AID 4: CONDITIONS UNDER WHICH THE VARIOUS TYPES OF MOTORPROTECTION SHOULD BE SPECIFIED FOR USE AT SAUDIARAMCO INSTALLATIONS, COMPILED FORM SADP-P-113,NEMA MG-1, AND ESTABLISHED ENGINEERING PRACTICES


Overload Protection Requirements

Motors less than 1 kW (1.5 HP)- Built-in thermal protection devices.

Low voltage (<600 volts) motors above 1 kW (1.5 HP)

Combination starters- Three thermal overload devices that are ambient-compensated.One device should be located in each phase.

Power circuit breakers- Three transformer-fed, ambient-compensated, thermal overloadrelays. One device should be located in each phase.

2.3 and 4 kV motorsNEMA Class E starters- Three current transformer-fed, ambient-compensated thermal overload

relays.

Power circuit breakers - <1,000 kW (1500 HP)- Three current transformer-fed, ambient-compensated thermal overload

relays.

Power circuit breakers >1,000 kV (1500 HP)- RTD-based protection - stator RTDs.

6.6 and 13.2 kV- RTD-based backed-up by overcurrent relays.




WORK AID 4 (Cont'd)

Short Circuit

Motors 600V and below- Adjustable pickup, instantaneous, and overcurrent relays. One device should

be located in each phase.

Motors 2.3 kV and 4 kV

Nema Class E2 starters- Fuses in combination with thermal overload relays.

Circuit breaker starters- Three adjustable pickup, instantaneous, overcurrent relays. Onedevice should be located in each phase.

Motors 6.6 kV and 13.2 kV- Three adjustable pickup, instantaneous, and overcurrent relays.One device should be located in each phase.

Ground Fault

One instantaneous overcurrent ground sensing relay for all motors with a 50/5 windowtype current transformer.

Current Unbalance

2.3 kV motors and below- No additional protection is required.

4 kV, 6.6 kV, and 13.2 kV- Negative phase sequence voltage relay with timer.

Vibration

Horizontal motors- 750 kW (1000 HP) - 3000 kW (4000 HP) - 1 seismic detector on each bearing.

Vertical motors- Two seismic detectors on the top bearing.




WORK AID 4 (Cont'd)

Alarm levels and shutdown levels are listed below:

Horizontal Motors with Proximity Probes

Motor Synchronous Speed RPM

Alarm LevelMicrometers

Shutdown LevelMicrometers




360018001200

75

90

100

100

125

150




Horizontal Motors with Seismic-Type, Casing Mounted Probes

360018001200900 and below

50

75

90

100

75

100

125

150







Vertical Motors with Seismic-Type, Casing Mounted Probes

Motor Synchronous Speed RPM

Alarm LevelMicrometers

Shutdown LevelMicrometers

360018001200900600 and below

100

125

140

150

275

190

200




150

175

225




WORK AID 4 (Cont'd)

Bearing Failure

RTD's will be used for detection and protection purposes.- RTD's will be platinum, 3-wire type, calibrated to 100 ohms at 0oC.

Horizontal Motors- 185 kW (250 HP) to 370 kW (500 HP) - mounting provision must be included,

but no RTD's.

- >370 kW (500 HP) - one RTD for each bearing.

Bearing alarm and motor shutdown for all motors that are larger than 370 kW (500HP).

Stator Winding Protection

Temperature detection is accomplished by RTD.

Motors 375 kW (500 HP) to 7500 kW (1000 HP) - two RTD's per phase.Motors 7500 kW (10,000 HP) and above - three RTD's per phase.

RTDs will be the following type.

RTDs will be platinum, 3-wire type, calibrated to 100 ohms at 0oC.

Shutdown and alarms

Plant-attended - alarm onlyPlant-unattended - alarm and shutdown

Shutdown and alarm levels

Motor Insulation Class Alarm or Trip SettingDegrees C

Maximum AllowableInsulation Temperature

Degrees CBFH

120145170

130155180




WORK AID 4 (Cont'd)

Undervoltage

Motors less than 2.3 kV require no special undervoltage protection.

Motors 2.3 kW and higher require a short time delay undervoltage relay.

Overvoltage and Surge Protection

The following surge protection requirements are found in 17-SAMSS-502:

Motors that are rated 3730 kw (5000 hp) and larger and motors that are rated 13.2 Kvwill be furnished with surge protection. The surge capacitors will be the last devicesthat are connected to the motor leads before the leads enter the stator. The connectionleads to the capacitors will be 120 SQ MM (4/0 AWG). The surge capacitors will berated as follows:

Motors Rated Voltage (KV)4.06.613.2

Capacitance (Micro-Farads)0.50.50.25

Maximum Operating Voltage (KV)4.577.915.2

Continuous Temperature Rating (Deg C) 606060




Surge arresters will be of the gapless zinc-oxide type, and they will be rated asfollows:

Motor's Rated Voltage (KV)4.06.613.2

Arrester Voltage Rating (KV RMS)4.57.515

Maximum Discharge Voltage at7.412.224.2

1500 (CREST KV)Continuous Temperature Rating (O C)

606060

Surge arrestors will be mounted in the terminal box and all connection leads will beless than 60 cm (24 in) from arrestor to motor terminal.

All motors should have a relay to trip the motor at 110% of the motor's nameplate ratedvoltage as indicated on the nameplate.




WORK AID 4 (Cont'd)

Motor Stalling

Where the possibility of motor stalling exists and start time is less than still time, use one ofthe following:

- Thermal overload set to trip after starting current delay time.

- Start sequence timer to prevent tripping overload upon start if motor comes upto speed.

- Start timer that prevents tripping during starting.

For motors that are rated above 4000 kW (5360 HP) or that have a start time that is longerthan the allowable locked rotor time, apply one of the above methods and use a timer that isset so that, if rotation does not begin within a set time, the motor will trip.

Differential Protection

Motors >3700 kW (5000 HP) will require differential protection.

One relay will be applied for each phase.

Synchronous Motors Extra Protection

If necessary, provide the following protection.

- Out of step- Sudden restoration power- Over voltage- Reverse power- Loss of excitation- Over-excitation- Under-excitation- Diode failure




GLOSSARY

ANSI American National Standards Institute

AWG American Wire Gauge

CACA Closed Air Circuit Air-Cooled

centistokes (cSt) The International Standards Organization's metric measure of alubricant's viscosity.

centrifugal force The force that tends to impel a thing or parts of a thing outward from thecenter of rotation.

critical speed A speed at which the amplitude of the vibration of a rotor that resultsfrom shaft transverse vibration reaches a maximum value.

diode A semiconductor device with two terminals that exhibits a nonlinearvoltage - current characteristic.

dynamic load The load to which the bearings can be subjected while achieving arating rated life of one million revolutions.

EXD Explosion Proof Motor. This type of motor is enclosed in a case that iscapable of withstanding an explosion of a specified gas or vapor thatmay occur within it. It prevents the ignition of a specified gas or vaporsurrounding the enclosure by sparks, flashes, or explosion of the gas orvapor within the enclosure. This type of motor operates at such anexternal temperature that a surrounding flammable atmosphere will notbe ignited.




EXN Non-sparking motor.

heat exchanger A device that is used to transfer heat between two fluids without directcontact between them.

hygroscopic Readily taking up and retaining moisture.

IEC International Electrotechnical Commission.

ingress The act of entering.

IP Ingress Protection.

journal bearing A bearing that supports the cylindrical journal of a shaft.kVA Kilovolt Ampere.

locked rotor The steady-state current that is taken from the line while the rotor iscurrent locked and the rated voltage is being applied to the motor.

natural frequency The frequency at which an object vibrates because of its own physicalcharacteristics when it is distorted and then released while it isrestrained or supported at specified points.

NEMA National Electrical Manufacturers Association.

overload Operation of equipment in excess of normal, full load rating.

saybolt universal A standard measure of a lubricant's viscosity.seconds (SUS)

second harmonic The second component frequency of a harmonic motion that is anintegral multiple or the fundamental frequency.

short circuit An abnormal connection of relatively low impedance, that is madeeither accidentally or intentionally between two points of differentpotential that results in a high current flow.




sleeve bearing A bearing with a cylindrical inner surface in which the journal of a rotorshaft rotates.

sound intensity The density of sound power at a point away from the source. Thisdensity of sound power is expressed in watts per square meter. Soundpower that is radiated by a source may be derived by integration of theintensity over an enclosed hypothetical surface of measurement.

sound level A weighted measure of the amount of noise that is produced by amachine at a given point.

sound pressure The level of pressure in the sound-conducting medium that results fromlevel (SPL) the sound intensity at the concerned point.

sound power A property of the noise that is produced by a machine that is(SWL) independent of the environmental conditions or the distance from the

machine.

TEFC Totally Enclosed Fan-Cooled.

thyristor A bistable semiconductor device that comprises three or more junctionsthat can be switched from the off-state to the on-state or vice versa.Such switching occurs within at least one quadrant of the principalvoltage-current characteristic.

Documents

Aramco Engineering - Evaluating Motor Specifications