INDUSTRIAL PROJECT REPORT

FAILURE ANALYSIS IN CRANE DUTY SLIP RING MOTORS.

SUBMITTED BY:- GUIDED BY:-Twinkle Singh Mr. D.K. VarshneyShatabdi Sengupta (AGM)

Dept of Electrical and Electronics, Electrical Repair Shop

ACKNOWLEDGEMENTWe take this opportunity to express our profound gratitude and deep regards to our guide Mr. D.K. VARSHNEY for his exemplary guidance, monitoring and constant encouragement throughout the course of this project.

We also take this opportunity to express a deep sense of gratitude to Mr. KUNTAL BAGHEL, for his cordial support, valuable information and guidance, which helped us in completing this task through various stages.

 We are obliged to the officials and workers of ERS, for the insight provided by them and for letting us work on the equipments. We are grateful for their cooperation during the period of our assignment.

Lastly, we would like to thank BHILAI STEEL PLANT for incessant help and also for providing us with all the resources we needed for this project, without which the completion of this project would not have been possible.

SUBMITTED BY:- GUIDED BY:-Twinkle Singh Mr. D.K. VarshneyShatabdi Sengupta (AGM)

Dept of Electrical and Electronics, Electrical Repair Shop

STEEL AUTHORITY OF INDIA LIMITED (SAIL)

BHILAI

This is to certify that “Ms. Shatabdi Sengupta, Ms. Twinkle Singh”; students of 6thsem Electrical and Electronics Engineering, SRM University, NCR Campus have completed their Industrial Training Project on “FAILURE ANALYSIS IN CRANE DUTY SLIP RING MOTORS” at Bhilai Steel Plant. During the project they were highly responsive, dedicated and hard working. On the basis of their interest and devotion towards the assigned tasks and keenness to complete within stipulated period, I certify them as have completed the project successfully under my guidance. It was a great pleasure for me to guide them and share the knowledge.I wish them great success in life.

Date: 14/06/2014 Mr. D.K. Varshney

BHILAI STEEL PLANT –AN OVERVIEWBhilai Steel Plant, a unit of Steel Authority of India Ltd. is a public sector undertaking and was conceived under Indo-USSR Treaty in the 2nd Five year plan. This was in accordance with erstwhile government policy for strengthening economy and self reliance through development of core sector.

The plant is located at the central position of India, which is one of the major iron belt of India, and it is about 40 kilometres from Raipur, capital of Chhattisgarh. The captive mines of the plant located at Dalli-Rajahara supplies iron ore and lime stone used to be available from Nandini captive mines. At present lime stone is procured from outside. The other major raw material, coal is purchased from outside either through import or from indigenous market.

Bhilai Steel Plant, an integrated steel works, was commissioned in 1959 with production capacity of 1.0 million tonne of steel. In successive phases, capacity was enhanced to 2.5 and 4.0 million tonne in the year 1962 and 1984 respectively. Figure depicts facilities available with Bhilai Steel Plant for 7.0 mt production. As of now this is the largest steel plant in India with present capacity utilization more than 100% for three consecutive years.

Bhilai Steel Plant produces wide range of products. This includes Rails, Wire Rods, Plates and Merchant products. Commitment to quality and customer satisfaction has resulted in consistent R & D efforts culminating in development and commercialization of distinctive new grades like SAILMA, UTS-90 etc. Bhilai Steel Plant could dream and implement the project of long rail (260 meter long) in consistence with its reputation. This was a basic demand from Indian Railways for enhancement of country’s economy. Bhilai steel plant is planning to expand its production to 7.0 Mt by the year 2014-15. During its expansion plan all energy efficient technology will be installed, after this the energy consumption may come down to 5.2 gcal/tcs. Human resource management is exemplary in Bhilai Steel Plant. It is worthwhile to note that Bhilai Steel Plant registered maximum profit for 2012-13 also among all public sector steel plants.

The production indices of Bhilai Steel Plant for the year 2007-08 is 5.267670 million tonne of hot metal, 5.054645 million tonne of crude steel and 4.428861 million tonne of saleable steel.

The annual turnover of the company for the year 2007-08 is Rs. 16518 crores with net profit margin of Rs. 5366 crores.

Bhilai Steel Plant symbolizes Indian Industrial Growth. Many laurels were bestowed upon Bhilai Steel Plant. It has been honored 11 times with coveted “Prime Minister’s Trophy” as best Integrated Steel Plant of the country.

At Bhilai IS0:14001 has been awarded for Environment Management System in the Plant, Township and Dalli Mines. It is the only steel plant to get certification in all these areas. The Plant is accredited with SA: 8000 certification for social accountability and the OHSAS-18001 certification for Occupational health and safety. These internationally recognized certifications add value to Bhilai's products and helps create a place among the best organizations in the steel

industry. Among the long list of national awards it has won, Bhilai has bagged the CII-ITC Sustainability award for three consecutive years.

Units of Bhilai Steel Plant

SI No.

Department

Unit Capacity

1 Coke oven 8 Batteries of 65 ovens and 4.3 M high2 Batteries of 67 ovens and 7.0 M high

3.3 million ton of BF coke

2 Sinter plants1) 3 machine of 75 Sq M and 1 Machine and 1 Machine of 80 Sq M hearth area2) 1 Machine of 320 Sq M hearth area

8.3 Mt of sinter

3 Blast furnace 3 furnaces of 1033 cum3 furnaces of 1719 cum1 furnace of 2000cum

4.71 Mt of hot metal

4 SMS-1 4 TWIN HEARTH FURNACES 2.5 Mt of steel5 SMS-2 3 BOF of 100/130 T capacity 1.425 Mt of

steel6 Concast 3 single strand and 1 combo caster 1.425 Mt of

steel7 Blooming

and billet mill

1150 mm blooming mill1000/700/500mm conti.billet mill

2.15 Mt of bloom

8 Rail and structural

mill

950/800 2 high reversing mill 0.75 Mt of product

9 Merchant mill

350 mm cross country mill 0.5 Mt of product

10 Wire rod mill 4 strand continuous mill 0.4 Mt of product

11 Plate mill 3600 mm 4 high reversing mill 0.95 Mt of product

Bhilai Steel Plant - a symbol of Indo-Soviet techno-economic collaboration,is one of the first three integrated steel plants set up by Government of India tobuild up a sound base for the industrial growth of the country, The agreementfor setting up the plant with a capacity of 1 MT of Ingot steel was signedbetween the Government of erstwhile U.S.S.R. and India on 2nd February,1955, and only after a short period of 4 years, India entered the main stream ofthe steel producers with the commissioning of its first Blast Furnace on 4thFebruary, 1959 by the then President of India, Dr Rajendra Prasad.Commissioning of all the units of 1 MT stage was completed in 1961. A dreamcame true-the massive rocks from the virgin terrains of Rajhara were convertedinto valuable iron & steel.

In the initial phase the plant had to face many teething problems, mostlyunknown to the workforce at the time, But by meticulous efforts and team spirit,these problems were surmounted and the rated capacity production wasachieved only within a year of integrated operation of the plant.Thereafter, the plant was expanded to 2.5 MT capacity per year, and thento 4 MT of crude steel per year, with Soviet assistance.

All the units of the plant have been laid out in sequential formation

according to technological inter-relationship so as to ensure uninterrupted flowof in-process materials like Coke, Sinter, Molten Iron, Hot Ingots, as well asdisposal of metallurgical wastages and slag etc., minimizing the length ofvarious inter-plant communications, utilities and services.

BSP is the sole manufacturer of rails and producer of the widest andheaviest plates in India. Bhilai specializes in the high strength UTS 90 rails, hightensile and boiler quality plates, TMT bars, and electrode quality wire rods. It isa major exporter of steel products with over 70% of total exports from theSteel Authority of India Limited being from Bhilai.

The distinction of being the first integrated steel plant with all major production units and marketable products covered under ISO 9002 Quality Certification belongs to BSP. This includes manufacture of blast furnace coke and coal chemicals, production of hot metal and pig iron, steel making through twin hearth and basic oxygen processes, manufacture of steel slabs and blooms by continuous casting, and production of hot rolled steel blooms, billets and rails, structural, plates, steel sections and wire rods. The plant's Quality Assurance System has subsequently been awarded ISO 9001:2000.

Not content with the Quality Assurance system for production processes,Bhilai has one in for ISO 14001 certification for its Environment ManagementSystem and its Dali Mines. Besides environment-friendly technology like CoalDust Injection System in the Blast Furnaces, de-dusting units and electrostaticPrecipitators in other units, BSP has continued a vigorous forestation drive,Planting trees each year averaging an impressive 1000 trees per day in the steelTownship and mines.

A leader in terms of profitability, productivity and energy conservation, BSPhas maintained growth despite recent difficult market conditions. Bhilai is theonly steel plant to have been awarded the Prime Minister's Trophy for the bestintegrated steel plant in the country seven times.Bhilai Steel Plant, today, is a panorama of sky-scraping chimneys andblazing furnaces as a modern integrated steel plant, working round the clock, toproduce steel for the nation. Bhilai has its own captive mines spread over10929.80 acres. We get our iron ore from Rajhara group of mines, 85 kmssouth-west of Bhilai. Limestone requirements are met by Nandini mines, 20 kmsnorth of Bhilai and dolomite comes from Hirri in Bilaspur district, 135 kms eastof the plant. To meet the future requirement of iron ore, another mining siteRowghat , situated about 100 km south of Rajhara, is being developed; as theore reserves at Rajhara are depleting.

Bhilai expanded its production capacity in two phases - first to 2.5 MTwhich was completed on Sept. 1, 1967 and then on to 4 MT which wascompleted in the year 1988. The plant now consists of ten coke oven batteries.Six of them are 4.4 metres tall. The 7 metre tall fully automated Batteries No 9& 10 are among the most modern in India. Of Bhilai's seven blast furnaces,three are of 1033 cu. metre capacity each, three of 1719 cu. metre and one is2000 cu. metre capacity. Most of them have been modernised incorporatingstate-of-the-art technology.

Steel is made through twin hearth furnaces in Steel Melting Shop I as wellas through LD Convertor -continuous Casting route in SMS II. Steel gradesconforming to various national and international specifications are produced inboth the melting shops. Production of cleaner steel is ensured by flameenrichment and oxygen blowing in SMS I while secondary refining in Vaccum

Arc Degassing ensures homogenous steel chemistry in SMS II. Also in SMS II isa 130 T capacity RH (Ruhshati Heraus) Degassing Unit, installed mainly toremove hydrogen from rail steel and Ladle Furnace to meet present and futurerequirements of quality steel. Bhilai is capable of providing the cleanest andfinest grades of steel.

The rolling mill complex consists of the Blooming & Billet Mill, Rail &Structural Mill, Merchant Mill, Wire Rod Mill and also a most modern Plate Mill.While input to the BBM and subsequently to Merchant Mill and Wire Rod Millcomes from the Twin Hearth Furnaces, the Rail & Structural Mill and Plate millroll long and flat products respectively from continuously cast blooms and slabsonly. The total length of rails rolled at Bhilai so far would circumvent the globemore than 4.5 times.

To back this up, we have the Ore Handling Plant, three Sintering Plants - ofwhich one is most modern, two captive Power Plants with a generating capacityof 110 MW, two Oxygen Plants, Engineering Shops, Machine Shops and a hostof other supporting agencies giving Bhilai a lot of self-sufficiency in fulfilling therigorous demands of an integrated steel plant. Power Plant No.2 of 74 MWcapacity has been divested to a 50:50 SAIL/NTPC joint venture company.The plant has undertaken massive modernization and expansion plan toproduce 7.5 MT of hot metal by the year 2010.

HUMAN PROFILE

More than the machinery and processes, it is the men i.e. the engineers,technicians, skilled and unskilled workers behind them that constitute the fleshand blood of this steel plant.Bhilai at present has around 34000 persons to run this pulsating giant. Theculture which has today become the hallmark of Bhilai is a result orientedapproach to work. It is their effective and co-operative working relationshipnurtured in a spirit of dedication and enthusiasm that has shaped Bhilai's imagetoday.

Adjoining the plant, a modern township - Bhilai Nagar, having thespaciousness of a village and the cleanliness of a modern town is spread - overin 17 self sufficient sectors with schools, markets, parks and other facilities.Free Medical aid is given to all the employees and their dependents througha network of health centres, dispensaries and hospitals. Medical facilities areextended to retired employees & their spouses also.

INTRODUCTION TO ERSLooking towards any integrated steel plant scenario, the important contribution of an Electrical Repair Shop can very well be visualized. It is one of the major service shops which carries out repair of all electrical machines used inside the plant, there are around 42000 electrical machines installed and running continuously year's together, in different processes of production in Bhilai Steel Plant. General maintenance like cleaning, greasing and physical inspection / check-up is done by concerned shops where the machines are used. All major repairs like winding repairs, mechanical repairs, modification in existing winding, periodic overhauling and innovative repairs etc. are being carried out by Electrical Repair Shop either at site or at ERS.

SCOPE OF REPAIR

Electrical Repair Shop carries out repairs of motors, generators, welding and control transformers, welding rectifiers, load lifting magnets, brake & control coils of all types and capacities. It also manufactures a large no. of spare parts like contact materials, switches, components, bus bar, main pole coils and inter pole coils of DC machines, primary and secondary coils of welding and control transformers and various electrical spare parts.

Repair of load lifting magnets, welding transformers, control transformers and rectifiers are carried out in Magnet and Transformer Repair Shop (MTRS). MTRS also fabricates load lifting magnets of all varieties using all in house technology and resources including the design concept. Fabrication (including manufacture of coils) of welding transformers is also under taken in MTRS.

Traction motors and roll table motors from various Rolling Mills are overhauled in Motors Overhauling and Repair Section (MORS) adjacent to MTRS.

PRODUCTION TARGETS

E.R.S. on an averages repairs 350 motors and generators manufactures / reclaims 535 nos. coils, repairs 17 nos. load lifting magnets, 38 nos. welding & control transformers per month. The no. of machines and categories of repair in case of motors and generators is as follows:

Categories Upto 10 Kw 10 Kw and Above

Total  

Rewinding jobs         130 - 140        70 - 80 200 - 220 Medium repair jobs   70 - 80         90 - 100 160 - 180 Total     200 - 220       160 - 180 360 - 400

Repairs from outside agencies are arranged strictly on need basis. Certain repairs like core staggering, shaft change etc. for which repair facilities are not available in ERS are off loaded to outside agencies. The repairs are limited in number (5 - 10 machines per year) and applicable for big size and critical machines only.

MotorsAn electric motor is an electric machine that converts electrical energy into mechanical energy.

In normal motoring mode, most electric motors operate through the interaction between an electric motor's magnetic field and winding currents to generate force within the motor. In certain applications, such as in the transportation industry with traction motors, electric motors can operate in both motoring and generating or braking modes to also produce electrical energy from mechanical energy.

Found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives, electric motors can be powered by direct current (DC) sources, such as from batteries, motor vehicles or rectifiers, or by alternating current (AC) sources, such as from the power

grid, inverters or generators. Small motors may be found in electric watches. General-purpose motors with highly standardized dimensions and characteristics provide convenient mechanical power for industrial use. The largest of electric motors are used for ship propulsion, pipeline compression and pumped-storage applications with ratings reaching 100 megawatts. Electric motors may be classified by electric power source type, internal construction, application, type of motion output, and so on.

Devices such as magnetic solenoids and loudspeakers that convert electricity into motion but do not generate usable mechanical power are respectively referred to as actuators and transducers. Electric motors are used to produce linear force or torque (rotary).

DC motorsA DC motor relies on the fact that like magnet poles repels and unlike magnetic poles attracts each other. A coil of wire with a current running through it generates an electromagnetic field aligned with the centre of the coil. By switching the current on or off in a coil its magnet field can be switched on or off or by switching the direction of the current in the coil the direction of the generated magnetic field can be switched 180°. A simple DC motor typically has a stationary set of magnets in the stator and an armature with a series of two or more windings of wire wrapped in insulated stack slots around iron pole pieces (called stack teeth) with the ends of the wires terminating on a commutator.

The armature includes the mounting bearings that keep it in the centre of the motor and the power shaft of the motor and the commutator connections. The winding in the armature continues to loop all the way around the armature and uses either single or parallel conductors (wires), and can circle several times around the stack teeth. The total amount of current sent to the coil, the coil's size and what it's wrapped around dictate the strength of the electromagnetic field created. The sequence of turning a particular coil on or off dictates what direction the effective electromagnetic fields are pointed.

By turning on and off coils in sequence a rotating magnetic field can be created. These rotating magnetic fields interact with the magnetic fields of the magnets (permanent or electromagnets) in the stationary part of the motor (stator) to create a force on the armature which causes it to rotate. In some DC motor designs the stator fields use electromagnets to create their magnetic fields which allow greater control over the motor. At high power levels, DC motors are almost always cooled using forced air.

The commutator allows each armature coil to be activated in turn. The current in the coil is typically supplied via two brushes that make moving contact with the commutator. Now, some brushless DC motors have electronics that switch the DC current to each coil on and off and have no brushes to wear out or create sparks.

Different number of stator and armature fields as well as how they are connected provides different inherent speed/torque regulation characteristics. The speed of a DC motor can be controlled by changing the voltage applied to the armature. The introduction of variable resistance in the armature circuit or field circuit allowed speed control. Modern DC motors are often controlled by power electronics systems which adjust the voltage by "chopping" the DC current into on and off cycles which have an effective lower voltage.

Since the series-wound DC motor develops its highest torque at low speed, it is often used in traction applications such as electric locomotives, and trams. The DC motor was the mainstay of electric traction drives on both electric and diesel-electric locomotives, street-cars/trams and diesel electric drilling rigs for many years. The introduction of DC motors and an electrical system to run machinery starting in the 1870s started a new second Industrial Revolution

DC motors can operate directly from rechargeable batteries, providing the motive power for the first electric vehicles and today's hybrid cars and electric cars as well as driving a host of cordless tools. Today DC motors are still found in applications as small as toys and disk drives, or in large sizes to operate steel rolling mills and paper machines.

If external power is applied to a DC motor it acts as a DC generator, a dynamo. This feature is used to slow down and recharge batteries on hybrid car and electric cars or to return electricity back to the electric grid used on a street car or electric powered train line when they slow down. This process is called regenerative braking on hybrid and electric cars. In diesel electric locomotives they also use their DC motors as generators to slow down but dissipate the energy in resistor stacks. Newer designs are adding large battery packs to recapture some of this energy.

AC MOTORSAn AC motors is an electric motor driven by alternating current. It commonly consists of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field.

There are two main types of AC motors, depending on the type of rotor used. The first type is the induction motor or asynchronous motor; this type relies on a small difference in speed between the rotating magnetic field and the rotor to induce rotor current. The second type is the synchronous motor, which does not rely on induction and as a result can rotate exactly at the supply frequency or a sub-multiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet. Other types of motors include eddy current motors, and also AC/DC mechanically commutated machines in which speed is dependent on voltage and winding connection.

AC Motors are basically of two major types:- Synchronous Motors Induction Motors

SYNCHRONOUS MOTORSSynchronous motors run at constant speed called synchronous speed and as such find applications for mechanisms which have to run at constant speed irrespective of load.

Examples are• Motor-Generator sets• Reciprocating air compressors.• C.W. pumps

Synchronous speed is a function of supply frequency and no. of poles

Where

Ns = Synchronous speed, in revolutions per minute F = AC power frequency p = Number of poles per phase winding

The rotors are given DC supply to lock themselves in the synchronous speed. Therefore slip rings are used to supply DC.

INDUCTION MOTORS

Slip

If the rotor of a squirrel cage motor runs at the true synchronous speed, the flux in the rotor at any given place on the rotor would not change, and no current would be created in the squirrel cage. For this reason, ordinary squirrel-cage motors run at some tens of rpm slower than synchronous speed. Because the rotating field (or equivalent pulsating field) effectively rotates faster than the rotor, it could be said to slip past the surface of the rotor. The difference between synchronous speed and actual speed is called slip, and loading the motor increases the amount of slip as the motor slows down slightly. Even with no load, internal mechanical losses prevent the slip from being zero.

Actual RPM for an induction motor will be less than this calculated synchronous speed by an amount known as slip, that increases with the torque produced. With no load, the speed will be very close to synchronous. When loaded, standard motors have between 2-3% slip, special motors may have up to 7% slip, and a class of motors known as torque motors are rated to operate at 100% slip (0 RPM/full stall).

The slip of the AC motor is calculated by:

WhereNr = Rotational speed, in revolutions per minute.S = Normalised Slip, 0 to 1.

As an example, a typical four-pole motor running on 60 Hz might have a nameplate rating of 1725 RPM at full load, while its calculated speed is 1800 RPM.

The speed in this type of motor has traditionally been altered by having additional sets of coils or poles in the motor that can be switched on and off to change the speed of magnetic field rotation. However, developments in power electronics mean that the frequency of the power supply can also now be varied to provide a smoother control of the motor speed. This kind of rotor is the basic hardware for induction regulators, which is an exception of the use of rotating magnetic field as pure electrical (not electromechanical) application.

BASIC PRINCIPLE OF AC MOTORSAn electrical motor is such an electromechanical device which converts electrical energy into a mechanical energy. In case of three phase AC operation, most widely used motor is Three phase induction motor as this type of motor does not require any starting device or we can say they are self starting induction motor.

For better understanding the principle of three phase induction motor, the basic constructional feature of this motor must be known to us. This Motor consists of two major parts:

Stator: 

Stator of three phase induction motor is made up of numbers of slots to construct a 3 phase winding circuit which is connected to 3 phase AC source. The three phase winding are arranged in such a manner in the slots that they produce a rotating magnetic field after AC is given to them.

Rotor: 

Rotor of three phase induction motor consists of cylindrical laminated core with parallel slots that can carry conductors. Conductors are heavy copper or aluminum bars which fits in each slots & they are short circuited by the end rings. The slots are not exactly made parallel to the axis of the shaft but are slotted a little skewed because this arrangement reduces magnetic humming noise & can avoid stalling of motor.

Working of Three Phase Induction MotorProduction of Rotating Magnetic Field

The stator of the motor consists of overlapping winding offset by an electrical angle of 120°. When the primary winding or the stator is connected to a 3 phase AC source, it establishes a rotating magnetic field which rotates at the synchronous speed.

Secrets behind the rotation:According to Faraday’s law an emf induced in any circuit is due to the rate of change of magnetic flux linkage through the circuit. As the rotor winding in an induction motor are either closed through an external resistance or directly shorted by end ring, and cut the stator rotating magnetic field, an emf is induced in the rotor copper bar and due to this emf a current flows through the rotor conductor.Here the relative velocity between the rotating flux and static rotor conductor is the cause of electric current generation; hence as per Lenz's law the rotor will rotate in the same direction to reduce the cause i.e. the relative velocity.

Squirrel Cage MotorsA squirrel-cage rotor is the rotating part (rotor) used in the most common form of AC induction motor. It consists of a cylinder of steel with aluminum or copper conductors embedded in its surface. An electric motor with a squirrel-cage rotor is termed a squirrel-cage motor.Most common AC motors use the squirrel cage rotor, which will be found in virtually all domestic and light industrial alternating current motors. The squirrel cage refers to the rotating exercise cage for pet animals. The motor takes its name from the shape of its rotor "windings"- a ring at either end of the rotor, with bars connecting the rings running the length of the rotor. It is typically cast aluminum or copper poured between the iron laminates of the rotor, and usually only the end rings will be visible. The vast majority of the rotor currents will flow through the bars rather than the higher-resistance and usually varnished laminates. Very low voltages at very high currents are typical in the bars and end rings; high efficiency motors will often use cast copper to reduce the resistance in the rotor.In operation, the squirrel cage motor may be viewed as a transformer with a rotating secondary. When the rotor is not rotating in sync with the magnetic field, large rotor currents are induced; the large rotor currents magnetize the rotor and interact with the stator's magnetic fields to bring the rotor almost into synchronization with the stator's field. An unloaded squirrel cage motor at rated no-load speed will consume electrical power only to maintain rotor speed against friction and resistance losses. As the mechanical load increases, so will the electrical load - the electrical load is inherently related to the mechanical load. This is similar to a transformer, where the primary's electrical load is related to the secondary's electrical load.

This is why a squirrel cage blower motor may cause household lights to dim upon starting, but does not dim the lights on startup when its fan belt (and therefore mechanical load) is removed. Furthermore, a stalled squirrel cage motor (overloaded or with a jammed shaft) will consume current limited only by circuit resistance as it attempts to start. Unless something else limits the current (or cuts it off completely) overheating and destruction of the winding insulation is the likely outcome.In overall shape, it is a cylinder mounted on a shaft. Internally it contains longitudinal conductive bars (usually made of aluminum or copper) set into grooves and connected at both ends by shorting rings forming a cage-like shape. The name is derived from the similarity between this rings-and-bars winding and a squirrel cage.The solid core of the rotor is built with stacks of electrical steel laminations. Figure 3 shows one of many laminations used. The rotor has a smaller number of slots than the stator and must be a non-integer multiple of stator slots so as to prevent magnetic interlocking of rotor and stator teeth at the starting instant.[1]The field windings in the stator of an induction motor set up a rotating magnetic field through therotor. The relative motion between this field and the rotor induces electric current in the conductive bars. In turn these currents lengthwise in the conductors react with the magnetic field of the motor to produce force acting at a tangent orthogonal to the rotor, resulting in torque to turn the shaft. In effect the rotor is carried around with the magnetic field but at a slightly slower rate of rotation. The difference in speed is called slip and increases with load.The conductors are often skewed slightly along the length of the rotor to reduce noise and smooth out torque fluctuations that might result at some speeds due to interactions with the pole pieces of the stator. The number of bars on the squirrel cage determines to what extent the induced currents are fed back to the stator coils and hence the current through them. The constructions that offer the least feedback employ prime numbers of bars.The iron core serves to carry the magnetic field through the rotor conductors. Because the magnetic field in the rotor is alternating with time, the core uses construction similar to a transformer core to reduce core energy losses. It is made of thin laminations, separated by varnish insulation, to reduce eddy currents circulating in the core. The material is a low carbon but highsilicon iron with several times the resistivity of pure iron, further reducing eddy-current loss, and low coercivity to reducehysteresis loss.The same basic design is used for both single-phase and three-phase motors over a wide range of sizes. Rotors for three-phase will have variations in the depth and shape of bars to suit the design classification. Generally, thick bars have good torque and are efficient at low slip, since they present lower resistance to the EMF. As the slip increases, skin effect starts to reduce the effective depth and increases the resistance, resulting in reduced efficiency but still maintaining torque.Virtually every washing machine, dishwasher, standalone fan, record player, etc. uses some variant of a squirrel cage motor.

Slip ring motorsA wound-rotor motor is a type of induction motor where the rotor windings are connected through slip rings to external resistances. Adjusting the resistance

allows control of the speed/torque characteristic of the motor. Wound-rotor motors can be started with low inrush current, by inserting high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased. [1]Compared to a squirrel-cage rotor, the rotor of the slip ring motor has more winding turns; the induced voltage is then higher, and the current lower, than for a squirrel-cage rotor. During the start-up a typical rotor has 3 poles connected to the slip ring. Each pole is wired in series with a variable power resistor. When the motor reaches full speed the rotor poles are switched to short circuit. During start-up the resistors reduce the field strength in the stator. As a result the inrush current is reduced. Another important advantage over squirrel-cage motors is higher start-up torque.A wound-rotor motor can be used in several forms of adjustable-speed drive. Certain types of variable-speed drives recover slip-frequency power from the rotor circuit and feed it back to the supply, allowing wide speed range with high energy efficiency. Doubly fed electric machines use the slip rings to supply external power to the rotor circuit, allowing wide-range speed control. Today speed control by use of slip ring motor is mostly superseded by induction motors with variable-frequency drives.

An alternate design, called the wound rotor, is used when variable speed is required. In this case, the rotor has the same number of poles as the stator and the windings are made of wire, connected to slip rings on the shaft. Carbon brushes connect the slip rings to an external controller such as a variable resistor that allows changing the motor's slip rate. In certain high-power variable speed wound-rotor drives, the slip-frequency energy is captured, rectified and returned to the power supply through an inverter. With bidirectional controlled power, the wound-rotor becomes an active participant in the energy conversion process with the wound-rotor doubly fed configuration showing twice the power density.Compared to squirrel cage rotors and without considering Brushless wound rotor doubly fed technology, wound rotor motors are expensive and require maintenance of the slip rings and brushes, but they were the standard form for variable speed control before the advent of compact power electronic devices. Transistorized inverters with variable-frequency drive can now be used for speed control, and wound rotor motors are becoming less common.Several methods of starting a polyphase motor are used. Where the large inrush current and high starting torque can be permitted, the motor can be started across the line, by applying full line voltage to the terminals (direct-on-line, DOL). Where it is necessary to limit the starting inrush current (where the motor is large compared with the short-circuit capacity of the supply), reduced voltage starting using either series inductors, an autotransformer, thyristors, or other devices are used. A technique sometimes used is (star-delta, YΔ) starting, where the motor coils are initially connected in star for acceleration of the load, then switched to delta when the load is up to speed. This technique is more common in Europe than in North America. Transistorized drives can directly vary the applied voltage as required by the starting characteristics of the motor and load.This type of motor is becoming more common in traction applications such as locomotives, where it is known as the asynchronous traction motor.

CRANES DUTY SLIP RING MOTORSSlip ring Crane duty motors are specially designed for service on cranes and hoists. They can also be used for similar applications such as material handling equipments, and cranes of all types. These motors can serve as auxiliary motors in rolling mills or wherever intermittent duty drives are required.These are duty type rated motors developing high starting torque with low starting current. The motors are suitable for frequent starts/ stops and reversals. Also rapid acceleration is achieved by high pull out torque / rotor inertia ratio.Crane duty motors are used for following applications in majority: Cranes & hoists: long travel drive, cross travel drive, main hoist, auxiliary hoist. Material handling equipments: various conveyors Lifts Rolling mills.Manufacturing range for cast iron slip ring crane duty motors is from 112 to 400 frame. The same for squirrel cage motors in cast iron enclosure is from 80-400 & in aluminium enclosure is from 63-160. 

Crane duty motors are suitable for 3 phase supply and can be wound for any single voltage from 220 to 650 volts and frequencies from 50 Hz or 60 Hz. The rotor voltage is committed for stator supply of 415 V, 50 HZ.For other supply conditions, Rv to be confirmed from CG.

Slip ring motors are used in cranes as for the following reason : Conventional AC operated electric overhead travelling (EOT) cranes uses slip ring induction motors whose rotor windings are connected to a power resistance. Speed control is performed by changing the rotor resistance in 3 to 4 steps by power contactors. Reversing is performed by changing the phase sequence of the stator supply through line contactors. Braking is achieved by a plugging operation.

A crane control system has been developed using a variable voltage variable frequency drive and a programmable controller which has the advantage of continuous speed control; reversing is achieved by changing the phase

sequence through an inverter. The main advantages of this system are precise positioning, energy saving and increased motor life.

MAXIMUM PERMISSIBLE OPERATING SPEED 

All 4, 6, 8, and 10 pole motors are designed for withstanding an over speed of 2.5 times rated synchronous speed or 2000 rpm whichever is less.

Duty:

Operation of the motor at load including no load and de- energised period to which the motor is subjected, including the sequence and duration. 

Crane duty applications can be classified into duty types S2 to S10. The duty types are as per table 4 below: 

CONSTRUCTURAL FEATURES OF SLIP RING MOTORS

Mechanical Constructional details

Stator Frames and Enfield

Made of high quality cast iron conforming to IS:210, ribbed externally to ensure maximum heat dissipation. All components are machined on CNC machines ensuring concentricity and correct alignment. The windings and working parts are completely enclosed and air is forced over the stator body by fan, mounted on the shaft and protected by a cowl. The feet are integrally cast with the body. This ensures sturdiness and resistance to vibrations.

Stator and Rotor Cores

Both the stator and rotor cores consist of low loss and high permeability steel stampings which are assembled under pressure and rigidly secured by end plates. The cores are properly laminated to ensure low eddy current losses and to avoid the hysteresis losses the core is made up of steel.

Enclosure and Cooling

Standard crane duty motors have IP55 degree of protection as per IS:4691. IP 56 & 66 can be provided on request. The cooling code of motor is IC 411 as per

IS:6362.

Shafts and Bearings

The shaft is of high grade steel and of appropriate diameter to withstand the bending and torsional stresses. All shafts are ultrasonically tested for any minor flaw in the material. Shafts are machined to extreme fine limits to ensure fit and interchange ability of bearings. The motors are provided with single shaft extension. Special shaft extensions like:

1. change in diameter and length,2. taper shaft, with threaded end, and with hexagonal nut and lock washer

double shaft extensions, (cylindrical & taper) are available on request.

Motors are provided with deep groove ball bearings. They are mounted with extreme care in dust proof housing. (Refer bearing size table for details). Sealed bearings are provided for motors upto frame 225M. Grease lubricated bearings are used on frames 250 and above. The correct amount of grease is filled in the bearings during manufacturing. On line greasing facility is provided for 250 frame & above to facilitate greasing of bearings without dismantling the motor. For provision of insulated bearings, please refer to CG. 

Slip Rings and Brush gearing

All slip rings are made of cupro-nickel. The slip ring unit is having high insulation resistance ensuring minimum wear and breakdown. The brush holders are assembled as a complete unit which can be easily replaced. The slip ring enclosure is dustproof & has a cover with accessibility for inspection. The slip rings are epoxy moulded / fabricated type as per requirement. 

Electrical constructional details

Stator Windings

Stator winding (and rotor winding in case of slipring motors) consists of enamelled copper wire impregnated with superior quality class F varnish which is rigid at all working temperatures. Stators in motors with frame 280 & above are manufactured with vacuum pressure impregnation process as a standard. All rotors irrespective of frame size are processed with vacuum pressure impregnation. This gives added electrical & mechanical strength to the winding for high number of starts. The varnish has a high insulation resistance and

excellent resistance to moisture, saline atmosphere, acidic/ alkaline fumes, and also to oil and grease. The insulation gives complete reliability under all atmospheric conditions, including humid tropical climates. Stator and rotor wound packs are subjected to surge test before impregnation.

Rotor Windings

1.Wound rotor :

Rotors of slip ring motors are wound and impregnated similar to stator windings. In addition, rotor windings are braced with resiglass banding. This gives protection against centrifugal forces experienced by overhang during overs peed and frequent reversals. Gelcoat is provided on the winding overhang for better consolidation and protection from vibration. Strip wound rotors are provided on frames 355 and above.

2. Cage rotor :

Rotors are of pressure die cast aluminium up to 355 frame. They are designed for high starting torque, suitable for high number of starts. Cooling fins cast integrally with the rotor cage, improve the cooling action within the motor. All rotors (cage and wound) are dynamically balanced to comply with the requirements of IS: 12075.

Terminal arrangement

The terminals are mounted on a moulded base and are enclosed in a box having an inspection cover. In case of slip ring motors, three terminals for stator and three terminals for rotor are terminated in terminal box with adequate creep age and clearance. The rotor terminals are in the same box as those of the stator.Separate terminal box for rotor terminals is provided for NDW355LX & NDW400LX frames. This simplifies wiring and maintenance. 

FAILURES IN CRANE DUTY SLIP RING MOTORSMechanical Failures

1. Defects In Bearings

Bearing-related problems are among the most common causes of motor failures. At first inspection for bearings and housings is done to check dirt, damage,

dents, contamination, scratches or any other kind of distortion.

2. Stator and Rotor not aligned

Proper alignment of stator and rotor is important for proper functioning of the motor, else air gap may not be uniform and beyond a certain limit it may cause rubbing between the stator and the rotor.

3. Bearing Not aligned

Generally roller bearings have two parts- inner and outer circles. The axis of these two circles should superimpose each other for the proper alignment of the bearing. Misalignment may cause excessive heating and jamming during running.

4. Excess grease used

Bearings are enclosed between inner and outer grease cup. Grease inside this grease cup is used to lubricate the bearings but if excess grease is used then heat dissipation will not be possible and therefore the temperature of the bearing may increase more than 120°C, thus damaging the bearings.

5. Defects in Slip Rings

Slip rings are made up of copper or brass. The carbon brushes in contact with the rotating slip rings may at times damage the surface of the slip rings due to sparking. The erosion of slip rings causes further excessive sparking, damage to the insulation, development of short circuit or ground fault, poor machine balancing.

6. Shaft Bending

Major reasons of shaft bending are load jamming, improper handling,improper load alignment. Sometimes the weight of heavy rotors causes the shaft to deflect if kept stationary for a long period. This causes imbalance condition and occurrence of vibrations. This may also create problem in coupling the load with the motor.

7. Occurrence Of Vibrations

Vibrations in the machine may occur due to many reasons like loose foundation on the ground, loose internal components like stamping, winding/conductor insulation, wedges etc, large air gap between rotor and stator, no proper alignment of stator and rotor, any kind of fault in bearings and no proper

varnishing.

8. Defects in End cover

Due to any kind of mechanical failure the end covers may get damage and this may produce many types of failures related to bearing , stator-rotor alignment in the motor.

9. Problems in coupling

Coupling is used for building the mechanical connection between the load shaft and the motor shaft. The coupling of both the components is connected using nut bolts. Basically each of the couplings is attached to the respective shafts using a key and a key way. If any kind of defect occurs in the coupling like shearing of connecting bolts, wear out of coupling, keyway damage then problems may occur in the alignment of the motor shaft and the load.

Electrical Failures

1. Terminal Lead Failure

The three terminals of the motor may get damaged due to high voltage or current i.e due to heating or if the insulation class for these leads are low. Failure may also occur if proper clamping/tying is not done for the 3 leads. A centrifugal force is developed when the rotor is rotating and thus the unclamped leads may get damaged.

2. Winding Interturn

The winding interturn causes short circuit in the stator winding. 30 -40 % of faults in induction machine is due to interturn faults. Mostly interturn faults is caused by thermal, thermo-mechanical, vibrational and environmental stresses during operation.

3. Earth fault

If any of the part of stator or rotor windings gets connected to the earth due to

failure of insulation, this causes phase to earth fault in the faulty circuit.

4. Winding cut/open Fault5. Slip Ring connection 6. Insulation Fault between Slip Rings7. Slip Ring Earth Fault.