eeeaitsrajampet.comeeeaitsrajampet.com/files/a012mUNIT - 3.docx · Web viewUNIT – III. ENERGY EFFICIENT MOTORS. 3.1. INTRODUCTION. In a commercial building, lighting is predominant

Energy Auditing & Demand Side Management Energy Efficient Motors

UNIT – III3. ENERGY EFFICIENT MOTORS

3.1. INTRODUCTION

In a commercial building, lighting is predominant user of as it is on for a major portion of a

day. However, electric motor driving pumps and air-handling fans may also operate for a larger part

of the time or even continuously. Therefore in industry, power used by electric motors and other

processes generally will be the larger user of electric power, although lighting is still a major

consumer.

“Energy-Efficient motors cost more than motor first cost (Standard Motor)”. On a life cycle

basis, electric motor efficiency can be far more important than motor first cost.

During the period from 1960 to 1975, electric motors, particularly those in the 1 to 250-hp

range, were designed for minimum first cost. The amount of active material, i.e., lamination steel,

copper or aluminum or magnet wire, and rotor aluminum, was selected as the minimum levels

required meeting the performance requirements of the motor. Efficiency was maintained at levels

high enough to meet the temperature rise requirements of the particular motor. As a consequence,

depending on the type of enclosure and ventilation system, a wide range in efficiencies exists for

standard NEMA (National Electrical Manufacturers Association) design B poly-phase motors.

The minimum efficiency is the lowest level of efficiency to be expected when a motor is

marked with the nominal efficiency in accordance with the NEMA standard. This method of

identifying the motor efficiency takes into account variations in materials, manufacturing processes,

and test results in motor-to-motor efficiency variations for a given motor design.

The nominal efficiency represents a value that should be used to compute the energy

consumption of a motor or group of motors. For example, a standard 10-hp electric motor may have

an efficiency range of 81–88%.

At 81% efficiency,

Input to electric motor = 10*7460.81

=9210 W

Motor losses = 9210-7460 = 1750 W

At 88% efficiency,

Input to electric motor = 10*7460.88

=8477 W

Motor losses = 8477-7460 = 1077 W

Therefore, for the same output the input can range from 8477 W to 9210 W, or an increase in

energy consumption and power costs of 8%, to operate the less efficient motor.

P.SURESH BABU, AITS, RAJAMPET 57


3.1.1. WHY MORE EFFICIENT MOTORS?

The escalation in the cost of electric power that began in 1972 made it increasingly expensive

to use inefficient electric motors. From 1972 through 1979, electric power rates increased at an

average annual rate of 11.5% per year. From 1979 to the present, the electric power rates have

continued to increase at an average annual rate of 6% per year. The annual electric power cost to

operate a 10-hp motor 4000 hr/yr increased from $850 in 1972 to $1950 in 1980 and to over $2500

by 1989. By 1974, electric motor manufacturers were looking for methods to improve three-phase

induction motor efficiencies to values above those for standard NEMA design B motors.

Unfortunately, there is no single definition of an energy effective motor. Similarly, there are

no efficiency standards for stand NEMA design B poly-phase induction motors. Energy-efficient

motors (EEM) are the ones in which, design improvements are incorporated specifically to increase

operating efficiency over motors of standard design (see Figure 3.1). Design improvements focus on

reducing intrinsic motor improvements focus on reducing intrinsic motor losses. Improvements

include the use of lower loss silicon steel, a longer core (to increase active material), thicker wires (to

reduce resistance), thinner laminations, smaller air gap between stator and rotor, copper instead of

aluminum bars in the rotor, superior bearings and a smaller cooling fan, etc.,

1 21 41 61 81 101 121 141 161 18170

80

90

100

Motor Rating (KW)

Effi

cien

cy (%

)

High Efficiency Motors

Standard Motors

STANDARD (vs) HIGH EFFICIENCY MOTORS (Typical 3- Induction Motor)

High Efficiency Motors

Standard Motors

Fig. 3.1

Energy-efficient motors operate with efficiencies that are typically 4 to 6% higher than the

standard motors. In keeping with the stipulations of the BIS, energy-efficient motors are designed to

operate without loss in efficiency at loads between 75% and 100% of rated capacity. This may result



in major benefits in varying load applications. The power factor is about the same or may be higher

than for standard motors. Furthermore, energy-efficient motors have lower operating temperatures

and noise levels, greater ability to accelerated higher-inertia loads, and are less affected by supply

voltage fluctuations.

Measures adopted for energy efficiency address each loss specifically as under:

3.2. FACTORS AFFECTING EFFICIENCY AND LOSS DISTRIBUTION

3.2.1. Stator and Rotor I2R Losses

These losses are major losses and typically account for 55% to 60% of the total losses. I2R

losses are heating losses resulting from current passing through stator and rotor conductors. I2R

losses are the function of conductor resistance and the square of current. Resistance of conductor is a

function of conductor material, length and cross sectional area. The suitable selection of copper

conductor size will reduce the resistance.

i.e., R= ρ*lA

Reducing the motor current is most readily accomplished by decreasing the magnetizing

component of current. This involves lowering the operating flux density and possible shortening of

air gap.

Stator Current= I1 = input watts√3*Voltage* cos

When improving the motor performance, it is important to recognize the interdependent

relationship of the efficiency and the power factor. Rewrite the preceding equation and solve for the

power factor:

Power Factor= cos = output HP*746√3*voltage* I1 *efficiency

Therefore, if the efficiency is increased, the power factor will tend to decrease. For the power

factor to remain constant, the stator current I1 must decrease in proportion to the increase in

efficiency. To increase the power factor, the stator current must be decreased more than the

efficiency is increased. From a design standpoint, this is difficult to accomplish and still maintain

other performance requirements such as breakdown torque. However,

Input watts= output HP*746efficiency

(or)

I1= output HP*746√3*voltage* cos *efficiency



The rotor power loss is generally expressed as the slip loss:

Rotor loss= (HP output*746+FW ) S1 - S

S = Ns- Nr

Ns= Slip

where,

Nr = rotor speed, rpm

Ns = synchronous speed, rpm

FW = friction and windage loss

The rotor slip can be reduced by increasing the amount of conductor material in the rotor or

increasing the total flux across the air gap into the rotor. The extent of these changes is limited by the

minimum starting (or locked-rotor) torque required, the maximum locked-rotor current, and the

minimum power factor required. Rotor I2R losses are a function of the rotor conductors (usually

aluminum) and the rotor slip. Utilization of copper conductors will reduce the winding resistance.

Motor operation closer to synchronous speed will also reduce rotor I2R losses.

3.2.2. Core Losses

Core losses are those found in the stator-rotor magnetic steel and are due to hysteresis effect

and eddy current effect during 50Hz magnetization of the core material. These losses are

independent of load and account for 20-25% of the total losses.

The hysteresis losses which are a function of flux density, are reduced by utilizing low-loss

grade of silicon steel laminations. The reduction of flux density is achieved by suitable increase in

the core length of stator and rotor. Eddy current losses are generated by circulating current within the

core steel laminations. These are reduced by using thinner laminations.

However, because of variables in the processing of the lamination steel into finished motor

cores, the reduction in core loss in watts per pound equivalent to the Epstein data on flat strips of the

lamination steel is seldom achieved. Magnetic core loss reductions on the order of 15–40% can be

achieved by the use of thinner-gauge silicon-grade electrical steels. A disadvantage of the higher-

silicon lamination steel is that, at high inductions, the permeability may be lower, thus increasing the

magnetizing current required. This will tend to decrease the motor power factor.

3.2.3. Friction and Windage Losses

Friction and windage losses result from bearing friction, windage and circulating air through

the motor and account for 8-12% of total losses. These losses are independent of load. The reduction

in heat generated by stator and rotor losses permits the use of smaller fan. The windage losses also

reduce with the diameter of fan leading to reduction in windage losses.



The friction losses in the bearings are a function of bearing size, speed, type of bearing, load,

and lubrication used. This loss is relatively fixed for a given design and, since it is a small percentage

of the total motor losses, design changes to reduce this loss do not significantly affect the motor

efficiency. Most of the windage losses are associated with the ventilation fans and the amount of

ventilation required to remove the heat generated by other losses in the motor, such as the winding

power losses I2R, magnetic core loss, and stray load loss. As the heat-producing losses are reduced, it

is possible to reduce the ventilation required to remove those losses, and thus the windage loss can be

reduced. This applies primarily to totally enclosed fan-cooled motors with external ventilation fans.

One of the important by-products of decreasing the windage loss is a lower noise level created by the

motor.

3.1.4 Stray Load-Losses

These losses vary according to square of the load current and are caused by leakage flux

induced by load currents in the laminations and account for 4-5% of total losses. These losses are

reduced by careful selection of slot numbers, tooth/slot geometry and air gap.

Stray load losses are residual losses in the motor that are difficult to determine by direct

measurement or calculation. These losses are load related and are generally assumed to vary as the

square of the output torque. The nature of this loss is very complex. It is a function of many of the

elements of the design and the processing of the motor. Some of the elements that influence this loss

are the stator winding design, the ratio of air gap length to rotor slot openings, the ratio of the

number of rotor slots to stator slots, the air gap flux density, the condition of the stator air gap

surface, the condition of the rotor air gap surface, and the bonding or welding of the rotor conductor

bars to rotor lamination. By careful design, some of the elements that contribute to the stray loss can

be minimized. Those stray losses that relate to processing, such as surface conditions, can be

minimized by careful manufacturing process control. Because of the large number of variables that

contribute to the stray loss, it is the most difficult loss in the motor to control.

3.3. SUMMARY OF LOSS DISTRIBUTION

Energy Efficient Motors

Energy efficient motors cover a wide range of ratings and the full load efficiencies are higher

by 3-7%. The mounting dimensions are also maintained as per IS1231 to enable easy replacement.

As a result of the modifications to improve performance, the costs of energy-efficient motors are

higher than those of standard motors by about 30%. The higher cost will often be paid back rapidly

in saved operating costs, particularly in new applications or end-of-life motor replacements. In cases

where existing motors have not reached the end of their useful life, the economics will be less

positive.



Because the favorable economics of energy-efficient motors are based on savings in

operating costs, there may be certain cases which are economically ill-suited to energy-efficient

motors. These include highly intermittent duty or special torque applications such as hoists and

cranes, traction drives, punch presses, machine tools, and centrifuges.

In addition, energy efficient designs of multi-speed motors are generally not available.

Further, energy-efficient motors are not yet available for many special applications, e.g. for flame-

proof operation in oil-field or fire pumps or for very low speed applications (below 750 rpm). Also,

most energy-efficient motors produced today are designed only for continuous duty cycle operation.

Given the tendency of over-sizing on the one hand and ground realities like: Voltage, Frequency

variations, efficacy of rewinding in case of a burnout, on the other hand, benefits of EEMs can be

achieved only by careful selection, implementation, operation and maintenance efforts of energy

managers. Summary of energy efficiency improvements in EEMs is given in the following Table.

Within a limited range, the various motor losses discussed are independent of each other.

However, in trying to make major improvements in efficiency, one finds that the various losses are

very dependent. The final motor design is a balance among several losses to obtain a high efficiency

and still meet other performance criteria, including locked-rotor torque, locked-rotor amperes,

breakdown torque, and the power factor.

Table: Energy Efficient Motors

Sr. No. Power Loss Area Efficiency Improvement

1. Stator I2R

Use of more copper and larger conductors increases cross

sectional area of stator windings. This lowers resistance (R) of

the windings and reduces losses due to current flow (I)

2. Rotor I2R

Use of larger rotor conductor bars increases size of cross section,

lowering conductor resistance (R) and losses due to current flow

(I)

3. Iron

Use of thinner gauge, lower loss core steel reduces eddy current

losses. Longer core adds more steel to the design, which reduces

losses due to lower operating flux densities.

4. Friction & Windage Use of low loss fan design reduces losses due to air movement.

5. Stray Load LossUse of optimized design and strict quality control procedures

minimizes stray load losses.



3.4. CONSTRUCTINAL DETAILS

The efficiency of energy efficient motors is higher due to the following constructional features:

1. By increasing the amount of copper in the motor ( 60%) which reduces the resistance

(ohmic) loss in the winding & temperature rise. Performance improves because of increased

thermal mass.

2. Use of more & thinner laminations of high quality motor steel reduces core losses in the

stator and rotor.

3. Narrowing of air gap between stator and rotor increases the intensity of magnetic flux,

thereby improving the motor ability to deliver the same torque at reduced power. Increasing

the length of the stator and rotor increases the net flux linkages in the air gap to the same

effect.

4. More complex rotor bar designs enable good starting torque with efficient full speed

operation.

5. Improved overall design reduces windage losses and stray load losses.

Applications

Energy efficient motors hold their efficiency better at part loads enhancing their advantage

over standard motors. Economic benefits of installing energy efficient motors can be recognized in

three situations:

In a new application (plant expansion)

In lieu of rewinding of failed motors

Proactive replacement for in-service standard motors

Energy efficient motors are more cost effective than standard motors in the above cases. Efficiency

of EEMs is 4-6% higher compared to the efficiency of standard motors

Energy efficient motors run cooler, and therefore have potentially longer life than their standard

efficiency counterparts.

3.4.1. Three Phase (3-) Induction Motors

In the integral horsepower sizes, i.e., above 1 hp, three-phase induction motors of various types drive

more industrial equipment than any other means. The most common three-phase (polyphase)

induction motors fall within the following major types:

NEMA (National Electrical Manufacturers Association) design

B: Normal torques, normal slip, normal locked amperes

NEMA design A: High torques, low slip, high locked amperes

NEMA design C: High torques, normal slip, normal locked amperes

NEMA design D: High locked-rotor torque, high slip



Wound-rotor: Characteristics depend on external resistance

Multispeed: Characteristics depend on design-variable torque, constant torque, constant

horsepower

There are many specially designed electric motors with unique characteristics to meet specific needs.

However, the majority of needs can be met with the preceding motors.

3.4.1.1. NEMA Design B Motors

The NEMA design B motor is the basic integral horsepower motor. It is a three-phase motor

designed with normal torque and normal starting current and generally has a slip at the rated load of

less than 4%. Thus, the motor speed in revolutions per minute is 96% or more of the synchronous

speed for the motor. For example, a four-pole motor operating on a 60-Hz line frequency has a

synchronous speed of 1800 rpm or a full-load speed of

1800 – (1800 x Slip) = 1800 – (1800 x 0.04)

= 1800 – 72

= 1728 rpm

(or)

1800 x 0.96 = 1728 rpm

In general, most three-phase motors in the 1- to 200-hp range have a slip at the rated load of

approximately 3% or, in the case of four-pole motors, a full-load speed of 1745 rpm. Figure 3.2

shows the typical construction for a totally enclosed, fan-cooled NEMA design B motor with a die-

cast aluminum single-cage rotor.

FIGURE3.2 NEMA design B totally enclosed, fan-cooled poly-phase induction motor.

Figure 3.3 shows the typical speed-torque curve for the NEMA design B motor. This type of

motor has moderate starting torque, a pull-up torque exceeding the full-load torque, and a breakdown

torque (or maximum torque) several times the full-load torque. Thus, it can provide starting and

smooth acceleration for most loads and, in addition, can sustain temporary peak loads without

stalling.



FIGURE 3.3 NEMA design B motor speed-torque curve.

3.4.1.2. NEMA Design A Motors

The NEMA design A motor is a poly-phase, squirrel-cage induction motor designed with

torques and locked-rotor current that exceed the corresponding values for NEMA design B motors.

The criterion for classification as a design A motor is that the value of the locked-rotor current be in

excess of the value for NEMA design B motors. The NEMA design A motor is usually applied to

special applications that cannot be served by NEMA design B motors, and most often these

applications require motors with higher than normal breakdown torques to meet the requirements of

high transient or short-duration loads. The NEMA design A motor is also applied to loads requiring

extremely low slip, on the order of 1% or less.

3.4.1.3. NEMA Design C Motors

The NEMA design C motors is a squirrel-cage induction motor that develops high locked-rotor

torques for hard-to-start applications. Figure 3.4 shows the construction of a drip-proof NEMA

design C motor with a double-cage, die-cast aluminum rotor. Figure 3.5 shows the typical speed

torque curve for the NEMA design C motor. These motors have a slip at the rated load of less than

5%.



FIGURE 3.4 NEMA design C drip-proof polyphase induction motor.

FIGURE 3.5 NEMA design C motor speed-torque curve.

3.4.1.4. NEMA Design D Motors

The NEMA design D motor combines high locked-rotor torque with high full-load slip. Two

standard designs are generally offered, one with full-load slip of 5–8 % and the other with full-load

slip of 8–13%. The locked-rotor torque for both types is generally 275–300% of full-load torque;

however, for special applications, the locked-rotor torque can be higher. Figure 3.6 shows the typical

speed-torque curves for NEMA design D motors. These motors are recommended for cyclical loads

such as those found in punch presses, which have stored energy systems in the form of flywheels to

average the motor load and are excellent for loads of short duration with frequent starts and stops.



FIGURE 3.6 NEMA design D motor speed-torque curves: 5–8% and 8–13% slip.

The proper application of this type of motor requires detailed information about the system

inertia, duty cycle, and operating load as well as the motor characteristics. With this information, the

motors are selected and applied on the basis of their thermal capacity.

3.4.1.5. Wound-Rotor Induction Motors

FIGURE 3.7 Wound-rotor motor speed-torque curves: 1, rotor short-circuited; 2–4, increasing values of external resistance.



The wound-rotor induction motor is an induction motor in which the secondary (or rotating)

winding is an insulated polyphase winding similar to the stator winding. The rotor winding generally

terminates at collector rings on the rotor, and stationary brushes are in contact with each collector

ring to provide access to the rotor circuit. A number of systems are available to control the secondary

resistance of the motor and hence the motor’s characteristics. The use and application of wound-rotor

induction motors have been limited mostly to hoist and crane applications and special speed-control

applications. Typical wound-rotor motor speed-torque curves for various values of resistance

inserted in the rotor circuit are shown in Fig. 3.7.

As the value of resistance is increased, the characteristic of the speed-torque curve progresses

from curve 1 with no external resistance to curve 4 with high external resistance. With appropriate

control equipment, the characteristics of the motor can be changed by changing this value of external

rotor resistance. Solid-state inverter systems have been developed that, when connected in the rotor

circuit instead of resistors, return the slip loss of the motor to the power line. This system

substantially improves the efficiency of the wound-rotor motor used in variable-speed applications.

3.4.1.5. Multispeed Motors

Motors that operate at more than one speed, with characteristics similar to those of the NEMA-type

single-speed motors, are also available. The multispeed induction motors usually have one or two

primary windings. In one-winding motors, the ratio of the two speeds must be 2 to 1; for example,

possible speed combinations are 3600/1800, 1800/900, and 1200/600 rpm.

In two-winding motors, the ratio of the speeds can be any combination within certain design

limits, depending on the number of winding slots in the stator. The most popular combinations are

1800/1200, 1800/900, and 1800/600 rpm. In addition, two-winding motors can be wound to provide

two speeds on each winding; this makes it possible for the motor to operate at four speeds, for

example, 3600/1800 rpm on one winding and 1200/600 rpm on the other winding. Multispeed

motors are available with the following torque characteristics.

Variable Torque:

The variable-torque multispeed motor has a torque output that varies directly with the speed,

and hence the horsepower output varies with the square of the speed. This motor is commonly used

with fans, blowers, and centrifugal pumps to control the output of the driven device.

Constant Torque:

The constant-torque multispeed motor has a torque output that is the same at all speeds, and

hence the horsepower output varies directly with the speed. This motor can be used with friction-type

loads such as those found on conveyors to control the conveyor speed.



Constant Horsepower:

The constant-horsepower multispeed motor has the same horsepower output at all speeds.

This type of motor is used for machine tool applications that require higher torques at lower speeds.

Note:The construction details of Energy Efficient Motors (EEM)i.e., efficient or high efficiency 3-

Induction motors are shown above. Similar for other type of machines also but the design changes

have been made to the normal standard motors.

3.5. CHARACTERISTICS – VARIABLE SPEED, VARIABLE DUTY CYCLE DIAGRAMS

The single most potent source of energy savings in induction motor system lies not in the

motor, but rather in the controls that govern its operation. Adjustable speed, intelligent controls and

other ways of modifying or controlling motor behavior hold great promise for improving

performance and efficiency in drive systems.

In addition, the basis of rating specifies the type of duty:

Continuous duty

Intermittent duty

Varying duty

It is desirable to use standard motors for as many different applications as possible. Consequently,

general-purpose continuous rated motors should be used when

1. The peak momentary overloads do not exceed 75% of the breakdown torque

2. The root-mean-square (rms) value of the motor losses over an extended period

of time does not exceed the losses at the service factor rating

3. The duration of any overload does not raise the momentary peak temperature

above a value safe for the motor’s insulation system

3.5.1. Need for using Controls

Induction motors are well suited to single speed, constant output applications. However, there are

large numbers of motor/load/system combinations where single speed operation does not efficiently

meet the proves requirements, usually due to two common factors:

Oversized motor: Motors are often oversized for their loads causing not only reduced

efficiency, but also reduced power factor, and in many cases increased energy consumption

in the load because of reduced slip.

Varying load: Many applications require modulated output from a motor/load/system. These

systems are sized to provide the maximum output under the worst operating conditions, but

rarely require this much flow (output). The excess energy is wasted, usually be some form of

throttling.



Controlling motor speed to correspond to load requirements provides many benefits,

including increased energy efficiency and improved power factor. Adjustable speed capability can

significantly improve productivity of many manufacturing processes by reducing scrap, enabling

quality manufacturing during transition times and allowing more control over start up and shut down.

Following are the benefits of VSDs (Variable Speed Drives):

Matching motor and load to the output.

Improved process precision

Improved power factor

Improved tool life.

Increased production & flexibility

Faster response

Extend operating range

Electrical isolation

Driving multiple motors

Throttled load saving (throttling is the most energy inefficient operation)

Cube-law load savings (P α N3)

In many applications, the load imposed on the driving motor varies from no load to a peak

load. When the motor load fluctuates, the temperature rise of the motor fluctuates. When there is a

definite repeated load cycle, the motor size selection can be based on the rms value of motor losses

for the load cycle. However, normally, the losses at each increment of the load cycle are not

available to the user. Therefore, a good approximation for the motor size selection can be based on

the rms horsepower for the load cycle. The rms horsepower is then defined as that equivalent steady-

state horsepower that would result in the same temperature rise as that of the defined load cycle.

When making the rms calculation, it is assumed that, when the motor is running, the heat dissipation

is 100% effective.

However, when the motor is at standstill, the heat dissipation is severely reduced and is

limited to dissipation by radiation and natural convection. This can be compensated for by using an

effective cooling time at standstill of one-fourth of the total standstill time.

An important word of caution:

This method of selecting electric motors is not satisfactory for applications requiring

frequent starting or plug reversing or systems with a high load inertia.

3.6. RMS HORSEPOWER LOADING

There are a great many applications especially in hydraulics and hydraulically-driven

machines that have greatly fluctuating load requirements. In some cases, the peak loads last for

relatively short periods during the normal cycle of the machine. At first glance, it might seem that a



motor would have to be sized to handle the worst part of the load cycle. For example, if a cycle

included a period of time where 18 HP is required, then the natural approach would be to utilize a 20

HP motor. A more practical approach to these types of “duty cycle loads” takes advantage of an

electric motor’s ability to handle substantial overload conditions as long as the period of overload is

relatively short compared to the total time involved in the cycle.

The method of calculating whether or not the motor will be suitable for a particular cycling

application is called the RMS (Root Mean Squared) horsepower loading method. The calculations

required to properly size a motor for this type of application are relatively simple.

The RMS calculations take into account the fact that heat buildup within the motor is very

much greater at a 50% overload than it is under normal operating conditions. Thus, the weighted

average horsepower is what is significant. RMS calculations determine the weighted average

horsepower.

In addition to reducing the size and cost of a motor for a particular application, RMS horse

power loading increases the overall efficiency and then oversized motor is so working the motor

result in improved overall. The duty cycle has to be

Step Horsepower

Duration (Seconds)

1 3 3

2 7.5 10

3 2.5 12

4 12.5 3

Repeats ContinuouslyIn order to obtain the RMS horse power loading of the above data, the formula can be written

as,

RMS HP= √HP12* t1 + HP2

2 * t2 + HP32 * t3 + HP4

2 * t4 + …………+ HPn2 * tn

t 1 + t2 + t 3 + t 4 + …………+ tn

The easiest way to approach this type of calculation is to make several columns as shown below and

fill in the details underneath.

Step

Horsepower HP2

Duration

(seconds)

HP2xTim

e

1 3 9.0 3 27.0

2 7.5 56.3 10 563.0

3 2.5 6.3 12 75.64 12.5 156 3 468.8



.3Total 28 1134.4

RMS HP= √1134.428

= √40.5 = 6.4

At first glance, it appears that a 7.5 HP motor would be adequate to handle the loading

required by this duty cycle. One further check has to be made and that is to determine if the motor

has adequate pullout torque (breakdown torque) to handle the worst portion of the duty cycle (12.5

hp load for 3 seconds) without stalling. In this case, one would have to refer to the manufacturer’s

data for the motor and determine the percent of pullout torque that is available.

3.7. VOLTAGE VARIATION AND VOLTAGE UNBALANCE

NEMA Standard MG1 recognizes the effect of voltage and frequency variation on electric

motor performance. The standard recommends that the voltage deviation from the motor rated

voltage not exceed ±10% at the rated frequency. A certain degree of confusion may exist in regard to

the rated motor voltage since the rated motor voltage and the system voltage are different. The rated

motor voltage has been selected to match the utilization voltage available at the motor terminals.

This voltage allows for the voltage drop in the power distribution system and for voltage variation as

the system load changes.

The basis of the NEMA standard rated motor voltages for three- phase, 60-Hz induction motors is as

follows:

For single-phase, 60-Hz induction motors, the basis for standard rated motor voltages is as follows:

Poly-phase induction motors are designed to operate most effectively at their nameplate rated

voltage. Most motors will operate satisfactorily over ±10% voltage variation, but deviations from the

nominal motor design voltage can have marked effects on the motor performance. Following

indicates the type of changes in performance to expect with variation in the motor terminal voltage.

The following shows the effect on the efficiency and power factor in standard NEMA design

B motors and also in energy-efficient motors. It is important to note that the efficiency and the power

factor of energy-efficient motors are not as sensitive to voltage variations as standard motors.

In recent years, the trend in some areas is to decrease system voltage to reduce the system load. In



some cases, this reduction has been as low as 85% of the nominal voltage. For most electric motor

loads, this increases rather than decreases the electric motor input and increases the full-load

temperature rise. Also, the locked-rotor torque is severely reduced such that hard-to-start loads may

not start at the 85% voltage level. Following Figures. illustrate the effect of reduced voltage on

selected horsepower ratings of both standard motors and energy-efficient motors

Voltage unbalance degrades the performance and shortens the life of a three-phase motor.

Voltage unbalance at the motor stator terminals causes phase current unbalance far out of proportion

to the voltage unbalance. Unbalanced current lead to torque overheating, which results in a shorter

winding insulation life.

Voltage unbalance is defined by the National Electrical Manufacturers Association (NEMA)

as 100 times the absolute value of the maximum deviation of the line voltage from the average

voltage on a three-phase system, divided by the average voltage. For example, if the measured line

voltage are 462,463, and 455 volts, the average is 460 volts. In this case, the voltage unbalance is :

(460 - 455)460

×100 = 1.1%

It is recommended that the voltage unbalances at the motor terminals not exceed 1%. Common

causes of voltage unbalance include:

Faulty operation of power factor correction equipment.

Unbalanced or unstable utility supply.

Unbalanced transformer bank supplying a three-phase load that is too large for the

bank.

Unevenly distributed single-phase loads on the same power system

Unidentified single-phase to ground faults

An open circuit on the distribution system primary.



The efficiency of a rewound, 1800-RPM, 100-hp motor is given as a function of voltage unbalance

and motor load in the following table.

The general trend of efficiency reduction with increased voltage unbalance is observed for all motors

at all load conditions.

Table: Motor Efficiency under Conditions of Voltage Unbalance

Motor Load % of FullMotor Efficiency, %Voltage Unbalance

Nominal 1% 2%100 94.4 94.4 9375 95.2 95.1 93.950 96.1 95.5 94.1

Voltage unbalance is probably the leading power quality problem that results in motor

overheating and premature motor failure. If unbalanced voltages are detected, a thorough

investigation should be undertaken to determine the cause. Energy and cost savings occur when

corrective actions are taken.

Example

Assume that the motor tested as shown in the above table was fully loaded and operated for 8000

hours per year, with an unbalanced voltage of 2.5%. with energy priced at $0.05/kwh, the annual

energy and cost savings, after corrective actions are taken, are:

Annual Energy Savings = 100 hp x 0.746 kW/hp x 8000 hrs/yr x (100/93 – 100/94.4)

= 9517 kWh

Annual cost Savings = 9517 kWh x $0.05/kWh = $476

Overall savings may be much larger because an unbalanced supply voltage may power numerous

motors.

3.8. OVER MOTORING

“Rating of motor is higher than the required rating of the motor is called over motoring”.

In many instances, the practice has been to over motor an application, i.e., to select a higher-

horsepower motor than necessary. The disadvantages of this practice are,

Lower efficiency

Lower power factor

Higher motor cost

Higher controller cost

Higher installation costs



Consider the comparisons of the 40-hp motor that could have been selected based on the peak load

versus the 30-hp motor that can be selected on the basis of the duty cycle:

1. Motor cost: list price of standard open 1800-rpm drip-proof motor:

30 hp = $1160

40 hp = $1446

2. Control Cost: NEMA-1 general-purpose motor, 240-V starter:

30 hp, size 3 = $600

40 hp, size 4 = $1350

This results in a cost difference of $1036, or 59%.

Figure 3.8 shows the difference in the input watts and Fig. 3.9 the difference in the input kilovolt-

amperes for 30- and 40-hp motors operating at the same output. At loads above 36 hp, the input is

FIGURE 3.8 Power savings in watts for a 30-hp motor versus a 40-hp motor at the same load.

more favorable for the 40-hp motor. However, at loads below 36hp, the kilowatt and kilovolt-ampere

inputs are lower with the 30-hp motor.

In general, the larger the difference between the actual load and the motor rating, the higher

the input requirements for the same load.



FIGURE 3.9 Savings in kilovolt-amperes for a 30-hp motor versus a 40-hp motor at the same load.

3.8. MOTOR ENERGY AUDIT

Five Basic Concepts of Energy conservation in Drive Power are as follows:

Drive power is huge – Think big ,

Motors are part of a system – Think systems,

Optimize the application & process – Deliver service,

The further the downstream savings, the higher is the upstream benefits – Start down

stream,

Pursue integrated packages of savings opportunities rather than isolated measures

because many savings are Inter – Dependent – Integrate measures.

3.8.1. Energy Conservation in Electric Motors (Load Equipment)

For improving the efficient use of energy it is important to know the typical load on the

motor over its duty cycle. It would be misleading to measure only the current drawn under load and

draw conclusion. For example, a typical 25 HP motor draws about 60% of its rated full load current

when delivering only 45% of its rated output. Hence it is required to measure input power, current,

voltage, power factor, frequency and speed of operation.

Collection of nameplate details of motors and load equipment.

Measurement of voltage, current, power, apparent power, power factor, frequency and

annual operating hours for major loads.



Calculation of load factor for major loads.

Checking for light loads on large motors

Check if valves are always used for flow control in pumps, fans and blowers.

Check if flow from pumps, fans and blowers are changing continuously.

Check if the set discharge pressure is at the lowest permissible limit of operation in the

compressor.

Check for proper maintenance of major equipment, I. e. cleaning, measuring temperature,

dust, vibration, noise, lubrication and coupled condition.

3.8.2. Power Factor Correction at Motor end

As noted earlier, induction motors are characterized by power factors less than unity, leading

to lower overall efficiency (and higher overall operating cost) associated with a plant’s electrical

system, capacitors connected in parallel (shunted) with the motor are typically used to improve the

power factor. The impacts of PF correction include reduced KVA demand (and hence reduced

energy) , reduced voltage drop in the cables (leading to improved voltage regulation) , and an

increase in the overall efficiency of the plant electrical system.

It should be noted that PF capacitor improves power factor from the point of installation back

to the generating side. It means that, if a PF capacitor is installed at the starter terminals of the motor,

it won’t improve the operating PF of the motor, but the PF from starter terminals to the power

generating side will improve, i.e., the benefits of PF would be only on upstream side.

The size of capacitor required for a particular motor depends upon the no-load reactive KVA (KVAr)

drawn by the motor, which can be determined to not exceed 90 % of the no-load KVAr of the motor.

Higher capacities could result in over-voltages and motor burn-outs. Alternatively, typical power

factors of standard motors can provide the basis for conservative estimates of capacitor ratings to use

for different size motors. The capacitor rating for power factor correction by direct connection to

induction motors is shown in Table.



Capacitor Ratings for Power Factor Correction by Direct Connection to Induction Motors

Motor Rating (HP)

Capacitor rating (kVAr) for Motor Speed

3000 1500 1000 750 600 500

5 2 2 2 3 3 3

7.5 2 2 3 3 4 4

10 3 3 4 5 5 6

15 3 4 5 7 7 7

20 5 6 7 8 9 10

25 6 7 8 9 9 12

30 7 8 9 10 10 15

40 9 10 12 15 16 20

50 10 12 15 18 20 22

60 12 14 15 20 22 25

75 15 16 20 22 25 30

100 20 22 25 26 32 35

125 25 26 30 32 35 40

150 30 32 35 40 45 50

200 40 45 45 50 55 60

250 45 50 50 60 65 70

From the above, it may be noted that required capacitive KVAr increases with decrease in

speed of the motor, as the magnetizing current requirement of a low speed motor is more compared

to the high speed motor for the same HP. Since a reduction in line current and associated energy

efficiency gains are reflected backwards from the point of application of the capacitor, the maximum

improvement in overall system efficiency is achieved when the capacitor is connected across the

motor terminals, as compared to somewhere further upstream in the plant’s electrical system.

However, economies of scale associated with the cost of capacitors and the associated labor cost will

place an economic limit on the lowest desirable capacitor size.


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eeeaitsrajampet.comeeeaitsrajampet.com/files/a012mUNIT - 3.docx · Web viewUNIT – III. ENERGY EFFICIENT MOTORS. 3.1. INTRODUCTION. In a commercial building, lighting is predominant