Evaluation and Application of Energy Efficient Motors

8/3/2019 Evaluation and Application of Energy Efficient Motors

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Evaluation and ApplicationofEnergy Effcient Motors

GE Industrial Systems
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Motor Selection Based Only On Purchase Price CanBe A Costly Mistake

The 1992 Federal Energy

Bill (EPAct) is now thelaw of the land and there-fore there are no moreopportunities to achieveadditional savings.This couldnt be further from the truth. Technologyhas continued to advance, and manufacturers havemoved to new levels of premium efficiency products.However premium efficiency motors may not be foreveryone. Their use should be carefully examined toensure that the extra cost can be justified, and thatthe proper manufacturer is selected. Not allpremium efficiency motors have the same overallperformance. Premium efficiency motors can repre-sent an investment of up to 20% over the cost of ordi-nary energy efficient motors. While this premium maybe recovered in a reasonable period of time, the firstobjective should be to maximize the return on thisextra investment. To reach this goal, the user needsto understand motor efficiency, how the design couldadversely affect the total system, and how to carry outan economic evaluation.

Understanding Motor LossesMotor efficiency is simply of the watts output dividedby the watts input (Figure 1). This is better expressedas the watts output minus the losses, divided by thewatts input.

Efficiency = 746 x Hp OutputWatts Input

= Input - LossesInput

Figure 1. Efficiency Equation

The only way to improve efficiency is to reduce motorlosses. The components of motor losses can be broad-ly defined as no-load and load losses. Figure 2 showstypical loss distribution for a 4-pole motor.

No-Load Losses % Total Windage, Friction 14

Core Losses 16

Load Losses Stator I2R Losses 33 Rotor I2R Losses 15 Stray Load Losses 22

Total 100

Figure 2. Distribution of Losses

No-load losses typically account for 30% of the total,and include windage and friction losses plus corelosses. The windage and friction losses are mechani-

cal losses from bearing friction, plus fan and rotorwindage. Core losses are a combination of hysteresisand eddy current losses in the magnetic steel core.

Load losses comprise the remaining 70% of the total,and include stator and rotor I2R losses as well as strayload losses. Stator losses are computed as the productof stator input current (at load) squared and the sta-tor resistance at operating temperature. Rotor lossesresult from rotor currents, and are equal to the prod-uct of the induced rotor current squared and therotor resistance at operating temperature.

Stray load losses arise from additional harmonic and

circulating current losses in the magnetic steel andwindings. These losses are a result of design and man-ufacturing processes. Some of the factors which con-tribute to stray load losses are shown in Figure 3.

Number of Slots Stator and Rotor Slot Geometry Rotor Slot Insulation Air Gap Length Manufacturing Process Control

Figure 3. Stray Load Loss Factors
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Improving Efficiency Takes Know-howThe design engineer of energy efficient motors strivesfor optimization using techniques shown in Figure 4.

Selection of lamination and rotor steel is a key ele-ment of design. Low grade steel typically has losses inthe 3.0 watts/lb. range and costs approximately thesame as cold rolled steel. To reduce hysteresis andeddy current losses, manufacturers may choose toconstruct energy efficient motors with high gradesilicon steel. This steel has an electrical loss of 1.5watts/lb. and costs approximately 50% more thanstandard motor lamination steel.

Improved Steel Properties Thinner Laminations Increased Wire Volume Improved Slot Designs More Steel Improved Rotor Insulation System More Efficient Fan Design

Figure 4. Efficiency Improvement

To further reduce eddy current losses, high gradesilicon steel can be purchased in a thinner gauge.Typical lamination thickness is .018 inches for siliconsteel as opposed to .022 inches for others. Siliconsteel also has a surface coating of insulation to pro-

vide high inter-lamination resistance to eddy currents.By increasing the volume of copper wire, stator I2Rlosses can be reduced. To accommodate this increase,slot areas must also be increased by as much as 50%.To compensate for the increase in slot size and corre-sponding decrease in remaining active steel, themotors rotor and stator core are increased in size.

The challenge is to have the expertise to design amotor that achieves the desired efficiency withoutincreasing locked rotor current beyond the require-ments of NEMA Design B. If a manufacturer is unwill-ing to add the offsetting steel, efficiency gains are

simply traded for higher locked rotor amps. Thisapproach is acceptable for the motor, but the higherlocked rotor amps may be excessive for the existingcircuit breakers, incoming power lines or even thelocal distribution transformer. The user should evalu-ate the total system impact, since lower initial motorcost may be more than offset by needed additionalinvestments to upgrade other components.

Rotor I2R losses are improved through redesign of therotor slots to increase the conductor cross section. In

doing so, the rotor full-load speed is increased slight-ly. Again, the slot redesign must be made in such away as to continue to provide NEMA Design Btorques and locked rotor currents. This requires care-ful selection of the slot shape as well as its size.

Some losses in the motor come from unplanned con-duction paths resulting from normal manufacturingprocesses. One such path is along the rotor surfacewhere the rotor OD is turned down to provide a uni-form air gap. This is where manufacturing expertise iskey. Careful choice and control of the process arerequired to keep losses at a minimum.

Because of the lower electromagnetic losses in anenergy efficient motor, this type does not require thesame amount of cooling as a standard motor design.The designer can now optimize fan design to reducewindage losses and achieve quieter operation.

In summary, by optimizing the motor design and con-trolling the manufacturing process, losses aredecreased and efficiency improvements are gained,while allowing cooler running machine. As marketconditions dictate and materials and technologyimprove, further efficiency gains may be achieved.

Making The Standards Work For YouEfficiency terminology can be very confusing. NEMAStandard MG1-1998, Part 12 establishes the defini-tion, testing and labeling requirements for motor effi-ciency. This standard states that efficiency shall bedetermined at rated output, voltage and frequencyand shall be tested by IEEE Std 112 Method B using adynamometer.

Due to variations of materials, manufacturing process-es and tests, there will be variations in tested motorefficiency from one motor to another. As a result, thefull load efficiency for a large population of motors isnot a unique value, but is distributed over a range ofefficiency. The range of efficiency appears as a nor-mal distribution commonly called a Bell Curve.NEMA MG1, Part 12 establishes a series of nominalefficiencies and an associated minimum efficiency foreach nominal efficiency value.


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NEMA MG1, Part 12 and the efficiency tables pub-lished by NEMA for energy efficient and premiumefficiency motors identify the levels that must beequaled or exceeded for the motor to be classified asenergy efficient. These energy efficiency tables par-allel those found in the 1992 Federal Energy Act(EPAct) and the Canadian standard CSA 390-93. Asignificant point to understand is that the minimum

efficiency standard allows 20% greater losses than itsassociated nominal efficiency value (see Figure 5).

Nominal or Average expected

Minimum or MaximumGuaranteed Expected

20% LossDifference

Figure 5. Efficiency Terminology

In June of 2001, NEMA formalized a specificationdefining premium efficiency. NEMA PremiumEfficiencies were developed in cooperation with themembers of the National Electric ManufacturersAssociation (NEMA), major US conservation groupsand the Department of Energy. The specification fol-lows the definition for qualification of EPAct, but cov-ers a larger range of product than the 1200 horse-power covered by EPAct. The NEMA Premium stan-dard defines the nominal efficiency of motors rated1500 Hp and includes ratings at 4000 volts. (Seehttp://nema.org/publication/ei/oct00/premiummotor.htm)

With a majority of the NEMA Premium ratings requir-ing a 20% decrease in nominal motor loss comparedwith the EPAct values, NEMA Premium is importantto the user for three reasons. First, the nominal effi-ciency values are higher, offering the user greaterenergy savings. Second, some manufacturers offerreal premium motors with a guaranteed minimumefficiency with no more than 10% additional losses.This results in superior repeatability of efficiency val-ues and relates to higher efficiencies and greater sav-ings. Third, manufacturers offering motors with nomore than 10% additional losses are more likely toadhere to tighter manufacturing processes. This pro-vides higher product reliability and reduced potential

for user downtime cost exposure.

Nominal or Average expected

Minimum or MaximumGuaranteed Expected10% Loss

Difference

Figure 6. Efficiency Terminology

To maximize return on investment, users shouldalways specify motors which have guaranteed mini-mum efficiencies. In most cases these motors willhave the minimum efficiency value listed on thenameplate. For easy comparison of EPAct vs. NEMAPremium, refer to GE publication e-GEK-M1003.

In making a financial evaluation of motor alterna-

tives, the user should consider only guaranteed mini-mum efficiencies. This will maximize true return oninvestment. While this approach is conservative, itensures that the calculated savings are actually real-ized. Other critical reasons for evaluating energy effi-cient motors using guaranteed minimum efficienciesare as follows:

Nominal efficiency represents a band of efficiencies.The replacement motor could have an actual effi-ciency that differs from the nominal value. As aresult, the calculated savings may not be realized.

Demanding guaranteed minimum efficiency pro-

vides the user with a basis for rejection if the motorreceived fails to meet the guaranteed level. Thisforces the manufacturer to demonstrate how effi-ciency is verified, and ensures that the user receivesthe product performance paid for.

Finally, this allows the user to evaluate and purchasein confidence that the savings calculated will actual-ly be realized.

With this basic understanding of motor efficiency,lets look toward how it should be evaluated.

The Premium Efficiency Motor DecisionSpecification and installation of Premium efficiencymotors can yield attractive economic results com-pared to the use of standard energy efficient motorsfor the same installation.

To fully understand these benefits, the buyer shouldmake either a simple payback calculation or a com-prehensive economic evaluation including a life cyclecost analysis. Typically, as the quantity of motorsincreases and the value of the installation grows, amore detailed analysis is performed.


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How To Calculate Annual SavingsIn comparing the efficiencies of two motors, thebuyer must consider the type of motor involved, theannual hours of operation, motor load, electricalcosts, and the motor efficiencies. The same basic fac-tors apply whether the comparison is between a pre-EPAct motor repair and an EPAct motor, or betweenan EPAct and a NEMA Premium Efficiency motor.

Regardless of the comparison, it is essential that theefficiency values be on the same basis. You must com-pare nominal vs. nominal or guaranteed minimum vs.guaranteed minimum. As previously discussed, theguaranteed value will provide the most realistic analysis.

With that in mind, the equation in Figure 7 can beused to determine annual savings for two 50 horse-power, 1800 rpm, totally enclosed fan-cooled motorsoperating at rated load. The nominal efficiency valuefor the standard EPAct motor is 93 while the compa-rable value for the NEMA Premium efficiency motoris 94.5. If operated continuously (8,760 hours peryear) with an electrical cost of $.07/kWh, the annual

savings would be $390.

s = .746x Hp x L x C x N (100) (100)(EB) (EA)

s = .746x 50 x 100 x .07 x 8760 (100) (100)100 (93.0) (94.5)

s = $390 per year

s Annual savingsL Percent load divided by 100C Cost of electricity ($/kWh)

N Annual hours of operationEA Premium efficient motor efficiencyEB Standard motor efficiency

Figure 7. Annual Savings

Evaluating your electrical savings

based on a contract charge per kilowatt

can be misleading. In all likelihood,

the contract number will not include

demand charges, fuel adjustment

charges, possible connect charges or

taxes. Your real power cost is the

actual payment to the power company

divided by the total kilowatts used.

Calculating Simple PaybackUsing the equation and example in Figure 8, thebuyer can divide the price premium of the NEMAPremium motor by the annual savings to obtain sim-ple payback. Assuming typical motor prices of $1,524for the 50 Hp EPAct motor described above and$1,833 for the comparable NEMA Premium motor,the simple payback is .79 years.

Simple Payback Period = Price PremiumAnnual Savings

= $282/$390= .79 years

Figure 8. Payback Period

By substituting different power costs and annualhours of operation, the equations in Figures 7 and 8can be used to calculate payback under variousassumptions. As shown in Figure 9, the relationshipbetween power cost and annual hours of operation issignificant. Buyers should understand this relation-

ship to determine if premium efficiency motors are anattractive economic investment for a given application.

Simple Payback Period (Years)50 Hp, 1800 RPM, TEFC Premium Efficiency

Annual Hours Power Cost ($/kWh)of Operation .04 .06 .082,080 (1 shift) 5.3 3.6 2.74,160 (2 shifts) 2.7 1.8 1.38,760 (continuous) 1.3 0.8 0.6

Figure 9. Effect of Power Cost on Payback

Where power costs are low, premium efficiencymotors may still be attractive if the motor runs contin-uously. Conversely, where power costs are high, pre-mium efficiency motors may be justified even wherethey operate for only one shift. Each buyer shouldevaluate his applications and let the economicsdecide. We cannot overstate the importance of usingyour real power cost in any evaluation.


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Making A Life Cycle AnalysisAnnual savings and simple payback calculations willestablish whether or not the buyer should considerpremium efficiency motors. If the facts are favorable,a life cycle analysis should be completed to determinethe true economic benefits of specifying premiumefficiency motors.

By introducing the number of years of operation orthe period of evaluation into the annual savings calcu-lation, the user can define the savings over the life ofthe motors. This is easily done by developing an eval-uation factor (EF) as shown in Figure 10. Thismethod takes into consideration the cost of electrici-ty, annual hours of operation and the number ofyears over which the user evaluates the project orinstallation.

EF = C x N x n

EF Evaluation FactorC Average power cost ($/kWh)N Annual hours of operationn Number of years of operation

or period of evaluation

Figure 10. Evaluation Factor

The evaluation factor is then substituted in the annu-al savings calculation as shown in Figure 11 to deter-mine life cycle savings achieved by applying premiumefficiency motors.

LCS = .746 x Hp x L x EF (100) (100)(EB)) (EA)

LCS Life Cycle Savings

EF Evaluation FactorEA Premium efficient motor efficiencyEB Standard motor efficiency

Figure 11. Life Cycle Savings

Using the previous motor example and assuming aseven year period of evaluation, the evaluation factorwill be $4,292/kW as shown in Figure 12.

EF = .07 x 8760 x 7= $4,292/kW

Figure 12. Evaluation Factor

By substituting the evaluation factor in the earlierannual savings calculation, the life cycle savings of theexample will be $3,904 as shown in Figure 13.

LCS = .746 x 50 x 100 x 6132 (100) (100)100 (93) (94.5)

= $3,904

Figure 13. Life Cycle Savings

Depending on the individual user and the applicationunder consideration, evaluation factors can rangefrom $800/kW to $10,000/kW. In approaching theevaluation of premium efficiency motors, each usershould determine individual evaluation factors tofully understand the economic benefits of premiumefficiency motors in their operation.

It should be noted that the life cycle savings are notannual savings, but a pretax estimate of savings basedon the years of anticipated operation. This evaluationcan be expanded to include the time value of moneyand the expected escalation of power costs.

Choosing The Most Efficient MotorsOnce the buyer has decided that premium efficiencymotors are a sound investment, the right suppliermust be chosen to maximize the return on invest-ment. There are many motor suppliers who manufac-ture premium efficiency motors, but their efficiencyclaims may differ. It is important to make your ownsupplier selection and savings calculation. This is also

an excellent time to factor in guaranteed minimumefficiencies in order to be certain of realizing all thecalculated savings.

Using the previously calculated evaluation factor of$4,292/kW and comparing a motor to existing stan-dard efficiency levels shows the importance of evaluat-ing even a tenth of a point of efficiency difference.This calculation is done using known standard or pre-EPAct, EPAct and NEMA Premium efficiencies.

Figure 14 shows the difference in life cycle operatingsavings resulting from different nominal efficiencies.

Pre-EPAct EPAct NEMA Premium91% 93% 94.5%$5,615 LCS $2,732 LCS

Figure 14. Life Cycle Comparison of Known Standards at 50 Hp

Not all premium efficiency motors are the same, anddetailed comparisons should be made to assure thegreatest possible life cycle savings for premium effi-ciency motors. This comparison should go beyondthe efficiencies to include other design factors thatmay affect the users system.

This summary is even more significant when looking

at pre-EPAct motors. Although no longer legally avail-able from manufacturers, there are literally millionsof pre-EPAct motors running in industry today. Theywill ultimately fail and be evaluated for replacementor repair. If the life cycle factor is high enough, theuser may elect to retrofit prior to a forced downtime.

However the simplest evaluation remains Premiumvs EPAct motors.


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Based on the difference in the efficiencies of the pre-mium and the EPAct motors, the EPAct motor wouldcost an additional $2,732 to operate over the motorslifetime. For equal total cost of ownership, you wouldhave to reduce the initial price of the EPAct motor by$2,732. In fact, in this case the EPAct motor wouldcost more to own even if the motor were free!

This clearly demonstrates the importance of an effi-ciency evaluation, and that not all premium motorsare the same. When deciding if premium efficiencymotors are the right economic decision, it is impor-tant to keep the following points in mind:

Evaluate the financial benefits of premium efficiencymotors based on guaranteed minimum efficiencies.

Prepare a life cycle savings comparison to clearlyunderstand the total cost of ownership.

The total cost of ownership equals the purchaseprice plus the life cycle efficiency evaluation.

Analyze the other trade-off in manufacturers design

that could effect system needs for incoming power.

Dont Let First Cost Drive The DecisionIn addition to lower operating costs, premium effi-ciency motors offer additional value in terms oflonger life, application versatility and improved per-formance. Along with lower losses, premium effi-ciency motors have much lower temperature risethan standard motors. Consequently winding lifecan be up to four times longer, and lubricant lifetwice as long compared with a standard motor. Asshown in Figure 16, premium efficiency motors canprovide the user with application versatility in manyof the following severe operating conditions.

High altitude and high ambient conditions

Impaired ventilation

Frequent starting

Non-standard waveforms associated with variablefrequency drives

High load inertia

Greater stall capacity

Premium efficiency motors also operate morequietly and have lower no-load losses.

Figure 16. Temperature Effect on Lubricant and Insulation Life

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1

.5120100806040

Operating temperature (C)

Relativelifemultiplier

Energy $aver

Motor

StandardMotor

Bearing Lubricant Life

Motor Insulation Life

Energy $aver EPAct

All premium efficiency motors are not created equal.In designing for the optimum efficiency there aretrade-offs that can have dramatic effects on othermotor characteristics. For a given stator diameter, theengineer can adjust stator slot geometry, wire size,rotor slot configuration and rotor material. If opti-mum efficiency were the only goal, then relaxingother performance limits such as allowing higher

than NEMA B locked rotor amps, or possibly modi-fied torque characteristics may make possible ahigh efficiency design at low cost. However thesedesign trade-offs can have a dramatic impact on theapplication when this motor is used to upgrade anexisting system. The existing starter may be too small,circuit breakers may need to be replaced, and if thereis a significant change-out to improve total efficiency,power leads and even the distribution transformermay need consideration.

Not all motor manufacturers are members of NEMAand so may approach their designs differently. Lookcarefully at the motors being offered. Check, for

example, the calculated locked rotor amps to makesure they meet the NEMA design letter on the name-plate. Evaluate the power factor. A lower power factormay be worth sacrificing to gain higher efficiency.Motor efficiency can only be optimized by initialdesign, while power factor can be improved with aone time investment in the system. Efficiency savingsare then realized for the life of the motor.

We at GE are unique, with over 100 years of motordesign and application-related experience. We havethe expertise to make sure all of the additional bene-fits of Premium Efficiency motors add up to greatermotor reliability and the best overall return on your

investment. This experience extends beyond motorsalone, offering opportunities for further total systemproductivity by combining Energy $aver motors withthe option of a complete line of GE Drives.

All of these additional benefits clearly add up togreater motor reliability for the user and a betterreturn on your investment.


8/8He-GEA-M1019 (03/02) Fort Wayne, Indiana 46802

GE Industrial Systems

We bring good things to life.

For additional information on evaluating the replacement of failed motors with energy efficient motors, orretrofitting existing motors with energy efficient motors, contact your local GE Industrial Systems distributor

and ask for the following brochure:Impact of Rewinding on Motor Efficiency, GET-8014.For more information, log on to www.geindustrial.com

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Evaluation and Application of Energy Efficient Motors