Energy Efficient Electric Motors

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E N E R G Y M A N A G E M E N T

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Energy Efficient Electric Motors

BY

OSCAR

BRANDWK

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opportunity to

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lcctric m o t o r s a r e a m o n g t h c m o s t e n e r g y efficicnt deviccs m a n has

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ever created. Today, more efficicnt electric motors are available than ever before. Simple eco- nomics will justify investing in energy efficient mo- tors for most commercial and industrial applica- tions. Energy costs are on thc rise and conservation practices - such as the use of energy efficient motors -will help control futurc cnergy costs.

Conservation practices can hclp slow down elec- tric load growth and offset the need to add gcnera- tion capacity. Conservation can help improve pro- ductivity by using resources more efficiently and will also help to keep electric costs low. This is an agc of increasing costs of electricity due mainly to higher demands for a limited rcsource and increasingly higher capital costs of new power plants. These and other factors have cncouraged many utilities to de- velop conservation programs and incrrasc energy efficiency awareiicss among their customers. Ke- quired generationcapacity will be greatcr than avail- ablccapacityin theU.S. bytheYear2000,according to some projections, without conservation and &c-

tric power production. planning that is nccessary today in the utility industry.

Electric motors consumc 65 pcrcent of all thc kilowatt-hours produced by electric power plants i n thc U S . each year. Motors have become more spe- cializcd, hencc there are many types availablc today that servc a variety of functions. The focus of this article is on the thrrc-phasr squirrel cage induction motor, by far the most common type of motor in commercial and industrial facilities. l h e plant engi- neer and facilities manager can control their selec- tion of this type of motor and have a considerablc impact o n energy savings, cost and efficiency.

The induction motor is often referrrd to as the

“workhorsc of industry” becauseof its relativcly low cost, reliable service, minimum maintenance require- ments, ability to suit many applications and easy availability from supplicrs. In commercial and in- dustrial facilities, motors can account for 50 to 90 pcrcent of the total electrical usage i n driving pumps, compressors, material handling, process equipment, HVAC systems, refrigeration and fans.

Small improvements in motor cfficiency can re-

Average Motor Efficiencies

Standard 8 Energy Efficient Pb T

Efficiency Grip:

Motors account for as much as 90 percent o f t h e total electrical usage in commercial and industrial

applications. Even small

improvements in motor efficiency result in

substantial energy savings.

N V V E M B E R ~ D E C E M ~ E R 1992

92 91

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85 .-*

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5 7.5 IO IS 20 25 30 40 50 60 75 100 125 150 200 Motor Horsepower

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^{Energy Elfidenl}

SOUR[€ NORIHERN SlAlES WWER CO

suit in substantial encrgy savings. A facility motor inventory will prove to he surprising. The number of motors installcd in a typi- cal facility present an K X -

celleiit opportunity to i n - prove energy efficicncy. An inventory that identifies usage, sizc and loading is a good place to begin cco- nomic evaluations of ret- refit possibilities and cven downsizing wherc appro- priatc.

DESIGNS FOR EFFICIENCY

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Motor cfficiency is im- pacted by four main areas within thc electrical and mechanical design of the motor: IzR Iosscs, stray load loss, corc loss, and friction and wiiidagc IOSSKS.

AWE FACILITIES

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Thc main causes and methods for reduc- ing thcse IOSSKS are outlined here.

I’R Iosscs, occurring in thestator wind- ingsandrotor bars,varywithloadingand can rangc up to 15 percent in three-phasc motors. Thcsc losses arc caused by resis- tance to current flow i n thc stator and rotor conductors and increase with mo- tor load and tcmperaturc. I’R IOSSCS can be reduced by using larger diameter con- ductors in the motor windings, and by using more aluminum or copper i n the rotor to reduce resistance. Improvements in rotor configuration and slot dcsign can also reduce I’R losses.

Stray load losses, like lzR losses, iti- crrasc with loadcurrent butonlymakeup approximately 2 pcrcent of motor losses.

Stray load losses result from leakage flux, increasing with load currcnt.These losses can be reduced and controlkd by design optimization ofpriniarytosecondary (sta- tor to rotor) air gaps. This optimization will rcquire less magnctic flux for motor operation, reducing electric current.

Core Iosscs, ranging from 1 pcrcent to F percmt, are magnetic losses in the rotor and stator core causcd by encrgy loss through eddy currentsand hysteresis. Core losscan bc reduced by usingmorelamina- tions of rhinncr, highcr grade electrical steel i n the rotor and stator core.

The final major category of motor losses, friction and windagc, arc mechani- cal in naturc and arc caused by motor bearing friction and aerodynamic impacts of the motor cooling fan. Friction and windagc usually account for less than 1 pcrcent of total motor losses. Though lossesarc low in thiscategory, thereisstill room for iinprovemcnt byemployinghigh quality bearings and bearing matcrials, prccise shaft alignnicnt, and rcccnt im- provcincnts in cooling fan design.

Electric motor efficiency standards have been established by NEMA (Na- tional Electrical Manufacturcrs Associa- tion), an industry trade organization of manufacturers and motor repair person- nel. Thc standard, NEMA M G - 1 2 . 5 5 , Table 12-6c, indicatcs the nominal and minimumefficicncy levels required, bascd on IEEE Standard 112, Test Method R.

For a motor to he designated ‘‘energy efficient,” it should meet the NEMAstan- dard which has helped manufacturcrs, suppliers and purchasers by crcaring a common benchmark. It will be periodi- cally updated tn reflcct improvements in design, materials and manufacturing methods of the motor industry.

THE ENERGY EFFICIENT MOTOR PURCHASE Many times, first cost takes prccedence NoveMBEH/DrcEMsEn 1 9 9 2

Typical Motor Losses

50 hp, 3-Phose, AC Induction Motor

1% Losses 0%

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^6%

k\

hiltion a

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^{0 Percent} Motor Load ^{100 Winduge} 1.1%

Controlling losses:

Each of the fnny main types of electrical and mechanical motor losses can be reduced with the introduction of new designs and materials.

OVCI operating costs. The lifetime opcrat-

ing costs of an electric mntor can be SO timcs more than its initial purchase price.

For various reasons, operating costs arc often ignorcd during the purchasing deci- sion. Studies havc found that companics which d o not huy energy efficient morors may cite a varicry of reasons, such as disbelief o f savings potcntial, poor avail- ability, complacency, and stable energy Using the “Motor Comparison and Payback Analysis Rcport,” developed by thc Washington State Encrgy Office, it is cvident that timely payhacks arc avail- able o n iiivcstmeiits in energy efficicnt motors. When ccrtain operating charac- tcristics arc prcsent, thesc payback pcri- ods bccome evcn more attractive. If a motor has characteristics that include, but arc not hmitcd to, new installation, rrtrofit of a failed motoi-, extended oper- ating hours, run timc exceeding idlc rimc, consrant or high loads, largc horsepowcr, and high kilowatt-hour 01- demand charges, thcsc applications are good can- didates for an encrgy efficient motor.

By using energy cfficicnt motors in- stead of standard motors, it is possible to improve opcrating efficicncy by 2 to 10 percent, yielding cnergy savings that should get the attention of any facilities manager. In the two examples given on page F7 (“Average Motor Efficiencics”), approximately 5 percent savings resulted from using the energy cfficient motor design instead of the standard design.

This savings represented more than a

$10,000 lifetime opcrating cost savings for the SO hp motor and a n 11-month payback on the energy efficient motor

CUStS.

investment. A 1.5-year motor life was assumed.

Formulas, as picturcd on page 60, enableoperating cost comparisons o f t w o motors with different cfficiencics. Studics have indicated that most motors are underloadcd, averaging about 60 perccnt of rated load. The loading can be esti- mated for the example o r a simple strobe light test may be done to determine ap- proximate motor load. Annual savings can be calculated using Formula 11. A handy ruleofthumb inestimatiiigannual savings is thatanenergycfficientmotor in place of a standard motor will save 10 to 1.5 times thc horsepower rating of the motor in annual dollars. In the earlicr SO hp cxamplc, this workcd out to he a n annual savings of $655, o r approximately 13 times the horsepower.

An cncrgy efficient motor will provide higher annual savings if it is propcrly sized to match the load it will see. Proper sizing will hclp to optimize efficicncy and powcr factor.

Manufacrurers design motors for maxi- mum efficiency from 75 percent to 100 pcrceiit loading. In many applications, motors are operated below this optimal range. Motor operation below SO percent loading considerably reduces motor effi- cicncy. Powcr factor also decrcases, like efficiency, as thc load of the motor de- creases. Scvcral underloaded small mo- tors or one underloaded large motor may cause the powcr factor of an entire facility to drop below 90 percent. When power factor is below 90 percent, many electric utilities add a powcr factor charge into the monthly hilling component, increas- ing electric energy costs. To avoid this,

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MOTOR COMPARISON AND PAYBACK ANALYSIS REPORT

New Motor Purchase Analysis

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HORSEPOWER ENCLOSURE SPEED ANNUAL USE LOAD EFFICIENCY PRICE ENERGY COSTS DEMAND CHARGES 11 MOTOR REBATE

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RESULTS:

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MOTOR A MOTOR B

25.0 T E A 1800 rpm 8760 Hours

25.0 I E K 1800 rpm 8760 Hours

75% 75%

90.2% 94.1%

$861 $1190

S 0.0400/kWH

$ 5.65/kW Per Month S 30

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Annual Energy Use: 135843 kWH 130213 kWH 5630 kWH Annual Energy Cost: $5434 $5209 $225

Annual $6216 $269

Annual Demand Charges: $1051 $1008

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Power Costs: 56485 HE Motor Premium: $299 Simple Payback: 1.1 Years

MOTOR COMPARISON AND PAYBACK ANALYSIS REPORT

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HORSEPOWER ENCLOSURE SPEED ANNUAL USE LOAD EFFICIENCY PRICE ENERGY COSTS DEMAND CHARGES EE MOTOR REBATE

New Motor Purchase

Analysis

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MOTOR A -~

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50.0

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^{T E K}

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1800 rpm

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8760 Hours

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^75%

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^90.2%

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^$1750

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S0.0400hWH

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^{$5.65/kW Per Month}

i: $30

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MOTOR B .~

50.0 T E A

1800 rpm 8760 Hours 75%

95.0%

$2380

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SAVINGS Annual Energy Use: 271686 kWH 257954 kWH 13727 kWH Annual Demand Charges: $2103 $1997 $106 Annual Power Costs: $12970 $12315 $655 HE Motor Premium: $600

Simple Payback: 0.9 Years

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RESULTS: A B

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Annual Energy Cost: $10867 $10318 $549

SOFIWARE DEVELOPED BY: WASHINGTON STATE ENERGY OFFICE MOTOR EFFICIENU PROGRAM

The Buying Derision:

Motor analyses demonstrate the timely paybacks of energy efficient designs. In these examples, the energy efficient motor design resulted in a 5 percent savings over the staizdard design, representing more than a $10,000 liferime oyeratinx cost savings for the SO hp mutoy.

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AII’E FACILITIES

facilities like to keep their power factor over 90 pcrcmt and as close to 100 pcr- cent as possihle. This can he done with capacitors, and other ways, but many timescan be improved with proper motor sizing.

Design engineers will uftcn slightly oversize motors t o accommodate future production growth, intermediate over- loads and load fluctuations, to insure against motor failure in critical applica- tions and to increase motor life due to lower operating temperatures on the con- ductorinsulation. Goodengineeringprac- ticcs will usually krep the operating load in the optimal range of 75 percent to 100 percent loaded.

CALCULATING MOTOR LOAD USING A STROBE LIGHT

Motor load can be closely approximated using a strohe light to determine opcrat- ing rpm. The test is easily accomplished without interruption of power and re- quires no electrical connections. The test is safe and can be done by anyone, regard- lessoftechnical background. Onlya hand- held adjustablestrobelightanda calcula- tor are required to conduct a motor load test. Hand-held adjustable strobes are available,powered by llOVAC, forabout

$200. The strobe light should have a

digital readout to indicate motor operat- ingrpm.Todcrcrminc motor loading, the calculation below may be used. Motor load will be a percent of full load.

Measured rpm will he obtained with the strobe. Synchronous rpm is the speed of the stator magnetic field, and i n most U.S. motors is 900, 1200, 1800 or 3600 rpm. Nameplate rpm is the synchronous rpm less the “slip.” Nameplate rpm will always be slightly lower than synchro- iious rpm brcause the rotor of an induc- tion motor does not rotate at synchro- nous speed, but slightly lags. Therefore, if namcplate rpm is known, it is casy to determine synchronous rpm - i.e., 1765 rpm nameplate spced would he 1800 rpm synchronous.Nameplate rpm is thespeed of the rotor at full load.

To measure the operating rpm of the motor, the strobe is pointed at the rotat- ing shaft; by adjusting thc strobe, the rotating shaft of the motor can be “fro- zen” to make it appear as if it is no longer rotating. When this is done, the digital readout of the strobe will indicate thc

N O V E M B E l l / ~ ~ C E l v l B E l l 1992

operatingrpmofthe motor. This number should be recorded and entered into the calculation to determine motor load. For example: if a motor with a nameplate horsepowerof50anda nameplaterpmof 1,765 was tested and found to have a strobemeasuredrpmof 1,775, thismotor would have a calculated load of 71 per- cent of full load, o r 35 hp.

This type of motor hading test yields fairly accurate results, quickly, and does not require electrical connections o r in- terruption of power to the motor.

In conclusion, energy efficient motors have many advantages, with operating cost reduction as the most significant.

Opportunity for improvement exists. The nationwide saturation of energy efficient motors continues to increase but cur- rently only accounts for approximately 5 percent of installed motors. This satura- tion level is extremely low when com- pared to other conservation measures in commercial facilities such as lighting, which has a 21 percent saturation. En- ergy efficient motors now represent a bout 20 percent ofthree-phase motor sales. As users become familiar with the advan- tages of more efficient motors and iden- tify savings opportunities within their facilities, sales will increase.

To have a major impact on the nation- wide saturation of energy efficient mo- tors, users would havc to retrofit stan- dard working motors with an energy effi- cient equivalent. Because retrofitting an operating motor is not a priority for most facilities managers, many electric utilities are offering incentives to their customers who retrofit standard motors with energy efficient motors or who purchase energy efficient motors for replacement and new motor applications. Some utilities have been working with motor suppliers in their area to promote energy efficient motors, familiarize users with energy sav- ings and help make energy efficient mo- tors more widely available. Incentives from utilities can help reduce the price differential between standard and energy efficient motors.

Energy conscious plant engineers and facilities managers should consider energy efficient motors for all new motor installa-

PAYBACK WORKSHEET - ENERGY-EFFICIENT MOTORS

Formula I

Motor horsepower (hpl x % load x .746 kW/hp = k W x Operating hrs = kWH/yr.

1. Nomeplote - horsepower

2. Motor lood -estimated or token from design doto

3.

Horsepower to kW conversion

4. Motor kW. This may be rolculoted from Steps I , 2 and 3,or it moy be measured directly ot the motor or loud renter

5. Operoting hours per yeor - either estimoted or from operoting schedule 6. kWH per yeor used

by

the motor

Formula II

]

^{x S/kWH} = S/yr. Savings

[

^Standard :fficiency

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High Efficiency kWH x

SB #7 IB#8 IB#lO

1. Retiprocol of stondord efficiency or existing motor efficiency 8. Reciprocol of high effiriency motor to he substituted 9. Electric rote in S/kWH

10. Slyeor sovingr

Payback = $Cost + SSnvingr

Simple paybackis obtnined

by

dividingtheestimated cost by theestimateddollar sovingr/year.

1 I. Estimoted onnuol sovings with energy efficient motor

12. Ertimoted cost or cost difference between stondord motor and energy efficient motor 13. Simple poybock

SOUKI: NORTHERN STATES POWER (0,1992 tions, in equipment specifications, for re-

placement applications, as part of an en- ergy management program, and when stocking spare motors. A simple operating

Operating Cost Comparisons:

Energy usage and savings calculations are helpful in determining whether a motor application will stay within the company’s established payback and inuestment guidelines.

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NOVEMBERIDECEMBER 1992

Efficiency and Power Factor Vs. Motor load

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100 ^~

0 25 50 75 100 125 150

Load (Oh)

Achieving o Balonce:

Proper motor sizing helps to optimize efficiency and power faclor. Most electric mutors arc designed for maximum efficient)' herimen 7.5 p r c e n t and 100 percenl

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We Put Ourselves

in Your Shoes.

In today’s marketplace, thinking like you - the client- is cruciul Understanding the deadlines you face is m’ticd

Meeting your needs is

essential.

At OBrien ^& Gere Engineers, we do more than think like you.

We put ourselves in your shoes.

In fact, it’s a perfect

fit.

OBrien & Gere Engineers

-

working with you each step of the way.

For mnre information on the range of facilities engineering services we offer, call Deborah Hennessy, regional marketing representative, at (315) 437-6100.

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... and offices in major U.S. cities.

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AWE F A C I L I . I I E S

cost analysis on each motor purchasc will rnahlc rquipmcnt operating costs to bc closely trackcd and will identify efficicncy improvcmrnt opportunities.

REFERENCES

1 ) EI’RI, “ElectricMotors: MarkrtTrends and Applications,” EPRITR-100423, ResourceDynamicsCorp:VA, Junc 1992.

2 ) Northern States Powrr Co.-Wiscon- sin, Consumcr Affairs, Commercial and Industrial Refercnce Library.

3 ) Ontario Hydro, Motors Reference Guide, 2nd Ed., February 1990.

4) Washington Statc Energy Officc, “Elec tric Motor Database Rcport,” Ver. 1.

5 ) Northern States Power Co.-Wiscon- sin, Smart Business News, Vol. 111, No. 2, May J 991.

6 ) Nadel, S., Energy Efficient Motor Sys- tems: Handbook on Technology, Pro- rrams and Policy Opportunities, Amcri- can Council for an Energy Efficient Economy, 1991.

7 ) Encyclopaedia Britannica, Vol. 11, lSth Ed., 1990.

8 ) U.S. Ilepartment o f Enrrgy, Office of Enrrgy Markets and End USC, “Charac- teristics of Chmmercial Buildings 1986,”

DOFL3A-0246(86), Washington, DC.

Oscar Rrandserisan engineer with North- ern States Power Co. in Eau Claire, Wis- consin where he assists large industrial customers with energy management, cou- seruation and service requirements. Pre- viously, he worked as plarrt engiizeer, plant maintenance engineer, manufactur- ing engineer and produci application en- gineer for ]ohn Deere Haruester Works and RTE Cory. He holds a bachelor’s in plant engineering, industrial technology from the University uf Wisconsin-Stout.

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For more information on this article, drcle Render Service No. 133.