Energy Efficient Electric Motors, O BRANDSER

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Energy Efficient Electric Motors
Improvements in t..h...r..e...e...-.p....h...a...s...e........... i.n...d...u...c...t.i..o...n....m....o...t..o...r..s. present a strong o...p....p...o...r..t..u...n...i..t.y....t..o...... reduce plant operating costs. Ifficiency Gap: Motors account for as much as 90 percent of the total electrical usage in commercial and industrial applications. Even small improvements in motor efficiency result in substantial energy savings.
E lectric motors are among the most energy efficient devices man has ever created. Today, more efficient electric
tric power production planning that is necessary today in the utility industry. Electric motors consume 65 percent of all the
motors are available than ever before. Simple eco- kilowatt-hours produced by electric power plants in
nomics will justify investing in energy efficient mo- the U.S. each year. Motors have become more spe-
tors for most commercial and industrial applica- cialized, hence there are many types available today
tions. Energy costs are on the rise and conservation that serve a variety of functions. The focus of this
practices -such as the use of energy efficient motors article is on the three-phase squirrel cage induction
-will help control future energy costs.
motor, by far the most common type of motor in
conservation practices can help slow down elec- commercial and industrial facilities. The plant engi-
tric load growth and offset the need to add genera- neer and facilities manager can control their selec-
tion capacity. Conservation can help improve pro- tion of this type of motor and have a considerable
ductivity by using resources more efficiently and will impact on energy savings, cost and efficiency.
also help to keep electric costs low. This is an age of The Induction Motor is often referred to as the
increasing costs of electricity due mainly to higher "workhorseof industry" because of its relatively low
demands for a limited resource and increasingly cost, reliable service, minimum maintenance require-
higher capital costs of new power plants. These and ments, ability to suit many applications and easy
other factors have encouraged many utilities to de- availability from suppliers. In commercial and in-
velop conservation programs and increase energy dustrial facilities, motors can account for 50 to 90
efficiency awareness among their customers. Re- percent ofthe total electrical usage in driving pumps,
quired generation capacity will be greater than avail- compressors, material handling, process equipment,
able capacity in the U.S. by the Year 2000, according HVAC systems, refrigeration and fans.
to some projections, without conservation and elec- Small improvements in motor efficiency can re-
sult in substantial energy
savings. A facility motor
Average Motor Efficiencies Standard & Energy Efficient
inventory will prove to be surprising. The number of motors installed in a typi-
cal facility present an ex-
95 -
cellent opportunity to im-
94 -
prove energyefficiency.An
93 -
inventory that identifies
usage, size and loading is a
good place to begin eco-
nomic evaluations of ret-
rofit possibilities and even
downsizing where appro-
I I I I II1 I l l l l l
5 7.5 10 15 20 25 30 40 50 60 75 100 125 150 200
- Motor Horsepower =---- Standard
Energy Efficient
DESIGNS FOR EFFICIENCY Motor efficiency is impacted by four main areas within the electrical and mechanical design of the motor: IZR losses, stray load loss, core loss, and frictionand windage losses.
N O V E M B E R / D E C E M1B9 9E2R
The `main causes and methods for reduc-
ing these losses are outlined here. ILRlosses, occurring in the stator wind- ingsand rotor bars, vary with loadingand
Typical Motor losses 50 hp, 3-Phase, AC Induction Motor
can range up to 15percent in three-phase
motors. These losses are caused by resis-
tance to current flow in the stator and rotor conductors and increase with mo-
- I*R Losses 0% 6%
tor load and temperature. IZRlosses can
be reduced by using larger diameter con-
ductors in the motor windings, and by
using more aluminum or copper in the
rotor to reduce resistance. Improvements
in rotor configurationand slot design can
also reduce IZRlosses.
Stray load losses, like 12R losses, in-
creasewith load current but only make up
Percent Motor Load
I\ 100
fritfion & Windage 1.1%
approximately2 percent of motor losses.
Stray load losses result from leakage flux, increasing with load current. These losses can be reduced and controlled by design optimization of primary to secondary (sta-
Controlling Losses: Each of the four main types of electrical and mechanical motor losses can be reduced with the introduction of new designs and materials.
tor to rotor) air gaps. This optimization
will require less magnetic flux for motor over operatingcosts. The lifetime operat- investment. A 15-year motor life was
operation, reducing electric current.
ing costs of an electric motor can be 50 assumed.
Core losses, ranging from 1percent to times more than its initial purchase price. Formulas, as pictured on page 60,
5 percent, are magnetic losses in the rotor For various reasons, operating costs are enable operatingcostcomparisonsof two
and stator core caused by energy loss often ignored during the purchasing deci- motors with different efficiencies.Studies
througheddycurrentsand hysteresis. Core sion. Studies have found that companies have indicated that most motors are
Iosscan be reduced by using more lamina- which do not buy energy efficient motors underloaded, averaging about 60 percent
tions of thinner, higher grade electrical may cite a variety of reasons, such as of rated load. The loading can be esti-
steel in the rotor and stator core.
disbelief of savings potential, poor avail- mated for the example or a simple strobe
The final major category of motor ability, complacency, and stable energy light test may be done to determine ap-
losses, friction and windage, are mechani- costs.
proximate motor load. Annual savings
cal in nature and are caused by motor Using the "Motor Comparison and can be calculated using Formula 11. A
bearing friction and aerodynamic impacts Payback analysis report," developed by handy rule of thumb in estimating annual
of the motor cooling fan. Friction and the Washington State Energy Office, it is savings is that an energy efficientmotor in
windage usually account for less than 1 evident that timely paybacks are avail- place of a standard motor will save 10 to
percent of total motor losses. Though able on investments in energy efficient 15 times the horsepower rating of the
lossesare low in this category, there is still motors. When certain operating charac- motor in annual dollars. In the earlier 50
room for improvement by employing high teristics are present, these payback peri- hp&example,this worked out to be an
quality bearings and bearing materials, ods become even more attractive. If a annual savings of $655,or approximately
precise shaft alignment, and recent im- motor has characteristics that include, 13 times the horsepower.
provements in cooling fan design.
but are not limited to, new installation, An energy efficient motor will provide
Electric motor efficiency standards retrofit of a failed motor, extended oper- higher annual savings if it is properly
have been established by NEMA (Na- ating hours, run time exceeding idle time, sized to match the load it will see. Proper
tional Electrical Manufacturers Associa- constant or high loads, large horsepower, sizing will help to optimize efficiencyand
tion), an industry trade organization of and high kilowatt-hour o r demand power factor.
manufacturers and motor repair person- charges, these applicationsare good can- Manufacturers design motors for maxi-
nel. The standard, NEMA MG1-12.55, didates for an energy efficient motor.
mum efficiency from 75 percent to 100
Table 12-6c, indicates the nominal and By using energy efficient motors in- percent loading. In many applications,
minimum efficiencylevels required, based stead of standard motors, it is possible to motors are operated below this optimal
on IEEE Standard 112, Test Method B. improve operating efficiency by 2 to 10 range. Motor operation below 50 percent
For a motor to be designated "energy percent, yielding energy savings that loading considerably reduces motor effi-
efficient," it should meet the NEMA stan- should get the attention of any facilities ciency. Power factor also decreases, like
dard which has helped manufacturers, manager. In the two examples given on efficiency, as the load of the motor de-
suppliers and purchasers by creating a page 57 ("Average Motor Efficiencies"), creases. Several underloaded small mo-
common benchmark. It will be periodi- approximately 5 percent savings resulted tors or one underloaded large motor may
cally updated to reflect improvements in from using the energy efficient motor cause the power factor of anentire facility
design, materials and manufacturing design instead of the standard design. to drop below 90 percent. When power
methods of the motor industry. L
This savings represented more than a factor is below 90 percent, many electric $10,000 lifetime operating cost savings utilities add a power factor charge into
THE ENERGY EFFICIENT MOTOR PURCHASE for the 50 hp motor and an 11-month the monthly billing component, increas-
Many times, first cost takes precedence payback on the energy efficient motor ing electric energy costs. To avoid this,
MOTOR A 25.0 TEFC 1800 rpm 8760 Hours 75% 90.2% $861 $ 0.0400/kWH $ 5.65/kW Per Month $ 30
MOTOR B 25.0 TEFC 1800 rpm 8760 Hours 75% 94.1 % $1 190
Annual Energy Use: Annual Energy Cost: Annual Demand Charges: Annual Power Costs: HE Motor Premium: Simple Payback:
135843 kWH $5434 $1051 $6485 $299 1.1 Years
130213 kWH $5209 $1008 $621 6
SAVINGS 5630 kWH $225 $ 44 $269
MOTOR A 50.0 TEFC 1800 rpm 8760 Hours 75% 90.2% $1 750 $0.0400/kWH $5.65/kW Per Month $30
MOTOR B 50.0 TEFC 1800 rpm 8760 Hours 75% 95.0% $2380
RESULTS: Annual Energy Use: Annual Energy Cost: Annual Demand Charges: Annual Power Costs: HE Motor Premium: Simple Payback:
271686 kWH 25795.9 kWH
$1 997
$1 2970
$1 2315
0.9 Years
SAVINGS 13727 kWH $549 $106 $655
The Buying Decision: 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 standard design, representing more than a $1 0,000 lifetime operating cost savings for the 50 hp motor. 60 AIPE FACtLtTIES
facilities like to keep their power factor over 90 percent and as close to 100 percent as possible. This can be done with capacitors, and other ways, but many times can be improved with proper motor sizing. Design engineers will often slightly oversize motors to accommodate future production growth, intermediate overloads and load fluctuations, to insure against motor failure in crieical applications and to increase motor life due to lower operating temperatures on the conductor insulation. Good engineering practices will usually keep 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 strobe light to determine operating rpm. The test is easily accomplished without interruption of power and requires no electrical connections. The test is safe and can be done by anyone, regardless of technical background. Only a handheld adjustablestrobe light and a calculator are required to conduct a motor load test. Hand-held adjustable strobes are available, powered by 1lOVAC, for about $200. The strobe light should have a digital readout to indicate motor operating rpm. To determinemotor loading, the calculation below may be used. Motor load will be a percent of full load. &I Motor S nchronousr m -Measuredr m Load = Sichronoums:r - Nameplate loo Measured rpm will be obtained with the strobe. Synchronous rpm is the speed of the stator magnetic field, and in 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 synchronous rpm because the rotor of an induction motor does not rotate at synchronous speed, but slightly lags. Therefore, if nameplate rpm is known, it is easy to determine synchronous rpm - i.e., 1765 rpm nameplate speed would be 1800rpm synchronous. Nameplate rpm is the speed of the rotor at full load. To measure the operating rpm of the motor, the strobe is pointed at the rotating shaft; by adjusting the strobe, the rotating shaft of the motor can be "frozen" to make it appear as if it is no longer rotating. When this is done, the digital readout of the strobe will indicate the NOVEMBER/DECEM1B9E9R2
operating rpm of the motor. This number should be recorded and entered into the calculation to determine motor load. For example: if a motor with a nameplate horsepower of 50 and a nameplate rpm of 1,765 was tested and found to have a strobemeasured rpmof 1,775,this motor would have a calculated load of 71 percent of full load, or 35 hp.
Motor horsepower (hp) x Yo load x .746 kW/hp = kW x Operating hrs = kWH/yr.
B h p x wload x
= w kW/hp
kW x/4oDo/ Op. hrs./yr.=l41,776/ kWH/yr.
u h p x alood x
kW/hp = /
kW x u Op. h r s . / y r . = I kWH/yr.
SEE #2
SEE #3
SEE #5
SEE #6
This type of motor loading test yields fairly accurate results, quickly, and does not require electrical connections or interruption of power to the motor. In conclusion, energy efficient motors have many advantages, with operating Cost reduction as the most significant. Opportunity for improvementexists. The nationwide saturation of energy efficient motors continues to increase but currently only accounts for approximately S percent of installed motors. This saturation level is extremely low when compared to other conservation measures in commercial facilities such as lighting, which has a 21 percent saturation. Energy efficient motors now represent about 20 percent of three-phase motor sales. As users become familiar with the advantages of more efficient motors and identify savings opportunities within their facilities, sales will increase. To have a major impact on the nationwide saturation of energy efficient motors, users would have to retrofit standard working motors with an energy efficient 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 savings and help make energy efficient motors more widely available. Incentives from utilities can help reduce the price differential between standard and energy efficient motors. Energy conscious plant engineers and facilitiesmanagers should consider energy efficientmotors for all new motor installations, in equipment specifications, for replacement applications, as part of an energy management program, and when stocking spare motors. A simple operating
1. Nameplate- horsepower 2. Motor load - estimated or taken from design data 3. Horsepower to kW conversion 4. Motor kW. This may be calculatedfrom Steps 1,2 and 3, or it may be measured directly at the motor or load center 5. Operating hours per year - either estimated or from operating schedule 6. kWH per year used by the motor
Formula II
kWH x
[ - ] 1 Standard Efficiency
High Ef1ficiency x $/kWH = $/yr. Savings
[ F / k W H x jl/.ss/- [1/90] x j.051S/kWH = w / S / y r . S a v i n g s
[ TI i l k W H x
SEE #8
1 x
S/kWH = j / $ / y r . S a v i n g s
SEE #9
7. Reciprocalof standard efficiency or existing motor efficiency 8. Reciprocalof high efficiency motor to be substituted 9. Eledric rate in $/kWH 10. $/year savings
Payback = $Cost + $Savings
TI 1 yr = $
cost + $
/yr. Savings
SEE #I 3
SEE #I 2
Simple paybackis obtainedby dividingthe estimatedcost by the estimateddollar savings/year. 11. Estimated annual savings with energy efficient motor 12. Estimated cost or cost difference between standard motor and energy efficient motor 13. Simple payback
SOURCE NORTHERN STATES POWER(0.1992 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 investment guidelines.
N O V E M B E ~ D E C E M19B9E2R
Efficiencyand Power Factor Vs. Motor load
cost analysis on each motor purchase will enable equipment operating costs to be closely tracked and will identify efficiency improvement opportunities.
REFERENCES 1) EPRI, "ElectricMotors: Market Trends and Applications," EPRITR-100423, Resource DynamicsCorp: VA, June 1992.
2) Northern States Power Co.-Wisconsin, Consumer Affairs, Commercial and Industrial Reference Library.
Load (%)
SOURCE: POWERSMARTPROGRAM:HIGHEFFICIENCIMOTORS, B.C. MYDRO INOUSTRIAL AtARKEfING, VANCOUVER, BC, 1991. Athiewing a Balance: Proper motor sizing helps to optimize efficiency and power factor. Most electric motors are designed for maximum efficiency between 75 percent and 100 percent.
3 ) Ontario Hydro, Motors Reference Guide, 2nd Ed., February 1990. 4)Washington StateEnergy Office, "Electric Motor Database Report," Ver. 1. 5) Northern States Power Co.-Wisconsin, Smart BUSINESS NEWS, Vol. 111, No. 2, May 1991. 6) Nadel, S., Energy Efficient motor systems: Handbook on Technology, Programs and Policy Opportunities, American Council for an Energy Efficient Economy, 1991.
7 ) Encyclopaedia Britannica, Vol. 11, 15th Ed., 1990.
8) U.S. Department of Energy, Office of energy markets and End Use, "Characteristics of commercial buildings 1986," DOE/EIA-0246(86),Washington, DC.
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At O'Brien & 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-workingwith you each step of the way. For more information on the range of facilitiesengineering serviceswe offer, ca.UDeborah Hennessy, regional marketing representative, at (315) 437-6100.
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-Gr& Rider Service No. 132
Oscar Brandseris anengineer withNorthern States Power Co. in Eau Claire, Wisconsin where he assists large industrial customers with energy management, conservation and service requirements. Previously, he worked as plant engineer, plant maintenance engineer, manufacturing engineer and product application engineer for John Deere Harvester Works and RTE Corp. He holds a bachelor's in plant engineering, industrial technology from the University of Wisconsin-Stout. For more information on this article, circle Reader Service No. 133. N O V E M B E R I D E C E1M9 9B2E R


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