NEXT GENERATION NEMA PREMIUM® MOTORS SUBSTANTIALLYLOWER OPERATING COSTS

Copyright material IEEEPaper No. PCIC-2006-8

Juergen F. Fuchsloch

Siemens AG Automation & DrivesA&D SD SPAFrauenauracher Str.8091056 Erlangen, Germany

William R. FinleySenior MemberPower Conversion DivisionSiemens Energy & Automation, Inc.4620 Forest Ave.Norwood, OH 45212

Reinhard W. Walter

Siemens AG Automation & DrivesA&D SD SPAFrauenauracher Str. 8091056 Erlangen, Germany

Abstract: There has been a push in thepetrochemical industry to higher and higher efficiencies.This can be seen in the revisions to IEEE 841 which isexpected to push to NEMA premium as the standardsometime in the near future. Leading motormanufacturers have been motivated by these industry-driven efficiency improvements to investigate theoptimization of manufacturing processes and designs tofurther improve 30I induction motor efficiencies.

It's very important to make it fully understood to theindustrial users, how losses are generated and toidentify the levers that reduce these significantly, all atan acceptable cost for the investment of the motor. Lifecycle costs should also be investigated.

This paper will focus on new technologies, design andprocesses being introduced to improve efficiency of 30induction motors.

1. INTRODUCTION

For many years, the efficiencies of 30I induction motorshave been subject of numerous investigations toincrease efficiency values by minimizing losses duringthe operation. This has a lot of value knowing thatapproximately 57% of the generated electric energy inthe USA is utilized by electric motors powering industrialequipment [1]. In addition far more than 95% of anelectric motors life cycle cost is energy cost.Substantially lowering motor losses will have asignificant impact on the country's energy consumption.With still growing utility costs, it becomes obvious that itis important to invest in this subject. The sources oflosses in electric motors are many and need to belooked at separately, in detail, considering their origin,level, influence and possibility to be measured ratherthan just determined. All considerations need to bechecked for technical feasibility and commercial impact.Induction motors design principles have not changeddramatically over the years while the tools andknowledge of the engineers have improved considerablyFrom this better understanding improvements continuebeing found to reduce losses in electric motors. In orderto systematically analyze the root causes of losses andthe possibilities for improvements, it requires a completeand new approach, looking not only in the electrical areabut also into mechanical areas such as cooling,

temperature levels, O.D. vs. length ratio and othertopics. In this paper, we apply a systematic andoptimized new design approach for NEMA motors inframe sizes 143T through 256T.

II. A FEW BASICS OF INDUCTION MOTOR DESIGN

Induction motors generally have only few parts whichare actively influence their performance. Thecomponents are differentiated into "active parts" and"non-active parts". Active parts are the rotor and statorassembly. The cores are manufactured from laminatedsteel. The rotor cage on NEMA size machines (Lessthan 400 HP / frame size 449) is mostly made of die-cast aluminum. The stator winding is made of insulatedcopper wires. The non-active components are thehousing or frame, the bearing end shields, the fan, thefan cover, the terminal box and the shaft. The electricalperformance or generally the ability to improveperformance is proportional to the active volume of themotor. Due to defined frame assignments for eachhorsepower rating and speed by NEMA MG1, thefreedom to move within a dimensional envelope islimited to certain well defined maximum dimensions ineach direction (mostly shaft height and mountingdimensions). In order to increase the performance of amotor either rating or efficiency, requires basically anincrease of the active volume or an improvement intechnology.

Types and Origin of losses

There are several significant sources of losses in electricmotors which will be addressed in this paper for a betterunderstanding of their origin and their effects. Theselosses are basically described in the applicablestandards such as NEMA MG 1 and IEEE. Thesestandards drive the testing procedures used andtherefore the fundamental motor design. IEEE 112describes the losses considered to determine thenominal efficiency of a certain motor per NEMA MG 1. Italso describes the proper measuring method and howmotors shall be tested in order to receive comparabledata. The applicable tolerances are also defined. NEMAMG 1 defines the performance requirements in general.

Stator losses are losses which are generated within thestator winding during the operation of an inductionmotor. These losses are proportional to the square of

1-4244-0559-9/06/$20.00 ©2006 IEEE 1

the stator current and directly proportional to the statorresistance (conductor cross-section). They can beinfluenced by specific copper wire material and the crosssection of copper used.

Rotor losses are losses generated in the rotor cage ofthe squirrel cage. There losses are also proportional tothe square of the rotor current and directly proportionalto the bar resistance (conductor cross-section). Thelever to influence those losses is the rotor design, therotor cage material and the cross section of materialused. These are also referred to at times as slip losseswhich also correctly imply that these losses areproportional to slip. Slip and the associated losses canbe minimized by winding the stator stronger (higher flux)but limited not to exceed the allowable locked rotorcurrent. This is always a balancing act.

Core losses are a function of the magnitude of themagnetic field which is induced by the voltage andampere-turns in the stator as well as in the rotorlaminations. These losses cannot be separatelymeasured, but are part of the no load losses andsegregated out as defined by IEEE 112 Method B.

Windage & Friction losses are the losses which arebasically generated by the bearing friction and the powerthe shaft mounted fan is consuming. This can also besegregated out from the no load losses.

Stray Load Losses are losses which don't exist at noload but increase with load by the square of the rotorcurrent and as a result of the rotor slipping relative to thefundamental flux wave. These increases in load lossesresult from a multitude of sources. High frequencyharmonic Iron losses are created due to the harmonicscreated as a result of the slip and rotor slot chopping thefundamental flux wave along with the increasing rotorcurrent. The skin effect in the rotor bar due to the highfrequency harmonics, forces current into the upper partof the bar near the rotor OD thereby increasing losses.Increasing current in the rotor generate increased crosscurrents and losses in the laminations between bars,

In order to optimize the total losses for induction motors,it is very important to introduce an integral approachcovering the electrical and mechanical issues as well asevery individual loss type. This is to optimize a newdesign for a motor family rather than a frame size byframe size or rating by rating.

Fig. 1 Typical loss distribution

100% -

75%

'T 50%-

0FoWind. and friction

25% EnlronilssesoAddloa osss

0%4 P

1 10Output Power Rating in kW

100

III. LEVERS TO IMPROVE EFFICIENCIES

1. Increase amount of active material2. Utilize high performance lamination materials3. Lower the temperature level of the motors4. Optimize the stator/rotor geometries5. Optimize the air gap dimensions6. Optimize the lamination punching and stacking

process to prevent cross currents and harmonics,causing stray load losses and minimize burrs whichcan short out lamination increasing core loss.

7. Improve efficiency of fan assembly8. Use higher efficient bearings9. Increase the rate of heat transfer between active

parts and housing10. Limit the manufacturing tolerances, optimized

processes

All the levers have to be carefully looked at andinvestigated because most of them are not independentfrom each other, and may negatively influence theefficiency gains in one or the other case. Secondly thecommercial impacts have to be strongly consideredsince with the higher efficiencies there may be apremium paid, which will limit the savings seen...

IV. IMPACT AND DEPENDENCY OF EACH LEVER

1. Increase of active material leads to highermaterial cost and requires either longer coredimensions, or bigger diameters of the statorlaminations. The target is to find an optimized sizeof active parts O.D.'s which are suitable for everyrating, efficiency level and speed within a givenframe size. If this can be achieved, the number ofvariances concerning none-active components suchas bearing end shields, fans, terminal boxes etc.can be minimized. The extra cost of additionalactive material may be compensated by the savingsof manufacturing tools, reduced inventory, simplifiedservice and repair efforts and significantly reducednumber of technical options such as encoders,tachs, brakes, gear boxes and flanges. Some mayargue that you can just increase copper in the slotbut ever since the push to higher efficiencies theslot fill has been maximized and further increaseswould reduce reliability and increase manufacturingcosts. It is true that some older designs in the fieldcan be rewound with more copper but overall thesedesigns have much poorer efficiencies and mostprobably should be replaced with today higherefficiency motors for a better overall return oninvestment.

2. High performance lamination materials(permeability, losses, insulation) may individually bemore expensive relative to their base prices, butconsidering an integrated approach may lead tosavings in materials and improvements in motorperformance. Other process can also be consideredwhere the laminations are heated treated afterpunching to minimize mechanical stress and reducethe grain structure around the punched areas thatcan increase losses if this works into themanufacturing process. This lever combined withitem #1 listed above could compensate (part of) the

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additional material demand, but grant the sameadvantages as described in item 1.

3. Lowering the temperature level of the motors willhelp to reduce the Stator and Rotor 12R losseswhich are directly proportional to the temperaturerise. The temperature can be reduced bydecreasing any of the losses generated in the motorby the methods discussed to increase efficiency, orby improving the heat transfer from the active partsto the none-active parts of the housing and endshields where the heat can then be dissipated to thesurrounding air. This requires very precisemanufacturing processes for the housings/framesand contact between the OD of the stator core andthe frame is critical to achieve better heat transfer.There is always a concern that this may yield ahigher material or manufacturing cost for thesecomponents. But new technologies and improvedmanufacturing process may minimize the costimpact. Considering that the overall target is toreduce the losses and drive toward a unified designwithin each frame size, this may be a worthwhileexercise.

4. Optimized stator and rotor geometries are theplayground of the electrical engineers. They likespecial designs and configurations that optimizeselected materials and dimensions. In the past,most of this exercise was trail and error. Today,there are analytical tools that significantly reducedesign time and result in much quicker design tofinished prototype times and accurately predict theperformance. Optimization does not necessarilylead to higher material costs, but most frequently,far better utilization of materials.

5. Air gap dimensions are also a valuable item to beoptimized in order to obtain better performance w/onecessarily increasing the overall material cost.This can be simulated with applicable analyticaltools such as FMEA where the flux path betweenthe stator can be modeled and the rotor and statorslot size and count along with the air gap can besized to minimize the chopping of the flux as itcrosses the air gap resulting in harmonic rotor andstator surface losses along with harmonic losses inthe stator and rotor conductors. This is a verycomplex analysis that required experiencedengineers but can result in a reduction of losseswith out a significant increase in cost.

6. Optimization of manufacturing processes is theoption-of-choice to reduce stray load losses whichare hard to predict and in most cases, only to bedetermined by prototype testing though designcriteria and technology have been developed tominimize these losses Optimization considerationsare extremely important for the design of punchingtools, stacking processes, clamping or welding ofthe stator packs, treatment of the finished rotors,insertion procedure of stators into the housings,treatment of the finished wound stator cores andmany other small process steps.

7. Optimized fan efficiency can reduce the frictionand windage losses, especially for motors with

improved efficiencies. Optimized designs wouldconcentrate on minimizing motor skin temperaturelevels and reducing the fan diameter. The latterwould also result in lower noise. Of coarse there isa trade of here as to how small to go on the fan andhow low to go on air flow versus the increasedtemperature rise which will increase the 12R losses,but this can easily be calculated. It is important tonote that many of the smaller higher efficiencymotors run so cool that the fans may not be trulyneeded to meet the stator temperature riserequirements but it is easy to calculate the trade offbetween windage and the increase in 12R losses asa result of the increased temperature rise. It isimportant to also note that the resistance change isnot directly proportion to temperature rise but isproportional to (234.5 + Temp2) /(234.5 + Templ)where templ&2 are two different temperatures indegrees C. A change from a total temperature of100 to 110 degrees C will only result in a 3 %change in resistance and since the 12R losses arenormally less than 50 % of the total losses will resultin only a 1.5% change in losses. On a motor whichhas a 95% efficiency and 5 % losses, this wouldresult in approximately a .015x 5=.075% change inefficiency. Less than 1 tenth of a percent.

8. Higher efficient bearings. Although this losssegment is relatively small, there is still theopportunity to select bearing designs and systemsto reduce the losses created by bearing frictionthough with the higher quality precision groundbearings available today this is minimal. Probably abigger concern is whether to use sealed, shieldedor open bearings. The more open the bearings arethe easier the excess grease can be thrown fromthe bearings reducing losses. On sealed bearingsthe grease is held in the bearing keeping the losseshigh throughout there entire life, but of coarse herethe advantage is reduced maintenance andincreased cleanliness of the critical part of thebearings. These are not normally used on the largermachines where to maximize efficiency many havegone to either single shielded or open bearings toreduce the losses during the factory tests though intime, weeks or months, running in the field theselosses would have eventually reached these lowerlevels.

9. Increase the heat transfer rate between the activeparts and the housing components. The surfacetreatment of the stator and the insertion process ofthe stator into the frame need to be defined andcontrolled during the assembly processes. Also, thechoice of housing materials (for example analuminum frame), will have an improved heattransfer rate to reduce material costs, reduce fansize and/or increase the efficiency.

10. Limit the manufacturing tolerances to reduce theefficiency tolerance bands from two to one, betweennominal and minimum efficiency values. This is asignificant value for the owner/user of motors andtheir energy costs. Though statically if one has alarge sampling of motors the average should be ator near the nominal efficiency but it would bepossible in a small sampling to have on a motor

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with 95% nominal efficiency a difference of up to.5% losses between band 1 and 2(94.5 & 94 %respectively). This 9% increase in losses couldincrease you energy consumption by .5% overall.This may not seem like much on one motor butoverall if you purchase $10 million of energy eachyear this could result in a $50,000 a year savings.

Nameplate

Efficiency(in

Percent)

All of these factors of have differing levels of influencesto motor performance and different dependenciesbetween each other. It's critical to select the mostimportant ones and evaluate the significance of each, aswell as the commercial considerations. The magnitudeof the possible influences on the performance can beseen in the following graph outlining the various subjectsand their impacts on a typical induction motor

100

99

98

97

96

95

94

..r Perpetual motion

.Super ConductingI * Amorphous Steel Laminations

.,

. Today's Premium Efficiency

-1997 Energy Policy Act

-Old "Standard" Motor

-Historic - 1975

Fig. 3 Picture of typical efficiency levels over time

Aereas of Improvement

L<vE

Levers

Fig.2 Impact of the possible areas of improvement forinduction motors performance

Presently, there are basically two levels of efficiencies.One is the well-known EPAct, which are federallymandated efficiencies. The other is a NEMArecommendation called NEMA Premium®, that definesefficiency values for efficiencies significantly higher thanEPACT. NEMA Premium® has been well received in theindustrial motor market, especially by the petrochemicalindustry and other power consuming industries. Ofcourse there is a cost impact to building higher efficientmotors, and reducing losses, but this in not always aone-way street. Up to now, the generally acceptedpractice was to obtain higher efficient motors byincreasing the amount of the active material and toincrease the outer diameter of the active parts.Sometimes, this would result in going to the dimensionsof the next larger frame size. This, however, is over-proportionally increasing the materials and is asking foradapted housing designs in order to remain in theallocated frame size envelope. To improve this situation,it can be suggested to the leading motor manufacturers,to come up with a complete redesign of their motorproduct lines. This shall take in account the dramaticallychanging requirements or recommendations concerningenergy saving motors. The historic levels of efficiencies,basically determined the motor designs. Newrequirements concerning the efficiency levels have beenquickly introduced not reconsidering the complete motorlines.

If an integral approach is adopted for the completerange of NEMA motors to accommodate the latestrefinements concerning efficiencies, the results willeventually lead to an optimized series design, whichmay help to overcome some of the draw backsencountered for one or the other frame size or rating.Moreover, anticipating further increases in efficiencies,due to rising energy costs and/or diminishing primeenergy resources, this approach may even lead to astrategy of increasing the targets for efficiencies in orderto easily accommodate future requirements w/o acomplete redesign of the motors. Additionally, this mayhelp to support technical specifications from othernations or end users. For example higher HP-output insmaller frames or any other, none US, rating vs.efficiency allocation, can be accommodated in thosenewly optimized basic designs.

Fig. 4 International specs and demands

Realization

The integral approach for NEMA motors and the targetto find a new platform concerning the model philosophy,leads to investigation into more efficient active materialsfor stators and rotors. The materials are basicallylamination materials, winding materials and the rotorcage material. The biggest increase in conductivityoffers the use of copper in the rotors. The conductivity of

4

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0 >,CU 0

CL aE .4)4) .2> M:'Z LU4)CU

w

12201 MI

aluminum is about 34mS and copper offers 58mS. Anincrease of 70% is feasible.

Cpper rotorcage

10§Conductivity increases from

34 MS/m to 58 MS/m

Fig. 5 Conductivity increase copper to aluminum

Copper rotors are a well know technology by all themajor motor suppliers. However, the majority of copperrotor designs are applied to medium and high voltagemotors. In these motors, the copper rotors reduce therotor losses significantly, which is very important sincemedium and high voltage motors have large ratings tobegin with. These copper rotors utilize pre-manufacturedcopper bars that are generally driven into the rotor slotand soldered to the short circuit rings on both ends ofthe rotor. For NEMA motor ratings, at least for thesmaller frame sizes, a fabricated copper rotor designdoes not seem to be practical, due to the high laborcontent compared to the value of such a motor.Therefore, the process of choice would be a die castingprocess, similar to the well established process ofcasting aluminum rotors. Theoretically, the technology ofutilizing copper rotors (independent of the manufacturingprocess) lead to a solution for a platform design offeringEPAct and NEMA Premium® efficiency values in thesame overall dimensions of the active (and of coursenone active) parts. The reduction of losses for motorswith copper rotors yields efficiencies even beyond theNEMA Premium® recommendations, while staying withthe same platform as an EPACT motor utilizing analuminum rotor. Comparing the higher copper priceversus aluminum allows for this option. Comparingsimilar performances relative to the efficiencies of thecopper rotor and the aluminum rotor, cost is in favor forthe copper rotor design, even in light of current highcopper commodity prices. The advantage tends to dropoff significantly with increased frame sizes (>FS250), butthe option to stay within on product concept (platform)may also compensate for certain extra costs. Theplatform offers reduced variances in productcomponents, reduced number of manufacturing tools,optimized process times in manufacturing and testingand many other benefits.

significantly higher temperature levels are imposingextremely high stresses on the tool materials and thewhole process set-up. Support for the layout of this newprocess can be obtained by the CDA of USA. A non-profit organization to support the introduction anddevelopment of copper casting processes for differentapplications. There are several slightly different castingprocesses known, which have to be adapted to thespecifically required products supposed to be cast. Forsponsors of the CDA, access to the know ledge baseand the use of CDA patents is free of charge. For allmajor motor suppliers this support and patent use isavailable on request.

V. RESULTS OF PROTOTYPES BUILT AND TESTED

Several motors have been built and tested with verypositive results concerning the efficiency increase andthe shrinking of tolerance bands. In general, majorimprovements have been proven. Considering a maturecopper casting process, including the appropriate lifetime of the casting tools, this technology can today becalled state-of-the-art. Apart from the technicalchallenges to be overcome, there is also the cost issueto be considered. Due to the presently highly volatilecopper cost, the comparison of motors utilizing the wellestablished aluminum die cast process, with motors atthe same performance data, however in the copper casttechnology shows significant advantages relative to thedemand for active material and non-active material. Thecopper cast motors have significantly shortenedlamination stack lengths and can be accommodated insmaller frames. So the laminations are cheaper, thewinding copper usage is less. Overall, it can bepostulated that this advantage is greater, the smaller theframe size. In real terms, the break-even point is varyingaround the 250 frame size, heavily depending on thecopper price. For frame sizes larger than 250, it may notbe of significant advantage. However, it may still be theonly way to achieve efficiency values significantly aboveNEMA Premium(®.

Comparison Cu vs. Al Rotor Technology

1000

100

10

1

U,00

.2

0

U,00

.

The only technical challenge remaining is the coppercasting process. For the copper casting process, thereare some very specific differences to be considered. Incomparison to the well established aluminum diecasting, the biggest challenge appears to be to handlethe higher melting temperatures of copper. The meltingtemperature for aluminum is approx. 1,2210 F and forcopper the temperature is about 1,985° F. This

- Copper Rotor Motor-Aluminum Rotor Motor

CO C) CO CN C\ O C

CN CN CN C C/

LL LL LL LL

Frame Sizes

Comparison Cu vs. Al Rotor Technology

1000800600400200

0

- Copper Rotor Motor-Aluminum Rotor Motor

oco o oco o~ o oC) OD C) OD N C\ O) C)C

LL LL LL LL LL LL LL

Frame Sizes

Fig. 6 Comparison of Al and Cu rotor and frame size

5

Al housingAluminum Copper

, 21en 1 9'A 17o 1 5E 13o 1 1z g

.ss9\tb DsNQ\^ \ \N151 NI'lIN

g 14,0-

In 12,0-

100In

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60

Q V~~~~~~~~~~~~~~~~~~~~Nl

-+ NPEUltra NPE

lUItrN PE

/7i IEC6

,.NEMA

+ NPE

Ultra NPE

Fig. 8 General platform concept covering internationalspecs

Fig. 7 Reduction of losses using copper rotors 2p and 4p

VI. FURTHER PRODUCT OPPORTUNITIES

Applying a forecast to the findings and gains realized forthe copper rotor construction for NEMA motors in thesmaller frame sizes, it could lead to a new approach fora world-wide motor series. There is a lot of interest onincreasing efficiencies for induction motors, as alreadymentioned earlier in this paper. Considering that thisinterest is relevant in the NAFTA regions, as well as inthe European and Asian regions, the tasks for full-lineand global motor suppliers would be to design a basicmotor family covering most of the leading internationalspecifications and only adapting minor issues to thenational specs (see fig. 4). Limited to efficiencies thereare basically 4 leading classes known, Eff 1 and Eff2 forthe IEC world, and EPACT and NEMA Premium for theNAFTA world. Additionally there are somegeographically and nationally driven specs, which aremore or less covered by both.

If an overall approach for the induction motors isselected where the motor is designed for both the NEMAand IEC market utilizing both copper and aluminumrotors in addition to cast iron and aluminum frames withcommon components, it now becomes practical tosupport such a design platform concept. This wouldallow the motor manufacture to give the customer whatthey need whether it is high Efficiency, performance orlow cost in one product line series. The would bepossible since the advantages to the motormanufacturer due to this approach are also tremendous,due reduced variances (parts), saving in toolinvestments, reduced inventory, simple modifications,readiness for international business, sales andmaintenance training, simplified marketing and salesprocesses etc.

Fig. 9 Picture of a typical aluminum motor

VIl. SUMMARY

Current motor technology/offerings along with frameenvelope limitations generally limit efficiencies to NEMAPremium® or a little above. Emerging technologies,including copper rotor technology pushes the envelopeof today's product offering into new heights. As the costof energy, therefore, the cost of producing goods goesup, the importance of premium efficient motors and theirrelative operating costs become of increasing interest.The use of new or previously considered technologieswhich were considered too expensive could becomemore practical.

Vil. NOMENCLATURE

NPE Nema Premium EfficiencyOD Outer Diameter

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IX. REFERENCES

[1] IEEE, November 2001, IEEE-USA PositionStatement on Energy Efficiency

X. VITA

Juergen F.Fuchsloch graduated from University ofStuttgart with a Dipl.-Ing. degree in ElectricalEngineering. He has been in several positions ofengineering and product management for SiemensGermany since 1984.

William R. Finley received his BS in ElectricalEngineering from the University of Cincinnati, Ohio. Hejoined Siemens in 1974 and has been Manager ofEngineering and Technology since 1994. He haspublished more than 20 papers in various magazines,including IEEE PCIC, Pulp & Paper, and IAS conferencepapers. He won multiple awards including a First Prizeat the PCIC Conference for his paper "An Analyticalapproach to Solving Motor Vibration Problems." He isalso a member of various organizations within NEMA, aswell as Chairman of the International StandardizationGroup and Large Machine Group.

Reinhard W. Walter graduated from FHCoburg/Germany with a Master's Degree in electricalengineering in 1974. He joint Siemens AG in Germanyand held several positions in marketing and sales aswell as in product engineering. Lastly he was thegeneral project manager for the new Siemens NEMAmotor generation presently entering the market.

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