High Efficiency Motors

High Efficiency Motors - Performance' Economy & Reliabitity, bY OPtimisation P. Caselottir,A. Conchetto',P. J. Tavnert I MarelliMotori SPA 2 FKI Engineering

I

Introduction

and outputs with a tough The TEFC machine is designed to standard dimensions motor that can interchangeable enclosure and blow_ove. .ùling to give a rugged, be usedin a wide variety of applications' to achieve requirements Historically manufacturers hàne perfected their products with improving powermachines in resulted as economicallyas possible.This has i

weightratios,asshowninFigurel,takenfromGlew'Refl'soelectricmotors havebecomeprogressivelycheaper' TEFC motors has not However, the hgure shóws that the power-weight of machines, in recent years there.has been a -deteriorate. increased at the same rate as larger This is due to the poor heat transfer tendency for the power-weight to which have air-blown in TEFC machine, "onlpu.éd to modern large machines' of standards that confine internal active parts. It is also due to the limitations for manufacturersto outputsto particular frame sizes.Therefore it has been harder further' power-weight the make TEFC motors more competitive, by increasing machine' without deteriorating the noise and efficiency of the and the European The challenge of the efficiency initiatives in North America by adding power-weight reducing union is to improve performance without materialsand increasingcost. where the key area This has been the subjict of other papers, Haataia et al Ref2, loss' hasbeenidentified as reducing of heat_transferin the This paper suggeststhat addrJssingthe fundamentalproblem possible to improve TEFC machine, in addition to ràducing loss will make it products' competitive provide performanceand still

2 Heat Transfer & Temperature Rise in TEFC Machines compromise the The TEFC motor has evolved from earlier geometries,which need to protect designer,sneed for cooling air in the active parts with the user's been adopted in the the motor flom its enviroiment. The present geometry has

96 majority of motors in the range0.15-75kW. The problemwith the geometryis that all heat from the active parts, the core, rotor and stator windings, must be extracted fiom the interior of the machine, passedto the ribbed barrel and removed by in this arrangement: convectionby the blow-over air. There are four weaknesses r The internal air does not remove much heat flom the active parts. o Heat from the rotor to the ribbed barrel crosses3 interfaces,the air gap, the stator insulation and the core/barrel interface. r Production factors affect these interfaces and increase their thermal resistance. o The convectiveheattransferis inefficient. In consequence the bulk heattransfercoefhcientof the TEFC motor, relating total losses to stator winding temperaturerise and motor surface area, is typically l5W/m2K, much lower than for larger electricalmachinesat 300 Wm2K. There are some referencesthat addressone or all of the weaknessesin TEFC machines, for examplePickeringet al Refl3.

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Interaction betweenLosses.Efficiencv & PowerWeight

The lossesin a motor can be classifiedas follows: i. Load DependentLossesincluding: o Joule lossesin the statorwinding; . Joule lossesin the rotor winding; o Stray losses. ii. ConstantLossesof load, including: r Iron loss; . Mechanicallossesincludine fan. bearineand shaftseal. tolo ,n. Joule lossesrepresentthe In small and medium sized motors ], no*, significant proportion of total losses, typically around 50%. Therefore their reductionmust be one of the principal objectivesin raisingmotor efficiencies. Haataja in ReB implied the reduction of lossesis the only way to develop high efficiency motors and this must increasethe volume of active parts. He quotes Applebaum et al, Re?l, who show increasingefficiency for increasedvolume of active materials. But this is only so if currents, fluxes and materials remain constant. No selÈrespectingmanufacturer improves his machines by keeping every,thingthe same. This paper is arguingthat one can increasepower-weightby raising the bulk heat transfercoefficient in the machineand increaseefficiency.This could increaseloss density,reducingefficiency,but only if the materialsand designare not changedto challengethat increase. To investigatethis point one can comparemotors of standardand high efficiency designs, selectedffom the cataloguesof Europeanmanufacturers.We consider

97 identical constructions,using aluminium frames, to eriminate variability due to structuralmaterials,exposingthe variationdue only to the activeparts. Resultsof a comparisonbetweenstandarddesignsand an earty irigrr efficiency 2_ pole design are shown in Figure 2. This demonstrates, for thií range, higher efficiencies by reducing power-weight by at reast 20yo. Figure 3 compares the samedesignwith another,optimised,high efficiencydesign.ihis showsoptimised resultsachievedwithout the samereductionin power_weight.

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Production Factors & Optimisation

To reachhigher performancecompetitivelyone must optimise: i. Performanceparameterssuchas: o Losses& efficiency. o Noise. r Startingcurrent& torque. ii. Productionfactors o Improved processesto reduceloss. r Choice of core frame diameters. o Cost of production. The cost consistsofthree components: i. Material. ii. Direct labour. iii. Overheads. The first two are influencedby the processtech-norogies and the third is a function of the organisationof the company. To reducethosà first two, designand process choicesmust be made that minimise cost but still focus on p.ifor,nlnce. It is also necessaryto improve.the variability of processesand materials. The following areasfor optimisationhave been identified: i. Designcalculations; r Electromagnetic; o Thermal; o Ventilation. ii. Materials; r Use improved steels; o Low loss bearingseals; r Reducematerialvariance. iii. Processes; o Better impregnationto improve heatffansfer; o Limit tool wear to reduceloss& wear variance; o Improved statorwinding to reduceloss; o Improved rotor cageconstruction; r Improved core/barrelfit.

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Temperature Rise,Insulation & Reliability

An important performance area, linked to loss and efficiency, is temperaturerise in the insulation system. Modern insulation materials are capable of withstanding higher temperatures.Most Class F insulation systems are now capable of withstandingClass H. If a Class F seriesof motors were operatedto Class H an immediate improvement in power-weight could be achieved but at the expenseof higher Joule and iron lossesand a potentialdecreasein efficiency. On the other hand a high efficiency motor has lower lossesthan a standardmotor and has less thermal problems. Reducing temperature rise enables ventilation losses to be reduced, which in the case of 2 pole motors are a significant proportion of the total, enabling funher increasesin efficiency and a significant reductionofnoise. A reducedtemperaturerise also confersincreasedreliat ility, as predictedby the Arrhenius Law. This suggeststhat high efficiency rnachineswith modernhigher temperatureinsulationsystemswill have an extendedlife. On the other hand, for a motor supplied from an inverter, one must consider the dielectric withstand of the insulation on the life of the motor. It is noted that PWM inverters with IGBT technology have increasedthe speedof commutation.This imposes voltage spikes on the winding, whose severity are related to the commutationrise time and the length of cable feedingthe machine,see IEE Ref5. Therefore the insulation withstand needsto be improved for inverter supply. When taken with the lower temperaturesobtainable in motors of high efficiency this could convert into increasedreliability but motors on inverter supply experience additional heating that may efface that advantage.

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Industrial Experienceof Reliability

The reliability of a motor is defined by the Mean Time BetweenFailures(MTBF) AS:

MTBF (Motor) hrs:1/(Sum of FailureRatesfor each Subsystemof the Motor) The IEEE conducts surveys on industrial electrical plant reliability and has establisheda standard,Ref6. This describeshow to carry out a reliability survey and how equipmentcan be designedto achievehigh reliability. Ref6 also contains reliability surveys for elechical equipment including motors, published in the IEEE Transactions,for example RefsT-S.Their data gives us the opportuniry to consider the reliability of TEFC Induction Motors. We have reviewed those surveys and identified relevant results in Table I showing MTBFs of over 150000hrs.The Arrhenius Law gives a stator winding insulation life at Class F temperaturesof 100000hrs.An analysisof failure modesin service,Table II, taken

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from three sources,shows that bearings rather than windings are the principal failure mode. The higher MTBF figures found in the surveysthereforeshow that real winding life must be much longer than 100000hrs. So, although higher temperatureswould reduce insulation life, the failure modes show that this will have little effect in reducing the life of a motor, compared to the bearings. Similarly a reduction in reliability due to commutationpulsesis onlv goirrg to be significant if it substantiallyreduceswinding life. In other words raising motor temperaturesis both feasible from a winding point of view and is unlikely to significantlyreducereliability from currenttypical figuresof 150000hrs.

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Conclusions

Improving the efficiency of TEFC motorsdoesnot necessarilymean increasingthe volume of material and thereforecost, but does necessitateoptimising design and production factors. An important byproduct of such work could be reduced winding temperaturesand a potential increase in the reliability of the high efficiency motor. In fact operatingsuch motors at higher temperaturesis unlikely to reduce motor reliability below 150000hrs. However, reliability may be compromisedwhen the motor requiresan invertersupply.

Acknowledgements This paper is published with the permissionof the Directors of FKI Engineering and of MarelliMotori SpA.

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References

t 1 l Glew N, Design and manufactureof energy efficient and environmentally

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friendly large machines,IEE Colloquium 1999, pplll-115. Haataja J, PyrhonenJ, Improving three phaseinduction motor efficiency in Europe, the challengefor manufacturers,IEE EMD Conf, Cambridge, 1997, P u b l n4 4 4 ,p p 1 9 0 - 1 9 4 . Pickering S, Lampard D, Hay N, RoylanceT F, Heat transferfrom the stator end windings of a low voltage concentricwound induction motor, IEE EMD Conf, Durham,1995,Publn412, pp 477-481. Appelbaum J, Fuchs E F, White J C, Optimisationof three-phaseinduction motor design,IEEE Trans,Vol EC-z,1987,No 3, pp 401-422. Various papers, Effects of high speed switching on motors and drives, Birmingham,June1999,IEE Colloquium99l144. IEEE Std 493-1991, RecommendedPracticefor Desisn of Reliable Industrial and CommercialPower Systems.

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O'Donnell P, Report of large motor survey of industrial and commercial PartI, IEEE TransIA, Jul/Aug 1985,pp 853-872. installations. Thorsen OV, A survey on induction motors in offshore oil industry petrochemical industry gas terminals and oil refineries, IEEE Trans IA, Sept/Oct1995,pp I 186-l 196'

Table I: Industrial Sur Type Machine

MTBF, hr' Unit-Years No. of Surveyed Failures Surveyed, Industrial 1 0 6 3I1 89 0-1000VIEEE 1985,RefT I 080 AC Motors Induction 164972 832 l 5 6 5 8 RefS IEEE 1995, AC Motors L V l 1 - 5 0 k w r59021 921 16738 of above Sum LV AC Motors Source of Information

Table II: ComparingProportionsof Failures Proportion Proportion Subsystem Of Failures, of Failures, Customer FKI Measured Predicted '7 95Vo 5Vo Bearings 2Va 9Vo winding Stator l7o 6Vo Rotor Winding

,. Proportlon Of Failures

IEEE. RefS Measured 5lVa l6Vo 5Vo

Fig 1 Weight Per unit outPut as a function of year, Ref I F s 0 P40 fso È

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