Induction Motor Tradeoff for VSD Driven Pumps and Fans.pdf

2006 IEEE 24th Convention of Electrical and Electronics Engineers in Israel

Induction Motor Tradeoff for VSD Driven

Pumps and Fans

Teddy Nickolayevsky and Sigmond Singer, Member, IEEE

Abstract - In this paper a new approach in overall drive systemchoice for centrifugal applications is proposed. This approachlies in tradeoff between number of induction motor poles andoverall system efficiency and investment. The proposed techniqueof induction motor and proper drive selection based ontheoretical proof that high nominal speed motor have betterefficiency at any operated speed that low speed one in spite ofnominal torque is equivalent for both. In present article curves,that represents induction motor efficiency changes versus speedwhen it driven by VSD, theoretically calculated for various polenumber motors at various load types, speeds and VSD controltechniques. Based on calculated data energy efficiency andtradeoff analysis for various types of motors and loads wascarried out.

Index Terms - Induction motor, variable speed drive,efficiency, centrifugal load, tradeoff.

I. INTRODUCTION

THE three phase induction motor is a workhorse of modernindustry. In a last decade the VSD has been wide spread

in the industry according to its undoubted power savingfunctions and as a result cost benefit for end user. In spite ofmotor drive revolution the motor choice technique forapplication has not been changed. Up to this date most ofengineers following by conventional rule of motor choice forlow speed pumps and fans (centrifugal loads) - nominal speedand power of motor should exceed highest operating loadpoint and be as close as possible to the next catalogue power.

The objective of this paper is to present a new approach ininduction motor tradeoff for VSD driven pumps and fans. Themotor concerned not as stand alone unit, but as part of drivesystem and it choice based on tradeoff of overall systemperformance.

II. INDUCTION MOTOR EFFICIENCY CALCULATION

The efficiency of any electrical machine is a ratio of outputpower to the sum of output power and machine losses.According to IEC [1] and IEEE [2] standards the losses ofinduction motor are segregated as follow:1) Stator winding power loss Pc111

Manuscript received September 9, 2006.

2) Iron-core power loss Pfe,3) Rotor cage power loss PCU2-4) Mechanical loss Pmech.5) Additional load loss Padd.

In order to calculate each part of loss the geometry anddetails of the motor should be known. The example of themotor geometry is introduced in Fig. 1 and basic laminationnomenclature identified in Table I.

Fig. 1. Induction motor lamination geometry

A. Stator winding lossStator winding power losses Pcu1 is a loss due to copper

winding resistance and they calculated as follow:

Pcu= 3IRcul(105) (1)I

(2)

where I is the stator phase current and R1,(]o05) is the phaseresistance of stator windings corrected to 1050C, pcu is thecopper volume resistance at 200C (pcu=1.75.10-8 Q.m), I is theconductor length and q,, is the conductor cross-sectional area.The stator winding resistance adjusted to 1050C based on

assumption that motor have temperature rise of 800Caccording to class B insulation and ambient temperature is250C according to IEC [3] and IEEE [2] standards.

1) Conductor length calculation: Conductor length ofwindings can be calculated as product of one turn length Lwand number of turns in phase wl:

I = Lww1 (3)

WI

T. Nickolayevsky and S. Singer are with the Department ofInterdisciplinary studies, Faculty of engineering, Tel Aviv University, RamatAviv 69978, Israel (e-mail: teddynicgtau.ac.il).

TCZ16a

(4)

(5)Lw = 2(Lfe + Kendbw + 2B)

1-4244-0230-1/06/$20.00 )2006 IEEE

Rcul(105) ::::::::1.33pcu

270

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TABLE ILAMINATION NOMENCLATURE

Symbol Name6 Radial air gapqz1 Area of stator slotqz2 Area of rotor slotbol Stator slot openingbo2 Rotor slot openingbzl Stator tooth widthbZ2 Rotor tooth widthDin Stator inner diameterDont Stator outer diameterh,j Height of stator yoke11,2 Height of rotor yokehz1 Height of stator slothZ2 Height of rotor slotLf, Iron core lengthtzi Stator slot pitchtZ2 Rotor slot pitchZ1 Number of stator slotsZ2 Number of rotor slots

b Y11T(Din + hzl) (6)z

where T is the number of turns per coil, C is the number ofcoils per slot, a is the number of parallel brunches, Kend is theend windings length coefficient, bv is the average coil width,y' is the coil pitch and B is the end adjustment length. For lowvoltage motors with random windings B=10mm should beaccepted.The value of Kend depends on motor poles pair number and

it just empirical number. For low voltage motors with randomwindings which are not tape isolated the values are introducedin Table II according to Kopylov [4].

TABLE IIEND WINDINGS LENGTH COEFFICIENT FOR LOW VOLTAGE RANDOM WINDINGS

(NOT TAPE ISOLATED)2p 2 4 6 >8Kend 1.2 1.3 1.4 1.5

2) Conductor cross-sectional area calculation: Conductorcross sectional area can be calculated as follow:

0.43 AlTC (7)

where 0.43 is a slot fill factor and Al is a slot cross-sectionalarea.

B. Iron-core lossesHysteresis and eddy current losses are the main part of core

losses but not sole. The iron core losses also concludedsurface, flux pulsation and other losses [4-6], but the last onehas been neglected in present article due to it very low relativevalue.

1) Main iron core loss: The classical equation for the mainiron core loss PFemain of motor concludes the core loss per unitmass PBif at fixed magnetic flux density B and frequency fi,iron core weights mzl and mR] of yoke and teethes respectivelyand coefficient ka that consider core loss augment due topunching and other technology processes applied to steel

laminations.

PFemain PB f (50) (lf6J (kazimzI + karlmRI)

mzl = hzlbziZ,LfePfe

MRI = T(Dout-hrl )hrLjfePfe

(8)

(9)

(10)where Pfe is the iron core steel density (pfe=7700Kg/M3).

The value ofPBif can be found in catalogues of non orientedfully processed electrical steel manufacturers. In present paperall calculation was based on M530-50A steel typemanufactured by Surahammars Bruks AB (Sweden). Ratedpower supply frequency for all motors is 50Hz and the peakvalue of B at each part of lamination not exceeds 1.6T. Thus,as per manufacturer catalogue the pBif=pB 650=50 151W/kg.

According to Kopylov [4] the ka factor for stator teeth andstator yoke for motors up to 250kW can be accepted aska,z =1. 8 and karl= 1.6 respectively.

2) Surface iron core loss: The surface iron core loss occursdue to flux density fluctuation resulting from presence of slotsand slot openings. The losses per unit surface of the stator androtor teeth are given in following equations:

ps,A:: 0.kl ~(B0tZ2)2

Ps2 0k2K(10f572 (B0tz1)2

(1 1)

(12)_I-TDin _I-TDin

Z, Z2where pl and Ps2 are the surface per unit losses of stator androtor teeth respectively, kol and ko2 are the factors taking intoaccount unit loss increase due to processing of the stator androtor teeth (kO=kO2=1.4), Bo is the amplitude of the air gapflux density fluctuations.The amplitude of the air gap flux density fluctuations can

be found through the Carter factor kc and the air gap fluxdensity B, according to equation (13). The value of B6admitted as 0.75T in order to simplify and uniformcalculations and analysis.

Bo = (kc - I)B8 (13)

The Carter factor according to Levi [5] accounts for thepresence of slot openings. The Carter factor shows how muchincrease air gap ampere-turns if one of the surfaces has slotopenings and the other one is smooth in comparison with twosmooth surfaces. In our case both stator and rotor surfaceshave slots. Thus, the Carter factor is a product of two Carterfactors, which are calculated for each side separately.

cly( tZ1 53+bho ) (14)

kc2 =(11

tZ2b2 l-1bo2 )j,0 (15)

271


kc = kclkc2 (16)After per unit surface loses was calculated, the overall

surface losses for complete machine can be calculated as inequation (17):

Ps = psl (tz1- bOi)ZILfe + Ps2 (tz2- bO2)Z2Lfe (17)3) Flux pulsation loss: According to Levi [5] in machines

with slots both on the stator and rotor, the flux density in theteeth varies as the relative position of the teeth varies. Thefrequency of the pulsation in stator teethes depend on rotorteeth number while frequency of pulsations in rotor teethesdepend on stator teeth number due to they relative rotation.According to Postnikov [6], the amplitude of flux densityfluctuations in stator and rotor teethes Bpl and Bp2 respectivelycan be found as follow:

Bl =12 (K 1)2tZ1 (18)

Bp2=1 Bz2 t (Kc, 1)2 tZ2 (19)where Bzl and Bz2 are the stator and rotor teeth flux density.As mentioned above rated Bzl= Bz2=1.6T.

The pulsation losses in watts can be obtained fromequations (20) as summation of losses per unit weight of thestator and rotor teeth multiplied by they weight mzl and mz2.

Pp =Jfe A Bpi) mz1 +dfeA BJp2J MZ2

mz2 hZ2bZ2Z2LfePfe (21)

2I/f

fe 6pfe rfe (22)where 5fe is the eddy current loss constant for electrical steel,A is the electrical steel width and rf1 is the resistivity ofelectrical steel. For M530-50A steel A=0.OOSm andrf1=36 10-8 Q m.

4) Total iron core loss: After the all parts of core loss weredetermined, the sum of iron core power loss can be calculated.

Pfe PFemain + Ps + PP (23)

C. Rotor cage power lossAccording to IEC standards [1] and [3] the rotor cage losses

determined as product of electromagnetic power Pelm, whichcrosses the air gap from stator to rotor, and sleep s:

Pcu2 = S Pelm = s.(Pin Pcul Pfe) (24)

D. Mechanical lossThe mechanical losses are the result of friction in the

bearings, windage between stator and rotor and powerabsorbed by cooling fan. For TEFC motors, that have two ormore pairs of poles, according to Kopylov [4] the mechanicallosses can be obtained as in equation (25).

P 13(1 lD )60f Do4Pmech .~out)ylop out

(25)where D,,, is a stator outer diameter (Fig. 1).

E. Additional load lossAdditional load losses are the losses introduced by load in

active iron and other metal parts of other than the conductors.Due to difficulties and complexity of additional load losscalculation and measuring the most of national andinternational standards stay their value as percentage of inputpower. The IEC standard [1] stays that additional load loss isequal to 0.5% of the rated input power.

Padd = 0.005Pin (26)

III. EQUIVALENT MOTOR CIRCUIT PARAMETERS, CONTROLSTRATEGY AND TYPES OF LOAD

Voltage-fed VSD implicates output voltage amplitude andfrequency modulation according to required by load speed andpower. Thus, motor efficiency versus speed changecalculation requires not only load power, but also VSDsupplied voltage, input current and sleep values at any point ofload. This data can be obtained when known type of load,equivalent motor circuit parameters and VSD control strategy.

A. Equivalent motor circuit parameters and they variationswhen itfed by VSD

Elements Rc1, and R'CU2 of motor equivalent circuit (Fig. 2)are represent stator windings (paragraph II) and rotor cageresistances respectively and they don't vary within speedchange. Element R, is an equivalent resistance correspondingto iron core loss and it can be expressed as follow:

3E 2ag

rfe (27)

The air gap emf Eag and iron core losses are the functions offrequency. Thus R, is frequency (speed) dependent element ofequivalent scheme.

Rcul X1 R2' X'cU2

A-11 1 1;10 ,~~a 12 tR2(1-s)fs

Fig. 2. One phase equivalent circuit of induction motor

Element R'2(1-s)ls represents output electro-mechanicalpower PoutE (28), while the output mechanical power or shaftpower Pout calculated according to equation (29):

PoutE = 31'2 R'2 Iss

Pout PoutE-PmechPout = mT

(28)

(29)(30)

272


co2

(1-z

S)p (31)

From equation (30) by substituting (28-29) and (31) thetorque equation of induction motor can be written as follows:

isf 2 0f21~j~fPs3J~R2 1.3(1 - DOU()K D(UI (6042/-T Cf, s 221 - s IOf,

(32)The reactances Xl and X'2 are represents leakages of stator

windings and rotor cage, X>, is a magnetizing reactance. Allreactances are changes proportional to frequency (speed)variations. The nominal value calculation procedure ofelements X1, X'2, X,, and R'2 doesn't introduced in presentarticle due to it complexity and inconvenience. The valueshave been calculated by using Ansoft RMxpert 5.0 software.

B. Types ofload and theypower versus speed relations.Each type of load have its own torque-speed characteristic

T=f('w) and according to equation (31) they can be expressedas function of frequency fi. Most type of loads can beclassified into the three types:1) Constant torque (lift motion): T=const => Pnoto. fi2) Linear torque (piston pump load): T-f=> Pmotorfi3) Square-law torque (centrifugal pump or fan load): T fi

> PmotorfIt should be note that rated load point of each characteristic

corresponds with rated motor power and rated speed.

C. VSD control strategyThe equation (32) has three variables - "2, f' and s.

Frequency change proportionally to speed and irrespective oftorque and power. Therefore "2 and s are the parameters thatshould be controlled in order to receive required torque andpower to load.

In a constant torque loads control strategy is to keep 1'2constant in order to avoid thermal overheating and magneticsaturation of motor [7]. Sleep in this case is a single variablethat depended on output power and speed changes.

In case of variable torque loads sleep s should be kept as

10U_a

80

_8QI-'

E 40w201-

0 10 20 30 40 50

Frequency (Hz)Fig. 3. 1 OkW 1500 rpm motor efficiency versus frequency change forvarious types of loads and control strategies.

constant due to pure efficiency of "'2=const control strategyfor such kinds of load. In Fig. 6 introduced study casedependence of efficiency to frequency (speed) change forlOkW 1500 rpm motor driving constant torque and

centrifugal loads. As it can be seen from the figure, centrifugalload driving by s=cont control strategy have significantefficiency advantage over "'2=const strategy.

IV. LAMINATION GEOMETRY

For better understanding, efficiency calculations carryingout and following comparison and analysis, all otherparameters of motors, than power and sped, has beenequalized as more as possible. For this purpose have beenchosen five motors in IEC frame 315S (Table III). As it can beseen from the table, except equal frame size and torque,studied motors also have equal electrical steel core volume -identical stator outer diameter and core length. Laminationgeometry and core length have been accepted according toEurotranciatura S.p.A. - lamination Manufacturer Company.Motors windings have been designed and calculated by usingRMxpert 5.0 software.

V. RESULTS

In Fig. 4 introduced efficiency versus speed change of1lOkW 1500rpm VSD operated motor that drives varioustypes of load. The constant torque and centrifugal types ofload has similar efficiency versus speed characteristicswhereas linear load have better efficiency characteristic.

In Fig. 5 are represented efficiency-speed curves of twomotors that drives centrifugal load required 44kW at nominalspeed of 600rpm. The 44kW 600 rpm motor operates the loadby VSD in conventional way by reducing speed from nominalvalue of 600 rpm to the lowest required speed. The 55kW1000 rpm motor operates the load in such a manner that motorspeed has been reduced by VSD to 600 rpm by keepingconstant torque characteristic and below 600rpm the motoroperated according to variable torque load strategy. Similarcase curves are introduced in Fig. 6 where 55 kW 750 rpmand 37kW 500 rpm motors operates centrifugal load required37kW at nominal speed of 500rpm. As it can be seen fromfigures, in both cases high speed motor have efficiencyadvantage over the low speed one. Similar results have beenreceived from analogous studies on all introduced in Table IIImotors.

It should be notes, that motor speed can be reducedaccording to constant torque strategy only to half of a nominalspeed. Such limitation introduced in IEC [12-13] andproceeds from the torque limitation for self ventilated motorsin order to eliminate motor overheating.The disadvantage of high speed motor operating on low

speed load is a high nominal current, and as follow highpower loss in cables and increased power rate of VSD. Thisfact can lead to increasing of price of all drive system.

273

_~~~~~~~~~~~~~ .0 .-f Variable torque (pump) load

Constant torqUe loadControl strategy 1 = const

1 \ 8Variable torque (pump) load

If

f Li I


FramePower, kWNo. of polesSynchronous sped, rpmNominal torque, NmStator outer diameter, mmRotor outer diameter, mmRotor inner diameter, mmCore length, mmlamination catalog No. acc. to EurotranciaturaNumber of stator slotsNumber of rotor slotsCoil pitchNumber of coils per slotNumber of turns per coilNo. of parallel branches

96

S94o Constanttorque load5: 92 _*f

t#- _ \ Linea~~~~rqtue loadgo -;~~~~~~~etiua tou loa

300 500 700 900 1100 1300 1500Speed (rpm)

Fig. 4. Efficiency versus speed change of 1lOkW 1500rpm motor forvarious types of loads.

95

92

O86 K R4E 83LSw

80 7

77,150 300 450 600 750

Speed (rpm)Fig 6. Efficiency versus speed change of 55kW 750rpm and 37kW 500rpm motors operated 37kW 500 rpm centrifugal load.

VI. CONCLUSION

Efficiency change of VSD fed induction motors has beenstudied. It was found that high speed motors operated lowspeed pumps and fans have efficiency advantage over lowspeed motors. Proposed motor tradeoff lies in number ofmotor poles choose and overall system efficiency and

95 . .55kW 750rpm Motor

92

80t0

77150 300 450 600 750

Speed (rpm)Fig 5. Efficiency versus speed change of 55kW 750rpm and 44kW 600rpm motors operated 44kW 600 rpm centrifugal load.

machines for traction vehicles)", International ElectrotechnicalCommission, 1972, pp. 25-27.

[2] IEEE Standard 112-1996, "IEEE Standard Test Procedure for PolyphaseInduction Motors and Generators", IEEE standards Board, 1996, pp. 5-10.

[3] Intemnational Standard 61972, "Methods for determining losses andefficiency of three-phase cage induction motors", InternationalElectrotechnical Commission, 2002, pp. 9-23.

[4] I.P. Kopylov, B.K. Klokov, V.P. Morozkin, B.F. Tokarev,"Proyektirovaniye electricheskih mashin", Moscow, Vishaya schola,2002.

[5] E. Levi "Polyphase motors. A direct approach to their design", JohnWiley & Sons, 1984

[6] I.M. Postnikov, " Proyektirovaniye electricheskih mashin", Kiyev, 1960.[7] Bimal K. Bose, "Modern power electronics and AC drives", Prentice

Hall PTR, 2002, pp. 33-39[8] Technical specification 60034-17, "Cage induction motors when fed by

converters - Application guide", International ElectrotechnicalCommission, Third edition, March 2002, pp. 17

[9] Technical specification 60034-25, "Guide for the design andperformance of cage induction motors specifically designed forconverter supply", International Electrotechnical Commission, Firstedition, April 2004, pp. 10-19.

investment.

REFERENCES

[1] International Standard 60034-2, "Methods for determining losses andefficiency of rotating electrical machinery from tests (excluding

274

TABLE IIISTUDIED MOTORS DETAILS

IEC 315S11041500706.9500325115300IEC 315/4.3256048122104

IEC 315S7561000722.8500375115300IEC 315/6.375725610293

IEC 315S558750707.9500375115300IEC 315/8.375726082288

IEC 315S4410600708.6500375115300IEC 315/8.37572606282

IEC 315S3712500716.7500375115300IEC 315/8.325726052286


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Induction Motor Tradeoff for VSD Driven Pumps and Fans.pdf