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PRZEGLD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 2b/2013 103

Miralem HADISELIMOVI1,2, Ivan ZAGRADINIK2, Bojan TUMBERGER1,2

University of Maribor, Faculty of Energy Technology (1)University of Maribor, Faculty of Electrical Engineering and Computer Science (2)

Induction Machine: Comparison of Motor and GeneratorCharacteristics

Abstract . The aim of this paper is to present the difference of induction machine electromechanical characteristics in motor and generator operatingmodes. Firstly, the paper presents analytically derived equations, which are based on equivalent circuit and are used for calculation of inductionmachine breakdown torque. Secondly, a comparison of calculated and measured characteristics in motor and generator operating modes is

presented. Results of torque characteristics measurements, temperature rise tests and load tests are also provided. Finally the difference betweenthe calculated and the measured results is commented on and explained in detail.

Streszczenie. Celem artykuu jest zaprezentowanie rnicy charakterystyk elektromechanicznych maszyny indukcyjnej pracujcej w trybie silnika igeneratora. W oparciu o model matematyczny i wyniki bada przedstawiono charakterystyki momentu, temperatury i obcienia. (Maszynaindukcyj na porwnanie charakterystyk w try bie generatora i silnika)

Keywords: induction machine, motor, generator, analytical calculation, measurementsSowa kluczowe: maszyna indukcyjna, silnik, generator

Introduction

In some electrical drives the induction machine canoperate as a motor and as a generator (for example:elevators, electric vehicles, pumped storage power plantsetc.). In such drives the key question is which nominalpower of the induction machine is the right one. Namely, thenominal mechanical power written on the nominal plate ofan induction motor is not the same as in a generator modeof operation. The reason lies in the shape of torquecharacteristics of the induction machine. Breakdown torqueis not the same in motor and generator modes. Normallythe generator breakdown torque is two to four times greaterthan the motor breakdown torque. The slope of torquecharacteristics is therefore also not the same in bothmodes. For the same torque value the generated rotor

losses in the generator mode of operation are significantlylower than in the motor mode of operation. Consequently,the nominal power of the same induction machine in thegenerator mode of operation can be significantly higherthan in the motor mode of operation.Engineers and researches use different models andtechniques for designing electrical machines which includemagnetically nonlinearities of iron core [1, 2]. Ordinarily theyuse numerical or analytical approach, sometimes both ofthem [3-5]. This work provides an explanation of theanalytical approach for the calculation of inductionmachine's characteristics in motor and generator modes ofoperation. This paper focuses on key equations for thecalculation of breakdown torque based on induction motorsequivalent circuit [4, 6] and the calculation of all electricaland mechanical values at nominal power.

Induction machine torque characteristics theoreticalbackground

Induction machines can operate in different modesaccording to the torque characteristic. There are threeknown operating modes: breaking mode for slip valuesbetween and 1, motor mode for slip values between 1and 0, and generator mode for slip values between 0 and . All three operating modes are as part of torquecharacteristic shown in Figure 1.During the operation of the induction machine power lossesgeneration is reflected in temperature rise, which can bedifferent for motor and generator modes of operation.

Direction of energy flow and the corresponding Sankeydiagram, including power losses presentation, are shown inFigure 2.

In the process of designing an induction machine it is very

important to calculate precisely the electrical andmechanical values in the breakdown torque point. The lattercan be calculated with equation (1), which is deducedbased on the equivalent circuit [6] of the induction motor(Figure 3):

(1)2

s sb

2

m p UT

D

where the operator enables the calculation of thebreakdown torque in the motor mode (the + sign) and in thegenerator mode (the sign), sm is the number of stator

winding phases, p is the number of pole pairs, sU is the

stator winding voltage, is the angular frequency and thedenominatorD represents:

(2)

s 2 r 1 s 1 r 2

2 2

s 2 r 1 r

( - ) ( )

( - ) ( )s

D R X X X

R X X X

In equation (2) the sign dictates the solution of thequadratic equation and the local extremes of the torquecharacteristic, where elements

1 2, and represent the

correction factors for the calculation of the breakdown point:(3)

1 s 11 d

(4)2 1 1

a b

(5) 2 21 2

In equation (3)s is the factor of stator leakage, described

by (6):

(6) ss

m

X

X

and 1 1,a b and 1d are the corresponding proportionsobtained from the equivalent circuit:

(7) s1

Fe

Xa

R

(8) s1

m

Rb

X

(9) s1

Fe

Rd

R

Equation (1) and the corresponding formulas of elementsfrom equations (2) to (9) enable an analytical calculation ofbreakdown torque values.

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104 PRZEGLD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 2b/2013

Tested induction machineIn this paper the small squirrel cage induction machine withnominal power 5.5 kW and shaft height of 132 mmaccording to IEC was used. The induction machine wasdesigned for a small bio-gas power plant where a gas motoris used as a turbine. The tested machine works in powerplant as induction generator and is water-cooled with thetemperature of water between 50 and 60 degrees Celsiusand a corresponding flow of approximately 8 l/min. The

water cooling system is spiral shaped and built into the

stator housing. The induction machine is designed and builtfor an F insulation class. Thus a standard stator and rotorlamination with a combination of 36/48 slots made bylamination manufacturer Kienle + Spiess GmbH (IEC 132-41248) is used as the basis for the construction. Figures 4aand 4b present the shapes of the stator slot and the rotorslot respectively. The stator and rotor lamination are madefrom steel material M700-50A, which has 5.7 W/kg ofspecific core losses at 1.5 T.

Rotor rotates in directionof the rotating field

Generator Motor Break

Generator takeselectrical powerfrom the grid

Generatingelectrical power

in the grid

sElectrical power

2 1 0 1 2

s

Reversing point

Torque,Air-gap powerP

Mechanical power

Rotor rotates against directionof the rotating field

Ideal no-load point

Fig.1. Induction motor torque and power characteristics

Fig.2. Energy flow Sankey diagram: a) motor mode, b) generator mode

sI

sU

sR sjX

mjX

rjXrI

FeImI

FeRrR

s

mU

Fig.3. Induction motor equivalent circuit

a

360

QS

BNS

BSS

JN=12

36;

2,5

16,9 mm;0,6 mm;

mm;

1 3,88 mm;

2 2,71 mm;

QS

BSS

HUSHSS

R SR S

b

BSR

360

QR

JN2=0

48;

1,1 mm;

0,4 mm

16,2 mm

1 1,78 mm

2 0,88 mm

;;

;;

QR

BSR

HSR

HUR

R R

R R

Fig.4. a) Cross-section of stator slot, b) cross-section of rotor slot

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PRZEGLD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 2b/2013 105

Table I. Values in the motor mode, power 5.5 kW, voltage Y 400 V and winding temperature 95 CI(A) cos 1(min )n N (Nm)T b (Nm)T loss (W)P

11,18* 0,905 0,785 1420 37 88 150010,63 0,909 0,821 1419 37 89 1200

Remark: * measured results

Table II. Values in the generator mode, power 5.5 kW, voltage Y 400 V and winding temperature 95 CI(A) cos 1(min )n N (Nm)T b (Nm)T loss (W)P

9,95* 0,801 0,825 1556 41,3 365 11409,49 0,845 0,864 1560 39,3 233 875

Remark: * measured results

Comparison of measured and calculated resultsAll presented measurements in this paper were

performed by classical induction machine measurementsystem shown in Figure 5. It consists of tested inductionmachine mechanically connected by clutch with active loadmachine, which one is speed controlled by converter viaposition sensor. Tested induction machine torque ismeasured via force sensor by reaction method. All electricaland mechanical values of tested induction motor aremeasured by power analyzer which ones via GPIB interfacesend measured results to personal computer whereanalysis of measured data is performed.

Power analyzer

NOKIA

Autotransfromer

F 1 F 2 F 3 F 4 F 5 F 6

NORMAPOWERANALIZERD6100

I

U

P

2.20

230.9

0.750

A

VkW

Inductionmachine

Active break

GPIBinterface

Multifunctioninterface

Positionsensor

INDRAMAT

RAC 4

A1 1501

AC- MAINSPINDLE DRIVE

Converter

Measurementamplifier

100,00 Nm

Forcesensor

Thermocouplesinstruments (J-type)

Personalcomputer

Fig.5. Measurement system

For temperature measurement the J-typethermocouples with corresponding instruments were used.Distribution of the thermocouples inside and on the surfaceof the induction machine housing is shown in Figure 6.

T2T3

Tair

T5 T6

T1 T4

TwouTwin

T7T1 first phase stator slot

T2 first phase end windingT3 second phase end windingT4 second phase stator slotT5 side A end bellT6 side B end bellT7 housingTwin inlet waterTwou outlet waterTair air

Fig.6. Measurement of temperatures on motors housing

Figure 7 shows the measured and the calculated torqueand current characteristics of the tested induction machinewith nominal power 5.5 kW. From the presentedcharacteristics it is evident that the breakdown torque in thegenerator mode is greater than the breakdown torque in the

motor mode. From the measurements we obtain 88 Nm (at 1050 rpm) breakdown torque of the machine in the motor

mode and in the 365 Nm (at 2170 rpm ) breakdown

torque in the generator mode, i.e. with slip 0,3s or

0,47s . It turned out that the measured breakdowntorque in the generator mode is about 4 times larger than inthe motor mode.

0

10

20

30

40

50

60

70

80

90

100

0 500 1000 1500 2000 2500 3000

I(A)

n (1/min)

Icalc

Imeas

Fig.7. Measured and calculated current characteristics

350

300

250

200

150

100

50

0

50

100

0 500 1000 1500 2000 2500 3000

T(Nm)

n (1/min)

Tcalc

Tmeas

Fig.8. Measured and calculated torque characteristicsTo calculate the characteristics of the three-phase

induction machine, we used the emLook software, whichenables "click calculation" of characteristics. The calculationsoftware package uses a standard equivalent circuit(Figure 3) stated in IEC 60034-28 as a basis. Themeasurement and the calculation (with emLook) results forthe machine are provided in Table I for the motor operationmode and in Table 2 for the generator operation mode.

A comparison of the measurement and the analyticalcalculation data shows a significant difference between thetwo in the generator breakdown torque. The measuredbreakdown torque in the generator operating mode is about

4 times larger than in the motor operating mode. Ananalytical calculation of the breakdown torque with equation(1) for equivalent circuit (Figure 3) however returns only a

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106 PRZEGLD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 2b/2013

2.6 times higher value for the generator than for the motor.There are several reasons for the much higher measuredvalue of the breakdown torque bT than the calculated value

(Table II). The main reason is that the analytical calculation(emLook) in generator mode does not correctly take intoaccount the real reduction of leakage reactance for thebasic harmonic of the magnetic field. This is due the highercurrent in the rotor, which has as a consequence higher

local saturation in slot bridges. The motor thus has themeasured current in the breakdown point Ib 36 A, and thegeneratorIb 96 A; the increase is about threefold. Theleakage reactance is the same in the analytical calculationfor motor and for generator both in the nominal and thebreakdown operation points. This is the main reason for thedifference between the measured and the calculatedresults. Another increase of the breakdown torque ingenerator operation mode is the consequence of theinfluence of higher harmonics, which are being deduced ingenerator operation, i.e. added to the negative torque of thebasic harmonic for the generator, thus absolutely increasingit.

0

10

20

30

40

50

60

70

80

90

100

0:00 0:10 0:20 0:30 0:40 0:50 1:00 1:10

T(K),Tair(C),I(A)

t(h:mm)

Tair(C)

I(A)

T1

T2

T3

T4

T5

T6

T7

Fig.9. Results of temperature rise test in motor operation mode:

nominal power 5,5 kW (mechanical output power)

0

10

20

30

40

50

60

70

80

90

100

0:00 0:15 0:30 0:45 1:00 1:15 1:30 1:45 2:00

T(K),Tair(C),I(A)

t(h:mm)

Tair(C)

I(A)

T1

T2

T3

T4

T5

T6

T7

Fig.10. Results of temperature rise test in generator operationmode: thermal power 6,6 kW (electrical output power)

Because of a much higher torque characteristic ingenerator operation mode the losses of the machine arelower at the same output power. At the same load currentand the same cooling conditions the output power ingenerator operation mode is for the presented machine6.36 kW or by 15.6% higher than in the motor operationmode with 5.5 kW of power. The thermal power was

approximately 6.6 kW at the incoming water temperature 50C and flow of 8.3 l/min for F insulation class. Results oftemperature rise tests for motor and generator operatingmode are presented in Figure 9 and 10.

ConclusionThe paper presents an analytically equation for

calculation of induction machine breakdown torque in motorand generator operating modes. Special emphasis is given

to the comparison of machine characteristics in motor andgenerator modes, including a comparison of calculated andmeasured results for load conditions and torquecharacteristics. The differences between the calculated andthe measured results are commented on and explained indetail. From calculated and measured results is obviouslyseen that the induction machine is able to operate ingenerator mode with much higher power than in motoroperating mode. Reason lies in higher efficiency and lowerpower losses of generator which is reflected as lowerheating in comparison with motor.

The main focus of future research will be the calculationof torque characteristics and magnet circuit and equivalentcircuit parameters of an induction machine with finite

element method in order to improve the analyticalapproach.

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and taking into account change of steel losses in inductionmotors in process of their aging, Proceedings of InternationalIEEE Conference EUROCON 2009, Saint Petersburg, 2009, pp790-795

[2] V. Prus, A. Nikitina, M. Zagirnyak, D. Miljavec, Research ofrnergy processes in circuits containing iron in saturationcondition. Przeglad Elektrotechniczny, 2011, Vol. 87, Issue 3,pp 149-152

[3] P. Virti, Variations of Permanent Magnets Dimensions in AxialFlux Permanent Magnet Synchronous Machine, PrzegladElektrotechniczny, 2011, Vol. 87, Issue 12b, pp 194-197

[4] P. Virti, J. Avsec, Analysis of coreless stator axial fluxpermanent magnet synchronous generator characteristics byusing equivalent circuit, Przeglad Elektrotechniczny, 2011, Vol.87, Issue 3, pp 208-211

[5] Kent R. D., Analytic Analysis of Single- and Three-PhaseInduction Motors, IEEE Transactions on Magnetics, 1998, Vol.34, No. 5, pp 3721-3727

[6] M. Mlakar, M. Hadiselimovi, B. tumberger, I. Zagradinik,Determination of equivalent circuit parameters for three-phaseinduction motors: procedure in accordance with the iec 60034-28 standard, Journal of Energy Technology, May 2011, Vol. 4,Issue 2, pp. 31-46

[7] M. Zagirnyak, D. Mamchur, A. Kalinov: The theory andapplication of the induction motor diagnostic methods based onelectrical signal analysis, Journal of Energy Technology, May2012, Vol. 5, Issue 2, pp. 37-50

[8] Zagradinik I., Hadiselimovi M., Ritonja J., Slemnik B., TheemLook software package for the analytical and numericalanalyses of electrical machines, Prz. Elektrotech., 2010, Vol.85, Issue 12, pp 175-178

Authors: Miralem Hadiselimovi, Bojan tumberger University ofMaribor, Faculty of Energy Technology, Hoevarjev trg 1, SI-8270Krko, Slovenia; Ivan Zagradinik University of Maribor, Faculty ofElectrical Engineering and Computer Science, Smetanova 17, SI-2000 Maribor, Slovenia; E-mail: miralem.h@uni-mb.si.