IEEE Instrumentation and Measurement Technology Conference Ottawa, Canada, May 19-21, 1997

A Practical Set-up for a Standard Test Procedure on Polyphase Induction Motors

P. Van Roy, B. Renier, K. Hameyer, R. Belmans

Kardinaal Mercierlaan 94, B-3001 Heverlee, Belgium. Katholieke Universiteit Leuven, E.E. Dept., Div ESAT/ELEN

Phone ( + 132-1 6-32 10 20 e-mail: ronnie. belmans@esat. kuleuven.ac. be

Abstract - The paper describes a practical set-up for testing low voltage, polyphase induction motors. The measured efficiency of an induction motor depends on the method and the standard according to which the measurements are performed. A short description of different standards as IEEE 112 and IEC 32 is given and the main discrepncies are mentioned. The measurement equipment for electrical and mechanical quantities and the employed data aquisition system are discussed. Reference is made to industrial measurements. Experimental results are discussed in detail.

I . INTRODUCTION

Three phase, low voltage squirrel cage induction motors are commenly used in industry. In the past, they were mainly directly connected to the grid. Nowadays, they are more and more, and especially for new installations, converter-fed. With the increasing emphasis on energy conservation and the growing energy prices, efficiency has become very important. Both total efficiency and the efficiencies of the different components of the drive system are important. In that way, it can be seen were improvement is possible.

Efficiency values of induction motors given by the manufacturer are measured or calculated according to a certain standard. For induction machines, two important standards are the IEEE standard 1 12-1 991 and the IEC standard 34-2. Several national standards are harmonized with regard to one of those above. In most of the European countries, the standards are harmonized to IEC 34-2. Efficiency values obtained from different testing standards can differ by several percent as will be shown in the measurement results.

Fax ( + 132 16-32 19 85 www.esat. kuleuven.ac. be/elen/elen. html

The increasing attention to efficiency and the number of efficiency tests according to one of the appearing standards have forced the set-up of a semi-automatic measuring system.

If the system has to be used in the laboratory as well as in industry, the equipment must have as little interference with the drive as possible. In order to measure also the efficiency of other parts in the drive, it is possible to handle other signals as e.g. pressure and delivery in a pumping installation.

II. STANDARDS FOR EFFICIENCY MEASUREMENTS

The methods for efficiency measurements can roughly be divided into two categories: direct and indirect methods. The main difference between them is that in the direct methods the torque has to be assessed.

Two different standards are discussed: IEEE standard 1 12 and IEC standard 34-2.

A. IEEE standard I 12- 199 I

The IEEE 112-1191 consists of five basic methods to determine the efficiency: A, B, C, E and F.

In method A, the input and output power is measured and the efficiency is directly obtained. This method is only used for very small machines.

Method B is also a direct method, input and output power are measured, but also a separation of the losses is performed. The following losses can be defined (s = slip, corrected to specified temperature):

Pstator: stator 12R loss corrected to the

0 Pf,+,: friction and windage loss, as derived specified temperature

from the no-load test

207 0-7803-3312-8/97/$5.0001997 IEEE

0 P,,,,: core loss, as derived from the no-load test, for the appropriate voltage Protor: rotor 12R loss = (measured stator input power - stator 12R loss - Core loss) . s

0

0 Pstray: stray-load loss

From the apparent total loss, the stator and rotor 12R loss, the core loss and the friction and windage loss are subtracted to obtain the stray-load loss in that particular point. To reduce the influence of the measuring error, a linear regression versus the torque squared is made of the stray-load losses at different loads.

There the additional stray-load losses are zero at zero load, the values are corrected by:

A good measurement is defined if the correlation coefficient of the linear regression is larger than 0.9.

Method B is recommended for the testing of induction machines up to 180 kW.

Method C is a back to back machine test. The total stray-load losses are also obtained via a separation of losses for both motor and generator operation. Proportional to the rotor currents, the stray-load losses are then divided between the motor and generator.

Method E and method E l are indirect methods. The output power is not measured. In method E the stray-load losses are directly measured using the reverse rotation test. In method El , the stray-load losses are set to an assumed value. These values are given in table I as a percentage of the rated output power.

In method F and F1, the equivalent circuit of the machine is elaborated. The stray-load losses are again directly measured or in the case of F1 an assumed value is used.

There exist also some additional methods as e.g. the use of the equivalent circuit but calibrated at a load point.

Motors with ratings larger then 180 kW can be tested using methods B, C, E, E l , F or F1.

TABLE I ASSUMED VALUES FOR THE STRAY-LOAD LOSSES A S A

PERCENTAGE OF THE RATED OUTPUT (IEEE)

Motor rated power Stray-load losses relative t o the output power

0.750 - 90 kW 91 - 375 kW 376 - 1800 kW

1800 kW and larger

1.8 Yo 1.5 Yo 1.2 % 0.9 %

B. IEC 34-2

The efficiency measurement of a polyphase induction machine is performed determining the overall losses of the induction machine. The overall losses can be obtained from 'the summation of the separated losses or from the measurement of the overall losses. Overall loss measurements according to IEC 34-2 can be made by:

0

calibrated machine test

0

However, the method preferred by IEC 34-2 is the determination of the separated losses. Regarding stator and rotor conductor losses no corrections for the temperature of the windings are made.

Very important is the amount of the stray-load losses. These are assumed to be 0.5 percent of the rated input power for motors at rated load. They are also assumed to vary as the square of the stator current. The assumed percentage value does not depend on the motor rated power.

It is generally known that the amount of stray- load losses can be several percent. This will also be shown by measured results. The efficiency values in the catalogues according to IEC are thus overestimated. When comparing the 0.5 % to the values of table I , the stray-load losses using the IEEE standard are larger than those using the IEC. The IEC efficiency is thus larger when compared to the IEEE value.

braking test with torque measurement

mechanical back to back test electrical back to back test

Ill. MEASUREMENT SET-UP

The measurement set-up is shown in Fig. 1. The following measuring tools are used:

1 , Current probes Fluke 801-1 000s 2. Power Analyzer VOLTECH 3000A 3. Torque transducer Vibrometer TE11 1 1OOONm 4. Encoder with 60 pulses per rotation.

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The major features of the Voltech Power Analyzer 3000A are:

Voltage range: 0.5 to 2000 V pk current range: INT 0.05 to 200 A pk, EXT 6.25 mV to 2.5 V pk frequency range: DC and 0.1 Hz to 500 kHz accuracy: 45 Hz - 450 Hz: 0.05%, 1 HZ - 500 kHz: 0.1 %

The Power Analyzer is connected to the computer, using an IEEE 488.2 interface. The computer is equipped with a National Instruments IEEE interface board of type AT-GPIBTTNT, which allows a maximum data transfer rate of 1.5 M bytes/s.

In order to make a direct measurement, the mechanical output power has to be measured. This means measuring the torque. In the laboratory a standard high accuracy torque transducer is used, with contact-less transmission of the torque signal.

The torque transducer and other measuring devices for e.g. delivery and pressure in a pipeline, give analog signals. To protect the computer against possible overvoltages, an isolated amplifier is developed, using an Analog Device AD202 component. This component guarantees a galvanic separation between input and output. On the same circuit board, jumpers and trimming resistances were integrated, to allow a choice of amplification factors and a precise setting of both amplification factor and offset voltage.

To limit the possible effect of electromagnetic noise originating in e.g. frequency converters, the torque signal, which is a voltage signal, is transformed to a current signal.

,- motor r 3 ,- load

. . . . . . . . . PM3000A /_....._______._.........________. .................................... ,

IEEE 488.2 interface

3-phase sinusoidal supply

Fig.1. Experimental set-up.

The rotational speed is measured with an encoder which gives 60 pulses per rotation. The signal is converted to an optical signal, again to reduce emc problems. A PCB is developed to protect the AD card from overvoltages in the absence of a signal, by lowering the output voltage in this situation.

The actual AD conversion is integrated on the National Instruments AT-MIO-16X data acquisition board inside the computer. The main features of this card are:

e

e

e

e

e

e

e

e

A

number of analog inputs: 8 in differential mode, 16 in single-ended mode sampling rate: 100 ksamples/sec max. input voltage: +/- 35 V (powered on), +/- 20 V (powered off) input impedance: 1 GR in parallel with 50 pF offset error (max.): +/- 1.22 mV gain error (max): 0.0244 % of reading dynamic - slew rate: 5.0 V/msec bandwith: 650 kHz (gain = 1 ) number of counters - timers: 3 independent 16 bit counters/timers input voltage: min. -0.7 V - max. 7 V

LabVlEW program is developed which allows measurements using either one or two power analyzers, a counter and up to 5 analog inputs. For the counter and the other analog inputs, offset and scaling can be programmed. For the power analyzer, a choice between two and three watt-meter method is possible. Next to the Voltech power analyzer, a Yokogawa 2533 power analyzer can be used. This power analyzer uses the IEEE 488.1 standard. The settings for the power analyzer can be programmed.

The program measures V, A and W in either fundamental mode or total mode. When measuring the input to a PWM inverter, the PWM function of the Voltech power analyzer can be used. This guarantees the detection of the low frequency components as well as the high frequency components. From the torque and the speed, the mechanical power is calculated. Measuements can be made in individual points, or in a continuous series of points. The time in betweerr successive measurements can be programmed. Depending on the number of signals to be measured, the minimum time between measuements varies from 1.5 to 8 seconds. The program allows either a schematic display with immediate efficiency calculations, or a graphical display of 4 input signals. This allows the

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observation of e.g. input power during warming-up of the motor.

Measurements can be made with the motor directly connected to the grid. In such a case only one power analyzer is necessary. When supplied by a converter, using two power analyzers, it is possible to measure the total efficiency, as well as the motor and converter efficiency separately.

IV. MEASUREMENTS IN INDUSTRY

The equipment for measuring the electrical quantities is the same as in the laboratory. The torque however, is measured using strain gauge technology. During a stand-still, strain gauges are glued on the motor shaft and connected in a four arm bridge. A transmitter, which is mounted to the shaft provides the excitation voltage for the bridge and measures the deflection of the bridge. The signal is then frequency modulated (FM) and transmitted to a standing antenna. The energy for the rotating transmitter is provided via an inductive coupling using the same antennas at the same frequency as the carrier of the FM signal.

From the deflection of the bridge, the torque is obtained according to 131.

(3) 1

Tshaft = 2 E A * wp

where c L G=-

2( 1 -k v)

nd 16

wp =-

4 UA UB

E A =--

(4)

(5)

(6)

Experimental results on a standard 75 kW motor will be discussed as example. Results from IEEE standard efficiency calculation and IEC calculation will be compared.

Motor data: 7 5 k W 4 0 0 V A 1 4 0 A 50 Hz

0 990 r/min PF=0.82

A. Stator resistance

The reference stator resistance a t an ambient temperature of 27.8 OC was measured for all three windings. The average value was 34.40 ma. During warming-up, the stator resistance is measured every half hour, and compared to the reference resistance. This allows the temperature rise to be determined, using the temperature coefficient a (7):

R T ~ = R ~ l ( 1 +CL.@) (7)

Stator resistance after warm-up was measured as 45.45 ma. With a temperature coefficient of 0.003931 "C" for copper this leads to a temperature rise of 91.6 "C. This is consistent with class F duty.

B. Start-up current

The start-up current a t nominal voltage and no- load was recorded with a digital Le Croy 9304 oscilloscope (Fig. 2). The maximum peak value was 1 166 A, yielding a relative start-up current of 5.9. - ,

and

E : elasticity modulus v : Poisson's coefficient of the steel d : diameter of the shaft k : strain coefficient of the gauges UA : deflection of the bridge UB : excitation voltage

V. EXPERIMENTAL RESULTS

In the laboratory machines up to 100 kW at 1500 r/min can be tested. The normal test procedure to define the IEEE efficiency consists of no-load test, resistance measurement and load test. Extra tests can include start-up current measurement, temperature rise test and locked-rotor test.

7-Jun-96 9:09:42

F+qq-TTL] . . . . . . . . .,..I.. .... ...IL.. . . . . . . . . . . . , , . I . , . .

Fig.2. Start-up current.

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C. Short-circuit test F. IEC-method

This test was performed with locked rotor, and gave the results as mentioned in table II.

D. No-load test

Voltage, current and power are measured with disconnected load at various voltages. From the curve of input power minus stator I ~ R loss versus voltage squared (Fig. 31, core loss and friction and windage loss may be determined. Test results at nominal voltage are given in table Ill.

E. Load test

Load tests at 100 %, 75 %, 50 % and 25 % are perfomed.

The results are given in table IV. The direct efficiency value, calculated as shaft power versus input power, is given, as well as the IEEE standard efficiency.

It can be seen that in this case the IEEE-standard efficiency value equals the direct value.

TABLE II

SHORT-CIRCUIT TEST.

U,, [VI Isc [AI P,, [WI PF I 1 92.9 144.2 6644 0.29

3000

2000

P,I - 1 .5I2R 1 000

VI

0 [ 40000 80000 1 20000 160000

Voltage squared [V2]

The IEC norm 34.2 and 34.2A prescribes a stray- load loss of 0.5 % of input power a t full load, and proportional to the square of the relative current at other loads. The stray-load loss can be higher than this value.

The results with this method are given in table V. It can be seen that the stray-load losses as calculated are lower than those calculated according to the IEEE standard. The calculated efficiency is accordingly up to 1.3 % higher.

TABLE IV EFFICIENCY MEASUREMENT FOLLOWING IEEE STANDARD

Load [%I 25 50 75 100 U [VI 402.6 398.7 404.4 399.3 I [Af 69.0 89.4 116.9 148 Pin [Wl 22080 41280 61180 81220 PF [ I 0.46 0.67 0.75 0.79 T [Nml 184.2 363.2 543.9 724.5 n Ir/min 998 996 994 991

Pout [WI 19249 37878 56620 75189 slip [%I 0.002 0.004 0.006 0.009 Ploss [WI 2831 3402 4560 6031 Pstator [WI 324 544 932 1492 Pcore [WI 1609 1553 1635 1562 Protor [WI 40 157 352 703 Pfr+w IWI 762 762 762 762 PSI [WI 94 385 879 1511 Pin/Pout [YO] 87.2 91.8 92.6 92.6

linear regression on stray-load losses (IEEE-I 12): A: 0.002903 correlation coefficient: 0.999

Psl.corr. [W] 98 383 859 1524 PJoss.corr. IWI 2835 3399 4540 6044 Poutsorr. [Wl 19245 37881 56640 75176 Pout/Pnom [YO] 25.7 50.5 75.5 100.2 Eff. [%I 87.2 91.8 92.6 92.6

1

TABLE V

EFFICIENCY MEASUREMENT FOLLOWING IEC STANDARD

Load U I Pin Pout Pslsorr. Ploss.corr Pout.corr. PoutIPnom Eff.

25 402.6

69.0 22080 19249

88 2825

19255 25.7 87.2

50 398.7

89.4 41 280 37878

148 31 64

381 16 50.8 92.3

75 404.4 11 6.9

61 180 56620

253 3934

57246 76.3 93.6

100 399.3

148 81 220 751 89

406 4926

76294 101.7 93.9

Fig.3. No-load characteristic.

21 1

VI CONCLUSIONS

It has been shown that the efficiency value of an induction motor is not unequivocal, but depends on the standards used. The European IEC gives almost always an overestimate of the efficiency. According to IEC, motors from different manufacturers have the same efficiency, while in fact, there are differences, that are determined by the stray-load losses. The extend of these losses depends on the design of the machine and the quality of the manufactoring proces. The value is larger than the 0.5 percent of IEC. All this leads to the conclusion that one should work towards a worldwide uniform standard for the testing of electrical machines.

Due to the constant improvement in the accuracy of measurement equipment and especially of torque transducers, the indirect method should whenever possible be abandonned in favor of the direct method as IEEE 112 - B.

ACKNOWLEDGMENT

The authors are grateful to the Belgian F.W.0.-V. "Fonds voor Wetenschappelijk Onderzoek - Vlaanderen" for its financial support of this work and the Belgian Ministry of Scientific Research for granting the project IUAP No. P4/20 on Coupled Problems in Electromagnetic Systems.

REFERENCES

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[31

[41

[51

[61

[71

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[91

"Rotating electrical Machines - Methods for determining losses and efficiency of rotating electrical machines from tests," IEC Std 34- 2:1972. W. Nurnberg, R. Hanitsch, "Die Prufung elektrischer Maschinen," Springer-Verlag, 1 987. "Handbook of electric motors", ed. by Richard H. Engelmann, William H. Middendorf, Marcel Dekker Inc, 1995. K. Hoffman,"Eine Einfuhrung in die Technik des Messens mi t De h n u n g sm essst rei c h en, " Hottinger Baldwin Messtechnik, Darmstadt R. K. Muller, "Mechanische Grossen elektrisch gemessen,"Kontakt & Studium, Band 45, Mess- und Pruftechnik A.A. Jimoh, R.D. Findlay and M. Poloujadoff, "Stray losses in induction machines: Part I, Definition, Origin and Measurement," IEEE Trans. on Power Apparatus and Systems, Vol. PAS-I 04, No. 6, pp 1500-1 505, June 1985. N. E. Nilsson, " A comparison of ANSI and IEC standards for power station polyphase induction motors," IEEE Transactions on Energy Conversion, VOL 1 1 , No. 3, pp 500-507, September 1996. Gerald G. Gray, Walter J. Martiny, " Efficiency testing of medium induction motors, a comment on IEEE Std 112-1991, " IEEE Transactions on Energy Conversion, VOL 1 1 , No. 3, pp 495- 499, September 1996.

[l l "IEEE Standard Test Procedure for Polyphase Induction Motors and Generators," IEEE Std 1 12- 199 1 , IEEE Power Engineering Society, New York, NY.

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