8/6/2019 Testing of Electrical Motors at Variable Temperatures

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IntroductionGerman students already know: "Wer misst, misstMist!” Meaning: Those who take measurements,measure mistakes". In fact it is not quite as bad.The readings are usually correct and if all relevantparameters are respected, one can lead also

differing test results together. In the following theimportance of the knowledge of the often-neglected motor temperature is shown whendetermining the characteristics of electric drivesand what errors may occur in case of disregard.

1. Test for Electrical Drives

1.1 Conventional Procedures. The performance data of electric motors aredetermined in the majority of all cases by a braketest. Power input is calculated from current and

voltage. Torque and power output are measuredwith a brake. It is important to reach for each pointa persistent temperature. This may take manyhours and also means that every point of a torquecurve can have a different temperature. However,very often the motor temperature is not consideredat all. In most of the suppliers’ data on their motorsno temperature is mentioned for which the quotedperformance is valid. Some methods are on themarket with a speedy way of testing. In such acase the test temperature is completely unclear. Itis neither related to an operational temperature,nor is it constant.

1.2 M.E.A. Testing System  For several years an innovative motor tester ofM.E.A. Testing Systems Ltd. is on the market.This test system avoids any temperature confusionby testing a motor at constant temperature overthe entire speed range. During the test procedure,the motor is freely accelerated from 0 to no loadspeed. The load of the motor is only thepredetermined inertia of the rotor. From the

moment of inertia and the acceleration the torqueof the motor is calculated and also the poweroutput over the whole speed given. Since theentire measuring time depending on the motor sizeis usually in less than one second the motor hasno time to warm up. It remains at room tempera-

ture during the measurement or at a temperatureto which it was heated before starting the test. Insuch a way motor characteristics at constanttemperature can be obtainedThe method is very accurate, test results are very

well reproducible, regardless of the operator andindependent of any brakes, especially of theiradditional mechanical losses. The method clearlyshows the influence of the temperature on motorperformance and efficiency.

2. Comparison of Test Methods

2.1 Three Phase Induction Motor  If it comes to comparing tests on one and thesame motor with different test methods, or evendifferent examiners or test institutes, there willalways be differences. Usually the test method isimmediately made responsible for the deviations.Everyone has more confidence in the system he isused to work with. Other systems will bequestioned and are suspect of delivering wrongresults. In principle all measurements are usuallycorrect, but obtained under differing conditions.One of the often neglected influences is the motortemperature. One has to be aware of this and willthus be able to interpret differing test results.An example from the practice may explain this.

Fig. 1 shows the torque of a 3phase inductionmotor of 0.25 kW at 400 V determined by a M.E.A.system at different stator temperatures. Also thesame motor was measured with a Magtrol -dynamometer, and the results of the manufactureron a brake dynamometer are shown. It is alwaysthe same identical motor.MEA have tested at 3 different windingtemperatures of 20, 57 and 94 ° C. Thetemperature was determined by the M.E.A.MotorHeat process by means of the changingstator winding resistance.

The temperature dependent resistance changeleads to a different current drawn by the motorduring the free acceleration. A temperatureincrease from 20 to 94 ° C yields in a current

 

M.E.A. Testexample

Testing of Electrical

Motors at VariableTemperatures 

8/6/2019 Testing of Electrical Motors at Variable Temperatures

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reduction of about 20% (Fig. 2) and thus leads tocorresponding reduction in power input, torqueand power output of also 20 %. The efficiency of3phase induction motors remains independent ofthe temperature. As mentioned, the M.E.A. testsystem executes this whole measurementprocedure in less than a second. The enginetemperature can therefore be considered as aconstant. The dynamometer torque curves of themanufacturer were probably taken in thetemperature range 60 to 85 °C. The control

measurement with the Magtrol – system started at20°C. During the test the motor assumed atemperature of approximately 55 to 57°C. Theexample shows which different results may beobtained when the temperature is neglected. If themeasuring time is extended on the brake testbench the motor heats further up and the result willbe different again. With each test system identicalresults could be achieved by running the motor atthe same temperatures. Interestingly the efficiencyof 3 phase motors is independent of thetemperature. Without knowing that the same motorwas tested with different temperature conditions,one might start probably irrelevant considerationsand actions

Fig. 1: Torque vs. speed of a 3-phase induction motor 0.25 kW, 400 Vat different temperature conditions

Fig. 2: Current to Fig. 1 at various stator temperatures 

What does this mean? This means thatmanufacturers’ motor data without indication of themotor temperature are meaningless.If the motortemperatures during the test are neglectedarbitrary results may be achieved. Checks ofmotor users for a certain application - againwithout taking into account the temperature - maylead to flagrant deviations.3. Conclusion 

It is evident that tests at differing motortemperatures may lead to deviating results. Theinfluence seems to be widely unknown to motoruser in their specific applications andevensometimes to motor manufacturers. The newIEC Standard 60034-2-1 creates a certain clarity.Firstly, a coolant temperature of 25 ° C shall apply.For variations, there is a correction formula. Themotor shall be brought to persistent operationtemperature in the load point.With the M.E.A. test system it is possible to

measure a motor at constant winding temperature.

This may be ambient room temperature or anyelevated temperature. When the desired

temperature is reached, the motor is accelerated

and the readings are taken. In case a brake is used

for heating the motor the brake has to be

deactivated. Torque charts of a motor as a function

of temperature can be obtained in such a way. It is

important to have the exact information on the

temperature test conditions.

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