No Load Test of Induction Motor


     This test is similar to the no-load test performed on a transformer. The purpose of this test is to determine the no-load losses, no-load power factor, and constants Rand Xo of  the equivalent circuit.


     The no-load losses include the frictional windage loss, core loss, and a small copper loss in the  stator winding. As the slip at no-load is very small and negligible, the rotor copper loss is negligible as rotor current at no-load is very small and neglected.

No Load Test of Induction Motor

     The figure above shows the connection diagram and equivalent circuit of the motor at no-load. The normal voltage is applied to the stator winding and the rotor is allowed to rotate freely without any mechanical load on its shaft. Two wattmeters W1 and  W2 measure the power input to the motor.

No Load Test of Induction Motor

     Since the motor is not supplying any load, the no-load current is small and stator copper losses are negligible. The input power, Wo equals mechanical and core losses. To be more accurate the stator copper loss at no-load current should be deducted from the input when calculating the above losses.



Calculation of No-Load Test of Induction Motor :


Let ,

     W =  Power input 

     V =   Applied voltage per phase  

      Io  =   Applied current per phase under no-load conditions. 


Then the no-load power factor is ,

Where ,

        IW    =   Working component of no-load current Io
   
        Iµ   =   Magnetising component of no-load current Io


The no-load losses for different voltages my be plotted as, 

No Load Test of Induction Motor

     The below table helps you to note down the readings while performing the no-load test on the induction motor. Which makes it easier for the calculation of no-load losses and power factor.

S.No.

Stator Voltage (V1)

Stator Current (Io)

Rotor Current (Ir)

Wattmeter Readings
   W1         W2

Power Input (Wo)

Readings oberved from the test



    But there is a thing you have to remember while performing the no-load test on induction motor, i.e., changes in speed, power factor, stator current, rotor current due to fluctuations in applied voltage. When there is changes ( fluctuations ) in the applied voltage the readings noted from the test will be more accurate. Hence it also increases the accuracy of the calculations made from the test.


i. While performing a no-load test, the speed of an induction motor obviously remains constant during the test. As the speed of an induction motor changes when there is a heavy load on the motor. Therefore, here the applied voltage should remain constant so that the speed of the motor will remain unchanged.


ii. When there is an increase in the applied voltage to a larger value, the stator current that as to supply magnetising component of the stator winding will also increase. Thus, there we can see a fall in power factor.


iii. The torque of an induction motor is proportional to the rotor current and flux generated by the stator winding. Here flux in an induction motor will depend upon the amount applied voltage to the stator. When the applied voltage is kept minimum the torque generated will remain constant, but the rotor current will go on increasing.

 

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