Blocked Rotor Test on Induction Motor


     This test is similar to short circuit test on a transformer. The purpose of   this is to determine the total resistance ( R01 ) and reactance ( X01 ) of the motor as referred to stator.

Blocked Rotor Test  on Induction Motor


     The rotor, in this test is prevented from rotating by a brake and therefore it is called blocked rotor test. The induction motor in this case acts like a short circuited transformer.



Process of Blocked Rotor Test : 


     A low voltage 3-phase supply ( obtained from a 3-phase auto transformer ) is applied to the stator. So that full load current flows through the stator winding. The power input is measured by the two wattmeters. The short circuit current corresponding to the rated voltage can be calculated by direct proportion.


     In the blocked rotor test, it voltage applied to the stator terminals must be low otherwise it can leads to damage of  the stator winding. In this test, the low voltage is applied so that the there rotation of rotor and motor draws a full load current through the stator winding.



Calculation of Blocked Rotor Test :


     When rotor is blocked, slip s = 1 and the approximate equivalent circuit referred to stator is shown below.

Blocked Rotor Test  on Induction Motor

Let,

  Wsc  =  Total power input 

  Vsc =  Per phase values of applied voltage and  

   Isc   =  Per phase values of applied current under blocked rotor conditions.


Then power factor,


Total resistance as referred to stator,


Total reactance as referred to stator,


The curves from the blocked rotor test by varying the voltage is shown below.

Blocked Rotor Test  on Induction Motor



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