Importance of Blocked Rotor Test on Motors

A blocked rotor test is conducted on an induction motor. It is also known as short circuit test, locked rotor test or stalled torque test.

From this test, short circuit current at normal voltage, power factor on short circuit, total leakage reactance, and starting torque of the motor can be found.

The test is conducted at low voltage because if the applied voltage was normal voltage then the current through the stator windings would be high enough to overheat the windings and damage them.

The blocked rotor torque test is not performed on wound-rotor motors because the starting torque can be varied as desired. However, a blocked rotor current test is conducted on squirrel cage rotor motors.

Process of Testing of Blocked Rotor Test of Induction Motor

In the blocked rotor test, it should be kept in mind that the applied voltage on the stator terminals should be low otherwise normal voltage could damage the winding of the stator.

In block rotor test, the low voltage is applied so that the rotor does not rotate and its speed becomes zero and full load current passes through the stator winding.

The slip is unity related to zero speed of rotor hence the load resistance becomes zero. Now, slowly increase the voltage in the stator winding so that current reaches to its rated value.

At this point, note down the readings of the voltmeter, wattmeter and ammeter to know the values of voltage, power and current. The test can be repeated at different stator voltages for the accurate value.

Process of testing blocked rotor test of induction motor
Source: https://www.electrical4u.com/

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