Wednesday, 29 June 2016

Why not Blocked Rotor Test by Keeping Induction Motor Rotor Winding Open?

As discussed in the previous post Blocked Rotor Test of Induction Motor, the rotor is prevented from rotating and then a small voltage is applied to the stator sufficient to pass the rated current through the stator winding and then leakage impedance of Induction Motor is calculated.

The question may arise that why don’t we keep the rotor winding of Induction Motor open to prevent rotor to rotate and then perform Blocked Rotor Test?

The answer is quite simple. If the rotor of the induction motor is kept open, then no current will flow in the rotor. Consequently the rotor copper loss would be zero. The rotor would not rotate even if it is not blocked. Hence the power measured would not be equal to the total copper loss and therefore the wattmeter reading cannot be used to determine the equivalent resistance referred to the stator.

In short, there is no point in doing the Block Rotor test on an induction motor with its rotor kept open.

If we try to pass rated current through the stator winding with the rotor open, then we will observe the stator voltage required is much greater than the rated stator voltage. Thus we are likely to damage the stator winding insulation and may burn the stator winding.

Thank you!

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