A 3.3 KV, 900 KW, 3000 RPM (50 Hz) double cage motor with DOL start driving a centrifugal fan. Whenever the input voltage dips by 5 to 10% (due to starting other motors), this motor speed drops and stator current increases to twice the normal current. The motor does not regain speed nor does the current drops to normal value even after the voltage rises to the normal rated value. Because of high stator current, winding temperature shoots up. Stator winding is healthy. What could be the possible reasons for such a drop in speed and increase in current when the voltage dips (which is unavoidable)? Any method to check the condition of the double cage rotor winding ? Visibly no cracks are seen in the rotor cage windings.
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