Induction Motor Protection (Part5)
Abnormal Operating Conditions from Mechanical Side (Part1)
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Bearing failure or rotor jam causes excessive load torque on the motor. This is reflected in the increase in stator current. In order to discriminate between rotor jam and other operating conditions that can also cause over-current, the high current is not recognized as a jam condition unless the motor has reached its rated speed and the current is in excess of 20% of full load persisting for at least twice the locked rotor time setting.