In What Situation that an Induction Motor can be Accidentally Become a Generator?



The induction motor can contribute current to the faulted location during short circuit condition. Especially for a large induction motor, electrical designers should never neglect its contribution in sizing the exact rating of the protective device. It represents a small but important value that is needed to determine the maximum short circuit current available and thereby establishing the short circuit rating of electrical equipment. Regardless of the size or voltage rating of a motor, it can be demonstrated that motor contribution is present during a fault. 

During Normal Operation

During normal operation, a motor converts electrical energy into mechanical energy. Current flowing in the stator produces a rotating magnetic field with the poles facing toward the rotor. This rotating magnetic field induces a current into the rotor. A magnetic field with the poles facing out is produced in the rotor due to the stator induced current. This causes the rotor (motor shaft) to rotate. As long as the stator is supplied to a stable voltage supply, the motor shaft will continue to rotate. 

During Short Circuit Condition


During a short circuit condition, the system voltage will decay. A stable voltage supply no longer exists. The rotating magnetic field in the rotor will attempt to support the reduced voltage condition by becoming a power source.
At this moment the induction motor will behave as a generator and will contribute current to the fault location (see fault point 4).

Motor Contribution to Faulted System

The amount of current that the induction motor can contribute is equivalent to its locked rotor current which is 400% to 600% of motor FLA. (ANSI standard C37.010 [1]).

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