Motor contribution to fault current

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designer82

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I know that if a motor is running and there is a short circuit, that motor will spin in reverse and contribute to the fault.

What if there's a motor that's off and not running. Will it start, run backwards and contribute to the fault also?
 
No. Also, the motor doesn't spin in reverse to contribute to the fault. A motor contributes to the fault current because it has a coil of wire spinning in a magnetic field that takes a bit to slow down.

If you oversimplify the system, a motor and a generator are the same device. The only difference is where the power is applied and where it is used in the system. A generator applies mechanical force to spin a shaft, which turns a coil in a magnetic field, and electricity is taken out to be used by the system. A motor applies electricity to turn a coil in a magnetic field, and mechanical force is taken out through the spinning shaft to be used by the system. Same device, same components, same forces. The only difference is the way the energy is flowing through the system.
 
designer82 said:
I know that if a motor is running and there is a short circuit, that motor will spin in reverse and contribute to the fault.

What if there's a motor that's off and not running. Will it start, run backwards and contribute to the fault also?
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The direction of the motor rotation does not change. The inertia of the motor and the load keeps the motor spinning and turns it into an induction generator supplying additional fault current into the fault for a very short time.
 
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