When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected

Solution (By Examveda Team)

The correct option is "it runs as a reluctance motor at the same speed".

In a salient pole synchronous motor, torque is normally produced by the interaction between the stator rotating magnetic field and the rotor field created by DC excitation.

If the excitation is suddenly disconnected while the motor is running at no load, the rotor loses its field excitation. However, due to the salient pole construction, the rotor still experiences a torque known as reluctance torque, which arises because the rotor tends to align itself along the path of minimum reluctance of the stator field.

This reluctance torque is enough to keep the motor running in synchronism with the stator field, but only under light or no-load conditions. The machine in this case operates as a reluctance motor and continues to run at the same synchronous speed.

Why not the others?

"The motor stops" is incorrect, because reluctance torque prevents it from stopping immediately when excitation is lost at no load.

"It runs as a reluctance motor at a lower speed" is incorrect, because synchronous machines always run at synchronous speed, not below it.

"None of the above" is incorrect, because Option B correctly explains the behavior.

Therefore, the correct answer is it runs as a reluctance motor at the same speed.