Correct option is C
To analyze the behavior of a salient-pole synchronous motor under the given conditions, let's examine each statement carefully:
Initial Condition:
o The salient-pole synchronous motor is running clockwise at no-load.
o If the field current is reduced to zero, the excitation is lost, and the motor will lose synchronism. This means the motor will no longer operate synchronously and may start behaving like an induction motor (if it has damper windings) or may simply stop.
Reversing Field Current:
o When the field current is reversed, the polarity of the rotor magnetic field is reversed.
Now let's look at the options:
(a) The motor would stop:
If the motor loses synchronism when the field current is reduced to zero, it might stop. However, this does not consider what happens when the field current is reversed. So this option is partially correct but not entirely.
(b) The motor would run in the reverse direction:
Reversing the field current will reverse the direction of the rotor’s magnetic field. However, for the motor to run in the reverse direction, it would need to lose synchronism and then get re-synchronized in the opposite direction, which typically doesn't happen spontaneously.
(c) The motor slips by one pole-pitch and continues running at synchronous speed:
When the field current is reversed, the motor will experience a torque reversal. In this scenario, the rotor might slip by one pole pitch and continue running at synchronous speed in the same direction. This is a more plausible outcome since the field reversal would realign the rotor with the stator field after a slip.
(d) The motor slips by one pole-pitch and speed would be less than synchronous speed:
This is not likely because if the motor is running synchronously and the field is reversed, it will either realign to continue running synchronously or stop. Running at a speed less than synchronous speed indicates asynchronous operation, which isn’t typical for synchronous motors without external control mechanisms.
