The correct answer is C. excitation winding gets disconnected.
A synchronous motor is an AC electric motor that runs at a constant speed regardless of the load on the motor. The speed of a synchronous motor is determined by the frequency of the AC power supply and the number of poles in the motor.
The excitation winding is a DC winding that is used to create a magnetic field in the motor. The magnetic field from the excitation winding interacts with the magnetic field from the stator winding to create a rotating magnetic field. The rotor of the motor aligns with the rotating magnetic field and turns at the same speed as the magnetic field.
If the excitation winding is disconnected, the magnetic field from the excitation winding will disappear. The rotor will no longer be aligned with the rotating magnetic field and will stop turning.
The other options are not correct because they will not cause the synchronous motor to stop.
- Option A: Supply voltage fluctuates. If the supply voltage fluctuates, the speed of the synchronous motor will also fluctuate. However, the motor will not stop.
- Option B: Load in motor varies. If the load in the motor varies, the speed of the synchronous motor will also vary. However, the motor will not stop.
- Option D: Supply voltage frequency changes. If the supply voltage frequency changes, the speed of the synchronous motor will also change. However, the motor will not stop.