Correct option is C
1. Synchronous motor is not self-starting:
True. Synchronous motors are not inherently self-starting because the rotor needs to be brought up to synchronous speed by some external means before the synchronous torque can develop.
2. If the rotor poles are excited by field current and the stator terminals are connected to AC supply, the motor vibrates:
True. When the rotor is not at synchronous speed, the motor can experience vibrations because the rotor is not synchronized with the rotating magnetic field of the stator.
The statement that "if the rotor poles are excited by field current and the stator terminals are connected to AC supply, the motor vibrates" is true because, at standstill or non-synchronous speeds, the interaction between the stator's rotating magnetic field and the stationary or differently rotating rotor causes pulsating torques, leading to vibrations. These vibrations are typically mitigated by using starting methods that bring the rotor up to synchronous speed smoothly.
3. Variable frequency supply is used to start synchronous motor:
True. Using a variable frequency supply is a common method to start a synchronous motor, as it allows the motor to gradually reach synchronous speed without the need for an additional starting mechanism.
4. By using a frequency converter, a synchronous motor can be brought from standstill to its desired speed:
True. A frequency converter (or variable frequency drive) can smoothly bring a synchronous motor from standstill to its operational speed by gradually increasing the frequency of the supply.
