Explain why a synchronous motor is not self-starting.

A synchronous motor is not self-starting because it requires a rotating magnetic field to be present in order to start rotating. In a synchronous motor, the rotor must rotate at the same speed as the rotating magnetic field in order to maintain synchronism.

When the motor is not running, there is no rotating magnetic field present to induce a current in the rotor windings and create the necessary torque to start the motor. Therefore, an external force, such as a starting mechanism or an auxiliary winding, is needed to initially rotate the rotor and create the necessary magnetic field for the motor to start running.

Once the motor is running and the rotating magnetic field is established, the motor will continue to run in synchronism as long as the frequency of the power supply remains constant.

A synchronous motor is not self-starting because its rotor speed is directly proportional to the frequency of the AC power supply. When the motor is first connected to the power supply, the rotor is at rest and the stator field rotates at the synchronous speed. The rotor field, however, is not synchronized with the stator field and therefore does not experience any torque.

In order for the motor to start, an external torque must be applied to the rotor to bring it up to the synchronous speed. This torque can be provided by a starting winding or a damper winding.

Once the rotor is at the synchronous speed, the rotor field will lock into step with the stator field and the motor will continue to run at the synchronous speed.