What characterizes the operation of a synchronous motor?

• A. It can operate at variable speeds
• B. It operates at a constant speed synchronized with supply current frequency
• C. It requires no external power source
• D. It is never synchronized with the power supply frequency

Answer: (B)

A synchronous motor operates at a constant speed that is synchronized with the frequency of the supply current. This means that the motor's rotor rotates at the same speed as the rotating magnetic field produced by the stator, which is determined by the supply frequency. The relationship between the supply frequency (f) and the synchronous speed (N_s) is given by the formula N_s = (120 * f) / P, where P is the number of poles in the motor.

This characteristic allows synchronous motors to be used in applications where precise speed control is vital. Once the motor is started and brought up to synchronous speed, it maintains this speed regardless of the load, provided that the load does not exceed the motor's capability.

In contrast, some of the other characteristics associated with losing synchronism or operating under varying load conditions do not apply to synchronous motors. They are not intended to operate at variable speeds like an induction motor, and while they may require an external source for excitation in certain types, they still fundamentally operate based on the synchronization with the supply frequency, which defines their efficiency and effectiveness in specific applications.