Why do synchronous motors require a separate DC power supply for the rotor?

Synchronous motors require a separate DC power supply for the rotor primarily to maintain magnetic field strength. In these motors, the rotor is equipped with field windings that require direct current to create a strong and stable magnetic field. This magnetic field interacts with the rotating magnetic field produced by the stator, allowing the rotor to synchronize with the stator's rotation.

A stable magnetic field is essential for the synchronous operation of the motor, enabling it to run at a constant speed, which is determined by the frequency of the AC power supply, regardless of the load. By supplying DC to the rotor windings, the motor can sustain the required magnetic field strength, thereby ensuring efficient operation and consistent performance.

While other factors such as overheating, speed control, and maximum load operation are important considerations in motor performance, the primary reason for the necessity of a separate DC power supply in synchronous motors is to establish and maintain the robust magnetic field needed for synchronization with the stator.