Which of the following statements about synchronous motors is true?

Synchronous motors are indeed capable of functioning as both motors and alternators. This characteristic is due to their ability to synchronize with the supply frequency, allowing them to maintain a constant speed regardless of the load. As motors, they use electrical input to produce mechanical output, while as alternators, they can convert mechanical energy back into electrical energy when driven above synchronous speed. This dual role is significant in applications where both power generation and motor functions may be required.

In contrast, the other statements do not accurately reflect the nature of synchronous motors. For instance, they are not self-starting; typically, they need an auxiliary method or some other means to bring them up to synchronous speed before they can operate effectively. Additionally, synchronous motors do require a power supply to operate, as they utilize electrical energy to establish their rotating magnetic field necessary for functioning. They also cannot operate without an external load; although they can run unloaded, they are designed for applications that closely match their load requirements to maintain efficiency and operational stability.