Motors and Motor Protection
Synchronous motors are used for a small number of unique applications but are not nearly as prevalent as asynchronous motors. A synchronous motor operates at only synchronous speed. Motors defined as "asynchronous" operate at a speed somewhat below synchronous speed. The squirrel cage induction motor is a simple and rugged motor design. It is free of the slip rings found in wound rotor motors. The term "motor controls" encompasses a number of devices. In critically important applications, small motors may warrant a higher degree of protection. Motors may be protected by a variety of ancillary devices including protective relays or merely interlocks in the control circuit. Protective relays can detect specific problems related to an electrical circuit and provide electrical signals that can be interlocked with the control circuit. In addition, external interlocks are frequently entered into a motor's control circuit. The specific interlocks of course depend on the application.