Synchronous motor

Article By:

Smith, Richard T. Consultant, San Antonio, Texas.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.674900

An alternating-current (ac) motor which operates at a fixed speed proportional to the frequency of the applied ac power supply (commonly known as the synchronous speed). A synchronous machine can be operated as either a generator or a motor (and sometimes as a source of reactive power, fulfilling the function of a capacitor), depending only on its applied shaft torque (whether positive, negative, or zero) and its excitation. There is no fundamental difference in the theory, design, or construction of a machine intended for any of these roles, although certain design features are stressed for each of them. Operating within an electric power system, the machine may change its role depending on the prevailing power conditions. As a result, it is preferable to set up a common general theory for synchronous generators, motors, and capacitors. The distinction between generator and motor is merely a difference in the direction of the currents and the sign of the torque angles. See also: Alternating-current generator; Alternating-current motor

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