How a Synchronous Clock Works

Abstract

A synchronous electric clock is driven by a synchronous motor, and how this works is not obvious. The speed at which a synchronous motor rotates is determined precisely by its design and the frequency of the AC supply, hence the use of the adjective synchronous. If a running synchronous movement is examined all that can be seen is a rotating rotor which drives the hands through the reduction gear. In British made synchronous clocks the motor often, but not always, rotates at 200 rpm. Electric motors, including synchronous electric motors, are driven by magnetic forces produced by interacting magnetic fields. These magnetic fields are invisible. By contrast, a mechanical clock has an escapement which can be seen operating. How a synchronous motor works is described in detail, including the magnetic impulses which drive the motor. A synchronous motor is not inherently self starting. In a non self starting clock there is a manually operated device which runs the motor up to the synchronous speed in the correct direction, whereas in a self starting clock this is done automatically. Self starting methods rely on manipulation of magnetic fields, and two methods used in synchronous clocks are described. Designs of reduction gear used in synchronous movements vary widely. The striking and chiming work used in some synchronous clocks is described. The operation of a synchronous time switch is described.