Electric Clock - Synchronous Electric Clock

Synchronous Electric Clock

A synchronous electric clock does not contain a timekeeping oscillator like a pendulum, but instead relies on the oscillations of the AC utility current from its wall plug to keep time. It consists of a small AC synchronous motor, which turns the clock's hands through a reduction gear train. The motor contains electromagnets which create a rotating magnetic field which turns an iron rotor. The rotation rate of the motor shaft is synchronized to the utility frequency; 60 cycles per second (Hz) in North and South America, 50 cycles per second in most other countries. The gear train scales this rotation so the minute hand rotates once per hour. Thus the synchronous clock can be regarded as not so much a timekeeper as a mechanical counter, whose hands display a running count of the number of cycles of alternating current.

One of the gears turning the clock's hands has a shaft with a sliding friction fitting, so the clock's hands can be turned manually by a knob on the back, to set the clock.

Synchronous motor clocks are rugged because they don't have a delicate pendulum or balance wheel. However, they are vulnerable to power outages. A power interruption will cause the clock's hands to stop moving, so they will be behind the correct time when the power resumes. Many synchronous clocks have a "power interruption indicator" on their face; a colored disk operated by an electromagnet turns red if the power has been interrupted. Turning the setting knob resets the indicator.