Definition
An alternating current (AC) electric motor whose rotor turns at exactly the same speed as the rotating magnetic field produced by the stator. The rotational speed is locked to the frequency of the supply current and the number of poles in the motor, so it does not slow down under load until it stalls.
Plain English
An AC motor that always spins at a fixed speed set by the frequency of the electricity feeding it. As long as the load stays within its limits, the speed never drifts.
Context Anchor
Seen in aircraft electrical and instrument system descriptions, especially where a steady, timing-based motor speed is important.
Derivation
Synchronous comes from the Greek syn- (together) and chronos (time). Together it means 'happening at the same time.' The motor's rotor and the supply current's magnetic field move together in time, which is exactly what makes the speed constant.
Why Pilots Care
Several aircraft instruments and clocks rely on synchronous motors because their accuracy depends on a rotational speed that stays exactly proportional to AC frequency. If the AC frequency drifts, anything driven by a synchronous motor drifts with it.
Analogy
A synchronous motor is like a person marching exactly with a drumbeat. As long as they stay with the beat, their pace is steady and predictable.
Intuition Check
Synchronous does not just mean “happening at the same time” in a loose sense. Here it means the motor’s rotation is locked in step with the timing of the AC power.
Example Sentence 1
The aircraft's electric clock uses a synchronous motor, so its accuracy depends on the regulated frequency of the AC bus.
Example Sentence 2
Technicians bench-test the synchronous motor in the autopilot servo to confirm it maintains exact RPM across the voltage range.