synchro

A synchro is an electrical machine that converts mechanical shaft rotation into a proportional electrical signal, or vice versa. It transmits angular position data from one location to another with high accuracy. The device consists of a stator with three fixed coils arranged 120 degrees apart and a rotor that spins with the measured shaft. As the rotor turns, it modulates the magnetic coupling to each stator coil, producing a three-phase AC output whose amplitude and phase relationships encode the exact shaft angle.

Synchros operate on AC power, typically 400 Hz in aircraft and 60 Hz in industrial systems. The transmitter synchro sends position data; a receiver synchro at a remote location responds by rotating its shaft to match. A control synchro can compare two inputs and produce an error signal proportional to their difference. The accuracy depends on construction quality and excitation voltage stability, but well-maintained synchros resolve angles to within 10 to 20 minutes of arc.

The three stator leads carry signals called the reference phase and two control phases. When a receiver synchro receives these signals, electromagnetic forces develop in its rotor that naturally align it to match the transmitter shaft angle. This self-synchronizing property makes synchros valuable in systems requiring remote position indication without continuous electrical feedback loops. Antenna platforms, gun turrets, radar dishes, and flight control surfaces commonly use synchro pairs to transmit position data across the vehicle.

Practical constraints and failure modes

Synchros require careful mechanical coupling and bearing support; misalignment introduces friction that degrades accuracy and can damage bearings. Temperature changes shift the electrical characteristics and mechanical clearances. Salt spray and moisture corrode the slip rings and bearings, causing erratic output or complete failure. The name reflects the electrical principle: the rotor seeks synchronism with the stator field, pulling itself into alignment like a magnetic compass needle.

Synchros have been largely superseded by resolvers (which have only two stator windings and better linearity) and by digital shaft encoders in new designs. However, many installed systems remain in service, and synchro technology remains viable for high-temperature, high-reliability applications where solid-state electronics are unsuitable. Understanding synchro operation is essential for troubleshooting older industrial machinery and aerospace systems.