What is a Synchro? Is it related in any way to a stepper motor?

The term “synchro” is an abbreviation of the word “synchronous.” It is the name given to a variety of rotary, electromechanical, position-sensing devices.

A SYNCHRO is a motor like device containing a rotor and a stator and capable of converting an angular position into an electrical signal, or an electrical signal into an angular position. A Synchro can provide an electrical output (at the Stator) representing its shaft position or it can provide a mechanical indication of shaft position in response to an applied electrical input to its stator winding.

Synchro systems were first used in the control system of the Panama Canal, to transmit lock gate and valve stem positions, and water levels, to the control desks. It is commercially known as a selsyn or an autosyn.


Synchros can be thought of as “variable transformers” .When an AC voltage applied to the rotor shaft winding, it causes a change in the synchro’s Stator output voltage. In its general physical construction, it is much like an electric motor. The primary winding of the transformer, fixed to the rotor, is excited by a sinusoidal electric current (AC), which by electromagnetic induction causes currents to flow in the three star-connected secondary windings fixed at 120 degrees to each other on the stator. The relative magnitudes of secondary currents are measured and used to determine the angle of the rotor relative to the stator, or the currents can be used to directly drive a receiver synchro that will rotate in unison with the synchro transmitter. In the latter case, the whole device (in some applications) is also called a selsyn (self and synchronizing). When several synchros are correctly connected, all of the rotors will align themselves in the same angular position. This is useful, since when the angular position of one synchro is forced to change, it can drive another synchro to indicate the angular change.

With their rugged construction and high reliability, Synchros have been used since World War II as the “angle” transducer of choice for Military, Space and Aviation applications, where only the best will do.

The relation between a synchro and stepper motor is that the stepper motor is just a special type of the synchro. A stepper motor is designed to rotate through a specific angle (called a step) for each electrical pulse received from its control unit.






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