Synchros are typically used to feedback angular position of rotating equipment. A synchro is essentially a transformer whose primary-to-secondary coupling may be varied by physically changing the relative orientation of the two windings.
The general physical construction of a synchro is much like that of an electric motor. The primary winding of the transformer, fixed to the rotor, is excited by an alternating current. Due to electromagnetic induction, this causes currents to flow in three secondary windings fixed at 120 degrees to each other on the stator. The relative magnitudes of the secondary currents are measured and used to determine the angle of the rotor relative to the stator. The secondary currents can also be used to directly drive a receiver synchro that will rotate in unison with the transmitter synchro.
Relatively few synchros are produced nowadays, and the majority of production output is used for legacy systems. Synchros have largely been replaced by lower cost devices such as Zettlex inductive angle encoders.
Zettlex inductive angle encoders are based on similar induction principles to synchros and hence exhibit similarly reliable and highly accurate operation in harsh environments. However, rather than the traditional transformer constructions, Zettlex inductive angle encoders use an elegant inductive technique incorporating printed circuit boards as their main components. This results in smaller, lighter and less costly devices. Zettlex encoders also differ from synchros in that they do not require any electrical connection to their rotors.