A
synchro is, in effect, a
transformer whose primary-to-secondary coupling may be varied by physically changing the relative orientation of the two windings. Synchros are often used for measuring the angle of a rotating machine such as an
antenna platform. 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 an
alternating current, which by
electromagnetic induction, causes currents to flow in three Y-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 may be called a
selsyn (a
portmanteau of
self and
synchronizing).