Resolver (electrical)
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The resolver is a type of rotary electrical transformer that is used for measuring the angle of a rotating machine such as an antenna platform. The primary winding of the transformer, fixed to the rotor, is excited by a sinusoidal electric current, which by electromagnetic induction causes currents to flow in two secondary windings fixed at right angles to each other on the stator. The relative magnitudes of the two secondary currents are measured and used to determine the angle of the rotor relative to the stator.
Resolvers designed for terrestrial use tend to be driven at 60 hertz (the mains frequency in many countries), while those for marine or aeronautical use tend to operate at 400 hertz (the frequency of the on-board generator driven by the engines).