Polyphase coil

Polyphase coils are electrical coils (phases) connected together in a polyphase system such as a generator or motor. In modern systems the number of phases is usually three, or a multiple of three. Each phase carries a sinusoidal alternating current whose phase is delayed relative to one of its neighbours and advanced relative to its other neighbour. The electrical angles between the phases are usually equal, and always add up to 360 degrees. For example, in a three-phase system, the phases are separated from each other by 120°. The result of such an arrangement is a rotating magnetic field that is used to convert electrical power to rotary mechanical work, or vice versa.

Compared to single-phase motors and generators, polyphase coils are simpler, because they do not require external circuitry (using capacitors and inductors) to produce a starting torque.

Like all coils used in electrical machinery, polyphase coils (made from insulated conducting wire) are wound around ferromagnetic armatures with radial projections and maximum core-surface exposure to the magnetic field.

The use of polyphase coils in electrical power systems was pioneered by the engineer Nikola Tesla.