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Alternating current

The Italian inventor Galileo Ferraris invented a polyphase AC induction motor in 1885. The idea was that two out-of-phase, but synchronized, currents might be used to produce two magnetic fields that could be combined to produce a rotating field without any need for switching or for moving parts. Other inventors were the American engineers Charles S. Bradley and Nikola Tesla, and the German technician Friedrich August Haselwander. They were able to overcome the problem of starting up the AC motor by using a rotating magnetic field produced by a poly-phase current. Mikhail Dolivo-Dobrovolsky introduced the first three-phase induction motor in 1890, a much more capable design that became the prototype used in Europe and the U.S. By 1895 GE and Westinghouse both had AC motors on the market. With single phase current either a capacitor or coil (creating inductance) can be used on part of the circuit inside the motor to create a rotating magnetic field. Multi-speed AC motors that have separately wired poles have long been available, the most common being two speed. Speed of these motors is changed by switching sets of poles on or off, which was done with a special motor starter for larger motors, or a simple multiple speed switch for fractional horsepower motors.