A 40-kva 400-cycle aircraft alternator

ALTERNATING current has replaced direct current in large commercial power systems because large amounts of power can be transmitted long distances more economically; alternating voltages can be stepped up or down at will by a transformer; and the polyphase squirrel-cage induction motor, which employs no brushes, is so simple and reliable. These advantages, which also apply in aircraft use, have caused a long-continued interest in a-c systems.
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While this interest is particularly strong now, it is to be remembered that the use of a-c systems in aircraft dates back to World War I when wind-driven alternators with built-in spark gap were used to supply power for radio transmitters. More than ten years ago 600-watt alternators driven by a main engine through a constant-speed drive, were used by the Navy. Prior to the present war two different a-c systems were tried experimentally in different planes: one of these was a single-phase 800-cycle system, the other, a three-phase 120-volt 400-cycle system. Rectified a-c systems with 30-volt d-c outputs of 200–800 amperes have also been proposed.
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