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The variable speed constant frequency alternator system provides accurate frequency control independent of the shaft speed. The method employed is completely electrical in contrast with the more conventional mechanical speed control systems. The principle of operation depends upon a constant angular velocity magnetic field in the alternator armature, the angular velocity being the vector sum of the mechanical velocity of the shaft and the velocity of an electromagnetic rotating field induced by the excitation of a polyphase field winding. The polyphase field winding is driven by a variable frequency polyphase exciter, its frequency being directly proportional to the speed deviation from synchronous speed of the alternator rotor and shaft. The frequency control system as described in this paper is the open-loop type and, as such, accomplishes absolute frequency control with no errors arising from load or speed transients. An experimental brushless, constant frequency, three-phase alternator is discussed.