Position control using variable exciter feed through

DETAILED DESCRIPTION We claim: 1.
A motor generator system comprising: a prime mover for driving an output shaft; a main motor generator, having a rotor driven by the output shaft, for producing electrical power on a plurality of stator phase windings in response to the prime mover rotating the output shaft and for driving the output shaft in response to operation as a motor; an exciter, having a rotor driven by the output shaft, for applying excitation to a field winding of the main motor generator during operation for producing electrical power on the plurality of stator phase windings and for applying current to the field winding of the main motor generator to produce a magnetic field in the field winding during operation of the main motor generator as a motor; a time varying signal source for applying a time varying signal having a fundamental frequency to a field winding of the exciter; a main inverter, responsive to a rotor position signal representing a rotary position of the rotor of the main motor generator, for commutating application of current to at least one of the plurality of stator phase windings to control operation of the main motor generator during operation as motor while simultaneously maintaining one of the plurality of stator phase windings in a non-commutated state in which no current is caused to flow therein by the main inverter; and circuitry for demodulating an amplitude modulation of voltage across the non-commutated phase winding wherein the amplitude modulation of voltage has a frequency twice the fundamental frequency and is representative of the rotary position of the rotor of the main motor generator and producing the rotor position signal in response to the demodulated amplitude modulation of voltage to control commutation of current in at least one of the plurality of stator phase windings.
2.
A motor generator system in accordance with claim 1, wherein the circuitry comprises: a frequency doubler, responsive to the time varying signal source, for outputting a synchronization signal having a frequency of twice the fundamental frequency; and a synchronous detector, coupled to the synchronization signal and to the phase winding maintained in a non-commutated state, for detecting the amplitude modulation of the voltage at twice the fundamental frequency