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For use with an adjustable speed a-c electric motor comprising a stator having effective direct and quadrature axes and a rotor spaced from the stator by a gap, said stator including windings adapted to be connected to a source of excitation of variable frequency and amplitude, a scheme is provided

For use with an adjustable speed a-c electric motor comprising a stator having effective direct and quadrature axes and a rotor spaced from the stator by a gap, said stator including windings adapted to be connected to a source of excitation of variable frequency and amplitude, a scheme is provided that comprises means for deriving a "torque angle" feedback signal representative of the phase angle between current and flux that interact in the motor to develop electromagnetic torque tending to move the rotor relative to the stator, in combination with means responsive to the torque angle feedback signal for controlling the source of excitation so as to control the frequency of stator excitation as a function of the torque angle feedback signal. In one aspect of the invention, means is provided for controlling the source of excitation so as to control the amplitude of stator excitation as a function of a variable command signal that is prevented from deviating below a predetermined limit. In another aspect of the invention, the torque angle feedback signal deriving means comprises phase discriminating means arranged to produce a signal representative of the complement of the electrical phase displacement between first and second periodic signals, the first periodic signal being synchronized with the fundamental component of the actual flux produced across the stator-rotor gap in a predetermined one of said stator axes and said second periodic signal being synchronized with the fundamental stator winding current in the other stator axis.

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