Abstract: Stability in operating a synchronous stepping motor in mid-frequency is achieved by monitoring instantaneous power to the motor and adjusting the phase of the motor drive waveform as needed to maintain uniform speed. In an embodiment in which a pulse width modulator responds to a reference signal waveform, the phase of the reference signal waveform is varied to adjust the instantaneous power delivered to the motor.

Abstract: A drive circuit for an inductive load such as a synchronous motor responds to an analog signal representing a desired load current and an actual load current and selectively applies first and second potentials to the load. The circuit includes transistor switches for selectively connecting the load to the potentials in desired polarity and diode circuits for maintaining current flow through the load when the transistor switches are opened, thereby decaying the load current.

Abstract: A control signal for the drive of a synchronous motor is synthesized in response to direction of rotation and speed digital input signals. The synthesizer includes up-down counters for counting the speed input signal and a toggle circuit tracks the repetitive count of the counters. A digital to analog converter converts the count to an analog current. Distortion of the analog current can be provided to match the motor characteristics and to compensate for temperature variations. The distorted analog current is applied to an operational amplifier which has a non-linear feedback circuit including serially connected resistors and serially connected diodes with each resistor shunted by a diode to limit voltage drop thereacross. A multiplexer receives the output from the operational amplifier and provides the output control signal.