Importance

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Abstract

Apparatus is provided comprising a motor having a current applied therethrough. The speed of the motor is a function of the current passing through the motor. An error signal is produced representative of the difference between the motor speed and a selected motor speed. The current applied through the motor is modulated in accordance with the speed error signal and a signal representative of the current in order to control motor speed. With such arrangement, motor current variations or ripple due to rectification are detected and compensated for to allow the speed of the motor to be maintained constant despite such ripple. In accordance with a feature of the present invention, the motor has a speed related to the current passed therethrough. An error signal is produced representative of the difference or error between the motor speed and a selected speed. A switch allows current to pass through the motor or inhibits current from passing through the motor, selectively in accordance with a control signal. The control signal is produced in response to the speed error signal and a signal representative of the motor current. The control signal comprises a pulse train having a nominal duty cycle. The duty cycle is modulated in accordance with the difference between the speed error signal and the signal representative of the current. The frequency of the pulse train is selected to be much higher than the mechanical resonant frequency of a load driven by the motor. Thus, current is passed through the switch and the motor at a much faster rate than the mechanical resonant frequency. The vibration in such load, and concomitant hum, associated with pulsing current through the motor at a rate near the mechanical resonant frequency of the load is thus substantially eliminated.

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First Claim