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Simple Permanent Magnet Motor Speed Control Circuit

IP.com Disclosure Number: IPCOM000052129D
Original Publication Date: 1981-May-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 2 page(s) / 60K

Publishing Venue

IBM

Related People

Korba, LW: AUTHOR

Abstract

An arrangement is described which dynamically controls the speed of a motor by the use of a switched power control and a feedback loop which monitors the back EMF (electromotive force) of the motor. The motor is alternately connected to a power supply and sample and hold circuit under the control of a pulsewidth modulator. The width of the pulses of the modulator output are dependent upon the voltage fed back from the motor.

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Simple Permanent Magnet Motor Speed Control Circuit

An arrangement is described which dynamically controls the speed of a motor by the use of a switched power control and a feedback loop which monitors the back EMF (electromotive force) of the motor. The motor is alternately connected to a power supply and sample and hold circuit under the control of a pulsewidth modulator. The width of the pulses of the modulator output are dependent upon the voltage fed back from the motor.

In Fig. 1, a clock operating at a frequency higher than the response of the motor triggers a pulsewidth modulator. Pulses of varying widths but at the same frequency as the clock are generated and during these pulses, the motor is connected to the power supply. Since the response of the motor is slower than the applied pulse frequency, the motor tends to integrate or average these pulses of energy and the motor, therefore, runs smoothly. If the pulse width is increased while maintaining constant frequency more energy is supplied to the motor per unit of time and, therefore, the motor increases in speed.

During periods when there is no pulse, the motor is switched from the power supply to a sample and hold circuit. This circuit samples the voltage of the input signal and holds the measured voltage until the next sample period. Because of the inertia of the motor, it continues to run essentially at the same speed after being connected to the power supply. In this coasting condition, the motor acts as a generator and the back EMF can be sensed at the motor terminals. Since this volt...