Brushless dc motor speed control via power supply voltage level control

Abstract

The operating speed of a brushless DC motor is maintained at a desired speed by controlling the voltage level of power supplied to the motor. The motor includes a rotor carrying permanent magnets, cooperating electromagnets contained in a stationary armature and Hall effect devices for sensing the operating speed and position of the rotor. Circuitry connected to the Hall effect devices generates angular sector signals corresponding to activation periods or periods during which power is applied to the electromagnets to operate the motor. A processor connected to one of the Hall effect devices monitors the operating speed of the motor and generates an error signal representative of the difference between the operating speed of the motor and a desired reference speed. A power supply is controlled by the processor to generate power having a voltage level corresponding to the error signal. Switching transistors are connected between the electromagnets and the circuitry for generating the angular sector signals to apply power from the power supply to each of the electromagnets during the entirety of its corresponding activation period to maintain the motor speed at the desired reference speed.