Brushless permanent magnet DC motor - with Hall elements controlling differential amplifiers feeding field windings

摘要

In a small low inertia brushless motor, control may be effected by operational amplifiers instead of switching transistors. Hall elements at 90 deg. are associated with field windings also at 90 deg.. The Hall element output is fed through trimming resistors (15,16) to the input terminals of operational amplifiers connected in the differential mode. The resultant output is a 2-phase alternating voltage synchronised with the rotor, and is used as the supply for the field windings. Voltage and current feedback (21,25) for each amplifier gives the motor a shunt characteristic such that speed is dependent upon supply voltage. Reversal of the machine is also possible as it has the same operating characteristics in each direction.