Control of brushless DC motor, switching controlled by sensing duration of back EMF pulse in unpowered winding

摘要

control system for electronically commutated brushless DC motors of the type where at any instant one winding is unpowered and where the zero-crossings of the back EMF (BEMF) induced in the unpowered winding is used to control winding commutations is disclosed. In such motors, a current pulse CP is produced in the windings when the current is cut off by the commutation device, the energy of this pulse being dissipated in freewheel diodes in parallel with the commutation device. The duration of this pulse is measured and used to provide a measure of motor current. The measured pulse duration is compared with a stored maximum value so as to adjust the duty cycle of the pulse width modulation signal used to control the current in the stator windings.