Sensorless brushless DC motor

Claim 1

A control circuit for use with a multiphase sensorless brushless direct current motor including a plurality of stator coils having terminal voltages, and for controlling magnetization of said plurality of stator coils of the motor by power supply signals supplied from a commutation unit, said control circuit comprising: a phase voltage determining means for determining the phase voltage of each phase of the multiphase motor from the terminal voltages of said stator coils; a phase voltage sign detecting means for detecting the sign of the phase voltage of each phase determined by phase voltage determining means and for outputting a phase voltage sign signal having a high state and a low state; and a digital phase-shifter for generating a delayed signal which is delayed up to 90° with respect to said phase voltage sign signal and for supplying said delayed signal to said commutation unit comprising: a first counter for counting to a higher value when said phase voltage sign signal as said high state; a second counter for counting to a higher value when said phase voltage sign signal has said low state; comparing means for determining whether the value of one of said counters is at least equal to one-half the value of the other of said counters and less than the value of the other of said counters; clear means for resetting said other of said counters to zero when the value of said one of said counters is at least equal to one-half the value of the other of said counters and less than the value of the other of said counters; and a delay signal generating means for generating a first signal when said first counter is reset to zero by said clear means, and for generating a second signal of a different state from said first signal when said second counter is reset to zero by said clear means.