Brushless dc motor unit and method of driving the unit

A brushless dc motor and a control device therefor, the motor having 3-phase armature windings and a permanent-magnet rotor that is rotated by a rotating magnetic flux generated by the armature windings when energized, the control device comprising:

• (a) a dc power supply, with a negative terminal grounded, having a motor drive voltage;
• (b) a voltage divider for dividing said motor driving voltage to obtain a midpoint voltage that is one half thereof;
• (c) an electronic commutation circuit connected to said dc power supply, said electronic commutation circuit having a plurality of solid-state switching elements, connected in a 3-phase bridge circuit, each having a switching control terminal, for commutating said motor drive voltage and providing 3-phase voltages individually provided to said 3-phase armature windings so that said windings are energized;
• (d) a plurality of phase delay circuits for individually delaying, by an electric angle of less than 90.degree., phases of 3-phase back emf voltages generated in said 3-phase armature windings when said motor is in rotation;
• (e) a plurality of voltage comparators each having a first input terminal and a second input terminal, said first input terminals being individually connected to output terminals of said phase delay circuits;
• (f) a comparator reference voltage computation circuit for outputting a sawtooth-wave comparator reference voltage having a frequency proportional to a current angular speed of said rotor and an amplitude whose center voltage is equal to said midpoint voltage, said comparator reference voltage being commonly provided to said second input terminals of said voltage comparators, said comparator reference voltage computation circuit including:
• a first operation amplifier having a positive and negative input terminals, to which a basic reference voltage is alternately provided at an alternating frequency proportional to the current angular speed of said rotor so as to output a first rectangular-wave voltage having an amplitude of two times said basic reference voltage with a center voltage thereof grounded,
• a second operation amplifier for receiving said first rectangular-wave voltage and said midpoint voltage to a common input terminal thereof and outputting a second rectangular-wave voltage having an amplitude equal to the amplitude of said first rectangular-wave voltage but a center voltage thereof being said midpoint voltage, and
• a low-pass filter circuit for converting said second rectangular-wave voltage to said sawtooth-wave comparator reference voltage;
• and
• (g) a switching control device for receiving comparator output voltages individually from said voltage comparators and generating switching control signals in reference individually to said comparator output voltages so that said switching control signals are individually provided to said switching control terminals of said switching elements so as to control said commutation circuit.