Brushless motor speed detecting apparatus for reducing generation of spike voltage

A brushless motor speed detecting apparatus comprising:

• a stator having three-phase driving coils which are energized reciprocally in the positive and negative directions;
• a rotor having magnetic poles;
• three position detectors for obtaining three-phase signals corresponding to the rotation of the rotor;
• a first synthesizing circuit for synthesizing an output signal from the three-phase signals;
• a full-wave rectifying circuit for forming a full-wave rectifying signal from the output signal of the first synthesizing circuit;
• mode switching means for forming an energizing mode switching signal in a sine-wave like form according to the full-wave rectifying signal;
• a driving circuit for supplying currents to the driving coils and during supplying currents switching an energizing mode from a first energizing mode to a second energizing mode and vice versa according to the energizing mode switching signal, and
• a second synthesizing circuit for detecting induced electromotive voltages generated in the driving coils as the rotor rotates in the non-energizing section of the first energizing mode and synthesizing a speed signal based on the induced electromotive voltages.