Brushless dc motor control method and control device

A control device for controlling a voltage to be applied to a coil of each phase of a motor being a brushless DC motor by PWM drive, the device comprising:

• a power supply voltage detector configured to detect a power supply voltage to be used for driving the motor;
• and
• a controller configured to evaluate a speed reduction rate, based on a detected rotational speed of the motor, transition to a deceleration mode when the speed reduction rate is equal to or greater than a first threshold value, release the deceleration mode when the speed reduction rate is less than the first threshold value, and switch the PWM drive to one-side PWM drive when the power supply voltage reaches equal to or greater than a second threshold value during the deceleration mode to drive the motor.