The invention discloses a motor drive device. According to the motor drive device, after the rotation speed of a motor is set through different external circuits, the motor can be operated at a minimum rotation speed or a maximum rotation speed; when the motor is operated at the minimum rotation speed, with the motor drive device, the problem that the motor stops rotating for the reason that the pulse cycle of a PWM driving signal is shorter than the minimum torque of the motor is prevent, so that with a control mode, requirement for minimum heat dissipation is maintained with energy-saving function taken into account; when the motor is operated at the maximum rotation speed, the motor, with the same motor load, can achieve a maximum rotation speed application so that requirement for heat dissipation is met; besides, when the motor is operated at the minimum rotation speed or the maximum rotation speed, the motor is completely free of the influence of the PWM driving signals provided by external systems, so that the motor can be further applied with more flexibility.