Motor drive device, and compressor and refrigerator using same

A motor drive device for driving a brushless DC motor including a rotor and a stator having three-phase windings, the device comprising:

• an inverter for converting DC electric power to AC electric power and supplying the electric power to the brushless DC motor;
• a current phase detection unit for detecting a phase of a current flowing through the brushless DC motor;
• a speed command unit for indicating a command speed of the brushless DC motor;
• a position detection unit for detecting a relative position of the rotor of the brushless DC motor;
• a phase correction unit for outputting a commutation signal for switching a winding to be energized in the brushless DC motor based on:
• a detection result of the current phase detection unit;
• the command speed indicated by the speed command unit;
• and
• the relative position detected by the position detection unit,
• so that a phase of a current flowing in a predetermined winding of the brushless DC motor and a phase of a voltage hold a predetermined relation;
• and
• a drive unit for outputting a drive signal to the inverter based on the commutation signal output from the phase correction unit,
• wherein the drive signal indicates a supplying timing of the electric power supplied to the brushless DC motor by the inverter, and
• wherein the phase correction unit outputs the commutation signal so that:
• the motor drive device performs a feedback control when the brushless DC motor is under low-load conditions;
• and
• the motor drive device performs a synchronous drive when the brushless DC motor is under high-load conditions, and
• wherein the phase correction unit comprises;
• a first commutation unit that outputs a first commutation signal for switching a winding to be energized in the brushless DC motor based on a signal of the position detection unit and the command speed;
• and
• a second commutation unit that:
• obtains the phase of a current detected by the current phase detection unit, the command speed, and a time from a reference current phase to a commutation;
• calculates a finite difference by comparing the time with an average of the time of the past;
• calculates a corrected amount of the communication cycle based on the finite difference
• decides a corrected commutation cycle of a second commutation signal by adding the corrected amount to the commutation cycle;
• and
• outputs the second commutation signal for switching a winding to be energized in the brushless DC motor.