A method is provided for controlling operation of an air conditioning unit. The method comprises supplying line voltage to activate a motor configured to operate the air conditioning unit, and monitoring a supply of control voltage in order to control operation of the air conditioning unit, the supply of control voltage being derived from line voltage. In response to detecting a control voltage below a predetermined threshold and/or by a predetermined percentage, a time delay is initiated. The method further comprises deactivating the motor if a predetermined increase in control voltage is not detected before the time delay expires.

A motor drive device includes: a check performance unit configured to perform an abnormality check operation for at least one of an upper transistor, a lower transistor and a motor in a state where a second main electrode is pulled up or pulled down by bringing one of a second main electrode pull-down switch and a second main electrode pull-up switch into an on state; and a control circuit configured to control a combination of on/off states of the upper transistor, the lower transistor and a motor relay connected to the motor.

This motor drive device comprises: a motor drive circuit that drives a synchronous motor by supplying AC power to the synchronous motor via power lines; an overvoltage protection circuit that short-circuits between the phases of the power lines by turning on a semiconductor switch when driving of the synchronous motor by the motor drive circuit is stopped and prevents the occurrence of overvoltage due to a back electromotive force between the terminals of the synchronous motor; and a temperature estimation unit that calculates a temperature estimation value of the semiconductor switch by using a motor constant and a semiconductor switch constant, said motor constant including at least one of the inertia and back electromotive force constants of the synchronous motor, said semiconductor switch constant including the on-resistance, thermal resistance, and thermal time constant of the semiconductor switch.