Motor drive device having overvoltage protection circuit

A motor drive device comprising:

• a motor drive circuit configured to drive a synchronous motor by supplying AC power to the synchronous motor through a power line;
• an overvoltage protection circuit being provided between the synchronous motor and the motor drive circuit and, configured to short-circuit between phases of the power line by turning on a semiconductor switch when drive of the synchronous motor by the motor drive circuit is stopped to prevent generation of overvoltage due to back electromotive force between terminals of the synchronous motor, and
• a temperature estimation unit configured to calculate an estimated temperature value of the semiconductor switch that rises and then falls by turning on the semiconductor switch, by using a motor constant including at least one item out of inertia and a back electromotive force constant that are related to the synchronous motor, and semiconductor switch constants including on-resistance, thermal resistance, and a thermal time constant that are related to the semiconductor switch.