Electric motor drive device

An electric motor drive device comprising:

• a smoothing capacitor connected in parallel to a DC power supply;
• an inverter which has a plurality of semiconductor switching elements and converts DC power from the smoothing capacitor to multi-phase AC power, to drive an electric motor;
• a control device for drive-controlling the plurality of semiconductor switching elements with a predetermined control cycle, to control power-running operation and regenerative operation of the inverter;
• and
• an overvoltage protection unit having a discharge circuit which has a discharge resistor and a semiconductor switching element connected in series and which is connected between both terminals of the smoothing capacitor;
• and a first determination circuit for determining that voltage of the smoothing capacitor exceeds first set voltage, the overvoltage protection unit being configured to turn on the semiconductor switching element in the discharge circuit on the basis of the determination by the first determination circuit, wherein
• the control device includes a phase short-circuit control unit having a second determination circuit for determining that voltage of the smoothing capacitor exceeds second set voltage lower than the first set voltage, the phase short-circuit control unit being configured to bring the electric motor into a phase short-circuited state on the basis of the determination by the second determination circuit, and
• a determination period required for the second determination circuit to perform the determination is longer than a period required for the first determination circuit to perform the determination.