Electric motor drive device, control method of electric motor drive device, and electrically driven device

A control method of an electric motor drive device driving an AC electric motor including a plurality of coils connected in a star configuration,

• said electric motor drive device including
• an inverter including a plurality of arms, each of the plurality of arms corresponding to any one of the plurality of coils of the AC electric motor and including a first switching device connected to a positive rail and a second switching device connected in series between the first switching device and a negative rail, wherein for each of the plurality of arms, the midpoint of the first switching device and second switching device is connected with one end of the corresponding coil of the AC electric motor and
• a capacitor charged by a current supplied from a DC power supply supplying DC voltage between a neutral point to which the other ends of the coils of the AC electric motor are connected and a positive rail or negative rail of the inverter and passing through the inverter;
• said control method comprising:
• comparing the induced voltage generated at said AC electric motor with the power supply voltage supplied by said DC power supply and
• using field weakening control, which lowers the induced voltage to control the switching devices of the inverter so that the AC electric motor rotates at a designated speed, when the induced voltage generated at the AC electric motor is larger than the power supply voltage supplied by the DC power supply and, on the other hand, using voltage boosting control, which changes the amount of charging of the capacitor so as to change the voltage applied to the AC electric motor to control the switching devices of the inverter, when the induced voltage is smaller than the power supply voltage.