To safely release a brake when the position of a motor magnetic pole specified, particularly, even when the malfunction of an inverter device is detected or multiple motors are selectively driven by one inverter.

As shown in Fig. 1, the inverter device comprises a magnetic pole position estimating part 7 composed of a converter part 2, a smoothing capacitor 3, an inverter part 4, and an inverter control circuit 6 and estimating the position of the magnetic pole when a synchronous motor 5 is started and a brake control circuit 8 for controlling the brake 10 mounted on the synchronous motor 5. The inverter device further comprises a magnetic pole detection completion determination part 9 which, after an operating instruction from a host control device is input into the inverter device, monitors the elapse of a predetermined time, outputs when the estimation of the position of the magnetic pole of the synchronous motor is completed after the predetermined time is elapsed, and when the estimation of the position of the magnetic pole of the synchronous motor is not completed, outputs a brake release instruction to the brake 10.

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