PURPOSE: To operate another synchronous motor at twelve pulses after a synchronous motor operated by a commercial power supply is low-frequency started by adjusting output voltage from a load commutation type inverter device.

CONSTITUTION: When a synchronous motor 11 at terminal voltage higher than the terminal voltage of six-pulse power inverters 7, 8 is low-frequency started, the six pulse power inverters 7, 8 are used as one six pulse power inverter 12 by a changeover, circuit 51. That is, switches 27, 29, 30, 32 are opened and switches 28, 31 are closed while lead wires R3∼T3 are connected to the synchronous motor 11. When the synchronous motor 11 is variable-speed operated, on the other hand, the six pulse power inverter 12 is changed over to the two six pulse power inverters 7, 8 by the changeover circuit 51. That is, the switches 27, 29, 30, 32 are closed, and the switches 28, 31 are opened while lead wires R1∼T1 and R2∼T2 are connected to a synchronous motor 10.