PURPOSE:To hold down loss of a switching element(SE) to such a degree that the loss at a special operation mode may not exceed a loss at a normal operation mode, and then to increase an efficiency by setting a switching frequency(SF) of the SE according to an operation mode and by decreasing the SF at the special operation mode. CONSTITUTION:An SF setting device 2 sets an SFF of an inverter primary circuit element lower than that at a normal operation mode when an operation mode setting device 1 commands a special operation. A frequency/voltage setting device 4 sets an inverter frequency command (IF)F and an output power command E. A gate signal generating device 3 generates a gate signal of the inverter primary circuit element according to the SFF and IFF and the output power command E. Therefore, at the other operation modes than the normal operation mode, loss of a switching element is held down to such a degree that the loss at the other operation modes may not exceed a loss at the normal operation mode. As a result, the size of a cooling equipment of the element is not required to be enlarged and a high-efficiency and small-size equipment can be materialized.
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