Abstract: First and second inverters are each controlled based on pulse width modulation using a carrier signal. In order to decrease the noise caused by the first inverter and the second inverter, a frequency of a carrier signal of the first inverter and a frequency of a carrier signal of the second inverter are controlled. The frequency of the carrier signal of the first inverter is changed periodically or randomly within a first preset frequency range as time elapses. The frequency of the carrier signal of the second inverter is changed periodically or randomly within a second preset frequency range to avoid overlapping the first frequency range.