Patent ID: 8681517

Claim:
An AC converter that converts a single-phase input AC voltage with a frequency f 0 into a three-phase output AC voltage with a frequency f 1 (where f 1 is lower than f 0 ), the converter comprising: a switching section, which converts the input AC voltage in response to a control signal and which outputs the converted voltage to a phase that has been selected in accordance with the control signal; a filter section, which filters out high frequency components from the converted voltage, thereby converting the converted voltage into the output AC voltage; and a switching control section, which performs a pulse density modulation on a phase-by-phase basis and in response to a reference signal with the frequency f 1 , which is associated with the output AC voltage of each said phase, synchronously with a zero cross of the input AC voltage, thereby generating the control signal according a pulse generation status by the pulse density modulation and the polarity of the input AC voltage and sending out the control signal to the switching section, wherein the switching section includes a first type of switching element that applies a positive voltage to its associated phase if the polarity of the input AC voltage is positive and a second type of switching element that applies a positive voltage to its associated phase if the polarity of the input AC voltage is negative, and wherein if the polarities of the input AC voltage and the reference signal for use in the pulse density modulation are both positive, the switching control section outputs the control signal to turn the first type of switching element ON, and if the polarities of the input AC voltage and the reference signal for use in the pulse density modulation are negative and positive, respectively, the switching control section outputs the control signal to turn the second type of switching element ON, and if the polarities of the input AC voltage and the reference signal for use in the pulse density modulation are positive and negative, respectively, the switching control section outputs the control signal to turn the second type of switching element ON, and if the polarities of the input AC voltage and the reference signal for use in the pulse density modulation are both negative, the switching control section outputs the control signal to turn the first type of switching element ON.