in a process of changing an ac voltage from zero to + ed or from zero to − ed , a control method in accordance with some aspects of the invention detects voltage across the capacitors and when the detected voltage is lower than a predetermined voltage value , switching operation pattern capable of charging the capacitor is given to the switching elements during a short period of time ; when the detected voltage is higher than the predetermined voltage value , a switching operation pattern capable of discharging the capacitor is given to the switching elements during a short period of time . thus , the width of voltage variation of the capacitors is limited within a specified variation range .