an input rectifier supplies d - c energy for an auxiliary - commutated inverter . a logic circuit provides gating signals for the semiconductor switches in the input rectifier circuit and in the inverter circuit . a pre - charge circuit provides an independent path to charge the commutating capacitor in the inverter when the system is energized . the gating signals to the input rectifier are delayed by a predetermined time interval , allowing the commutating capacitor in the inverter to be charged and thus ensure effective commutation when the inverter commences operation .