An inverter circuit includes at least two semiconductor controlled rectifiers operatively connected between a source of direct current and an AC load to provide an output current with alternating polarity. A series combination of a commutation capacitor and a primary winding of a transformer connected across the semiconductor controlled rectifier applies reverse bias to the semiconductor rectifier. A first semiconductor uni-directional conductive element is connected in anti-parallel with each of the semiconductor controlled rectifiers to pass reverse current on commutation. The output current with alternating polarity flows through a secondary winding of the transformer to control the charging and discharging currents of the commutation capacitor flowing through the primary winding of the transformer.
This is a division of application Ser. No. 513,472, filed Oct. 9, 1974, now U.S. Pat. No. 3,942,094; which is a continuation of application Ser. No. 387,700, filed Aug. 13, 1973, now abandoned; which is a continuation of application Ser. No. 199,190, filed Nov. 16, 1971, now abandoned.
A series inverter, high voltage capacitor charging circuit includes a current sensor for sensing the current through the load circuit. The sensed current is used to vary the gating frequency of SCRs and to preclude firing of an SCR pair until another SCR pair is in a stable off condition. The gating of each SCR pair is delayed by an amount of time determined by a control capacitor charging circuit. That charging circuit is inhibited as long as one of the SCRs is conducting and once the load voltage has reached a predetermined level. The time constant of that charging circuit is longer if insufficient reverse bias is applied to an SCR for quick turn-off of the SCR. The current sensing thresholds of common base transistors depend on whether forward or reflective current is being sensed. A series capacitor in the load circuit is discharged prior to a first cycle of current flow from the dc supply.
A commutation circuit for thyristors alternately turned ON by pulse width modulation (PWM) for the generation of a fundamental sine wave is characterized by two separate primary windings associated with respective positive and negative thyristors with a common secondary winding at the output, and two commutating networks for the respective poles, each including an auxiliary thyristor for fixed-commutation and the resonant combination of a capacitor and an inductor. The thyristors, the commutating networks, the DC source, and the output transformer have a symmetrical disposition as well as symmetrical commutation steps.
