Chopper with adaptive energy commutation

In a power control system including a time ratio control electric power circuit connected between an electric load and a pair of d-c power busses of relatively positive and negative potential that are adapted to be connected to a source of d-c electric power, through an electric filter comprising at least a filter capacitor connected between the positive and negative busses, the power circuit including a cyclically turned-on main load current carrying thyristor and a commutation circuit operative to periodically turn-off the main thyristor, the commutating circuit including a commutation capacitor tht discharges during the turn-off process of the commutating circuit, the commutation capacitor being connected in a path that includes the d-c source and that conducts charging current during intervals when the main thyristor is turned-off, the improvement comprising:

• (a) inductive means serially connected in said charging current path;
• (b) controllable switch means connected in shunt with said inductive means and having a high resistance blocking state and a relatively low resistance conducting state;
• (c) unidirectional conducting means connected in said charging current path and poled to conduct said charging current;
• and
• (d) gating means coupled to said commutating capacitor and to said switch means for forcing said switch means to its conducting state in response to the voltage across said commutating capacitor attaining a predetermined variable magnitude, said gating means being coupled to monitor the magnitude of said main load current and to adjust said predetermined variable magnitude as a direct function of the magnitude of said main load current.