Abstract: The DC content of each phase of a three phase AC waveform is generated by partially filtering each phase and applying the filtered waveforms sequentially through a multiplexer to a sample and hold circuit which measures the instantaneous value of each phase at two points which are spaced an exact odd number of half-cycles apart. These instantaneous values are fed by an analog to digital converter into a microprocessor which averages the two readings to generate the DC content for each phase and compares the highest DC content with a threshold value. The excess of the highest DC content over the threshold value is integrated and an output signal is generated when the accumulated volt-seconds exceed a selected value.

Abstract: The firing pulses to the nonload carrying bank only of the naturally commutated thyristor power circuit of a static power converter are retarded to reduce or eliminate the interbank circulating current. In one form of the invention, half wave rectified load current signals are summed with the output waveform reference signal applied to the positive and negative bank firing pulse generators in a sense to retard the pulses in the nonload carrying bank. In another form of the invention, the inversion end stops are used to generate the retarded firing pulses in the nonload carrying bank.

Abstract: An arc quenching circuit senses initiation of an arc across the contacts of a contactor, commutates a capacitor discharge to the contacts that brings the contact voltage to zero for arc extinction and promptly recharges the capacitor to enable reoperation.