Cycloconverter system including means to derive a control signal representing fundamental converter output

Claim 1

In a cycloconverter system having positive and negative banks of gate controlled switching rectifier elements which are gated by firing circuits to convert a first alternating waveform from a source into a second alternating waveform that characteristically includes a desired fundamental frequency component and also harmonics of the fundamental component, which harmonics are substantially removed by a filter including a capacitor and the fundamental component is applied to a load connected across the filter capacitor, the improvement comprising: apparatus to derive, for control purposes, a signal that corresponds to the referred to fundamental component of the said alternating output current wave, including any distortion resulting from load switching and load nonlinearity, so as to permit detection of zero crossing points thereof, said apparatus comprising first means to develop a current proportional to alternating current in the filter capacitor and independent of load transients, second means to develop a current proportional to alternating current in the load, and third means to add vectorially said alternating currents from said first and second means, said filter capacitor being connected in a circuit branch across the load, said first and second means to develop currents comprising current transformers respectively associated with conductors in series with the filter capacitor in the branch across the load and with the load, and further comprising an additional filter to filter the current from said first means before reaching said third means.