GB1166841A - Improvements in or relating to Static Inverters - Google Patents

Abstract

1,166,841. Inverting circuits. PLESSEY CO. Ltd. 11 Jan., 1967 [21 Jan., 1966], No. 2814/66. Heading H2F. [Also in Division G3] A static inverter, of the type in which the third harmonic is eliminated from the output by combining two square waveforms which have the same frequency but are 60 degrees out of phase, is regulated by pulse width modulating each waveform or the combined output by a modulation signal of predetermined frequency and varying the degree of modulation to maintain a predetermined output, third harmonic cancellation being preserved by maintaining a fixed phase relationship between the modulating signal and the combined output. The Figure shows a three-phase arrangement in which the six square waveform outputs of a ring counter 16 are pulse width modulated at 18 by a trigger pulse generator 17 whose square-wave gating signal has its mark/ space ratio varied by a control 21 in accordance with variations of the output voltage of the inverter which is thereby maintained constant. The control 21 advances or delays the leading edge of the square waveform of generator 17 to vary its mark/space ratio. The fixed phase relationship is maintained by using a master oscillator 15 to supply the ring counter 16 and the pulse generator 17. After pulse width modulation the six ring counter outputs are amplified at 19 and then corresponding pairs are combined at 14 to produce a three-phase third harmonic free output which is filtered at 20 to produce a sinusoidal waveform. Transistors may be used as switching elements in the gating circuit 18 whose output may be switched off under overload conditions by protection circuits 22. Typical waveforms before and after pulse width modulation are illustrated and discussed, Fig. 1, 2 and 3 (not shown).