GB1213685A - Static inverter systems - Google Patents

Abstract

1,213,685. D.C. to A.C. converting systems. STANDARD TELEPHONES & CABLES Ltd. 14 Nov., 1968, No. 53991/68. Heading H2F. Thyristors in an invertor are controlled by logic circuitry to produce an output voltage waveform symmetrical about zero, each half wave of which consists of two pulses phase displaced by 60 degrees (at the fundamental frequency) as shown, Fig. 5, whereby third harmonics are eliminated since 60 degrees represents a half cycle of the third harmonic, the pulses having equal width # which can be varied between 0 and 57 degrees allowing for commutation time, by varying the leading or trailing edges. A single phase embodiment (Fig. 3) of the invertor comprises two half bridges 17, 18 each of which comprises a known arrangement (as shown, Fig. 4) of thyristors 1, 2 (or 3, 4), reactive current, feedback diodes 22, 23 commutating inductor 19 and commutating capacitors 20, 21. In operation gate pulses 1-4 (Fig. 5) are applied to respective thyristor switches 1-4 in a predetermined sequence to produce the desired double pulse waveform output from a transformer 7. Thus, Th1 and Th4 are made conductive simultaneously at time 2 to produce the first positive pulse. Th3 is turned on at time 3 to turn off Th4 and terminate the first pulse. Th4 is turned on again at time 4 thereby turning off Th3 and initiating the second positive pulse. Th2 is turned on at time 5 thus turning off Th1 and terminating the second positive pulse. At 180 degrees, time 6, to prepare for the next half cycle Th1 and Th3 are both fired and Th2, Th4 turn off. Th2 is then fired at time 7 turning off Th1 so that Th2, Th3 conduct simultaneously to produce the first negative pulse and so on. The logic control circuitry utilizes a ring counter, AND gates and flip-flops fed with pulses from an oscillator at six times fo, and is described with reference to Fig. 6 (not shown). The flip-flop outputs operate firing pulse control gates which control the application of firing signals from an hf source 40 to the thyristors gate electrodes, similar to Specification 1,097,891. Constant voltage output despite variations in the input voltage and load, may be provided (Fig. 7, not shown); the output voltage being compared with a reference voltage and the error voltage used to operate a phase shifter to vary the mark/space ratio of the oscillator output pulses and thereby the invertor output voltage. Alternatively the phase shifter may be operated by a manually controlled voltage. A three-phase embodiment is described with reference to Fig. 9 (not shown) wherein a threephase motor is supplied, while in Fig. 8 (not shown) embodiment the fifth and seventh harmonics are also substantially eliminated. Substantially sinusoidal output can be obtained by filtering.