GB1381260A - Electrical power conversion systems - Google Patents

Abstract

1381260 Converting F L F STEGHART 24 Nov 1971 [24 Nov 1970] 55785/70 Heading H2F In a frequency changer of the A.C.-D.C.-A.C. type, a starting circuit is arranged to initiate operation of the driver for the D.C.-A.C. section after a predetermined number of trigger pulses for said D.C.-A.C. section have been generated. General.-Fig. 2 shows a single-phase converter or one phase of a three-phase converter. A.C. at terminals A, N is converted to D.C. by means of a bridge (Fig. 3, not shown) of two thyristors and two diodes, the output being filtered (Fig. 4, not shown) and fed to an inverter (Fig. 5) comprising a bridge of thyristors CSR1 ... CSR4 triggered by a driver (Fig. 8, not shown). The pulsed output of the inverter is converted to sine by means of a filter (Fig. 7) having current limiting facilities, giving an output from a transformer to terminals A, N of the required frequency. The output is monitored and regulated by controlling the conduction angle of the thyristors in the A.C.-D.C. section. The frequency of the inverter is determined by means of a unijunction transistor oscillator feeding a ring counter comprising three bi-stable circuits (Fig. 9, not shown), power supply for the inverter driver being turned on by a start circuit (Fig. 11) after counting a predetermined number of clock pulses. Inverter.-Fig. 5 shows the inverter comprising a bridge of four thyristors CSR1 ... CSR4 having series diodes D5 ... D8 to prevent discharge of commutating capacitors C1, C2 through the load. Output filter and current limiting.-Harmonics in the output are removed by means of a series circuit L1, C1 (Fig. 7) tuned to the output frequency and a parallel tuned circuit (Fig. 6, not shown) across the load. A further series circuit, tuned to the third harmonic, may also be connected across the load. In the event of excessive load current, the diac breaks down, triggering a triac which minimizes load current by connecting inductor L2 across part of the series circuit C1. Starting circuit.-Fig. 11 shows a diode pump D1, D2, C1, C2 which counts inverter trigger pulses and, after a predetermined number, triggers a programmed unijunction transistor PUT1 which turns on a thyristor CSR1 supplying power to the inverter driver. Stop circuit.-Fig. 12 shows a circuit for monitoring a trapezoidal wave from the A.C.- D.C. converter. In the event of supply failure, transistor T1 becomes turned off, allowing capacitor C1 to charge sufficiently to turn on unijunction transistor UJ1 which triggers a thyristor CSR1 so as to apply a clamp to the D.C. side, preventing application of power to the inverter until the D.C. filter has completely discharged. This circuit prevents possible commutation failure in the inverter due to low voltage operation. (For Figures see next page.)