GB1176484A - A Control Circuit for Static Converters - Google Patents

Abstract

1,176,484. Converting; control of D.C. motors. ENGLISH ELECTRIC CO. Ltd. 18 Oct., 1967 [20 Oct., 1966], No. 47066/66. Headings H2F and H2J. In a control device for a static converter, firing pulses are withdrawn from one or more controlled rectifiers in response to inverse voltage occurring during the interval when said rectifier(s) is conditioned to conduct. The system is applicable to control of a system (Fig. 1, not shown) in which the outputs of a pair of polyphase bridges are connected in parallel opposition to a D.C. motor so as to provide forward and reverse power to the motor. In order to increase motor speed, the firing angles of the thyristors from the appropriate bridge are reduced in the normal manner but to decrease motor speed, the conducting bridge is first blocked and the other bridge conditioned to invert so as to absorb energy from the slowing motor. When the desired speed has been reached the inverting bridge is blocked and the other bridge rectifies again at the appropriate firing angle. In order to reverse the motor the initial procedure is adopted again but the inverting bridge is maintained in that state until the motor stops and is then conditioned to rectify. In such a system, the withdrawal of firing pulses from the rectifiers of one of the bridges must only occur when those rectifiers are non-conductive. Reduction in motor speed or in supply voltage during this time causes an inverse voltage to appear across one or more rectifiers. This inverse voltage is detected by diodes D2, D3 and causes a short circuit to appear across the primary of transformer T1. The rectifier firing pulses are supplied via capacitor C3 to a NOR gate N1 whilst firing pulses delayed by 60 degrees are supplied to a NOR gate N2, gates N1, N2 being cross-coupled to form a bi-stable circuit. Auxiliary 15Á, 5 kc./s. pulses supplied to the base of transistor TR1 result in inverse pulses being passed via the short-circuited transformer T1 and Zener diode ZD1 to a further NOR gate N3. This gate thus only provides an output of such pulses if the controlled rectifier 1 has inverse voltage during a period between the firing time and a time 60 degrees later. This output is used to withdraw the firing pulses from one or more of the controlled rectifiers. The circuit shown in Fig. 5, may be connected to one or more of a number of parallel thyristors isolated by respective current-limiting reactors.