GB1317455A - Ignition phase angle control system for a contr.rectifying device - Google Patents

Abstract

1317455 Automatic current and power regulation DANFOSS A S 21 May 1970 24644/70 Heading G3X [Also in Division H2] A circuit for supplying ignition control pulses to thyristors via which power is supplied to a load (1-3), Fig. 1 (not shown), comprises a condenser 15, a unijunction transistor 17 for discharging the condenser to provide an ignition control pulse when the voltage across the condenser reaches a predetermined value and a transistor 53 which is rendered conductive at the end of each half cycle of an A. C. supply to ensure that the condenser is discharged at the beginning of each half cycle of the alternating voltage. Each phase R, S, T of the A. C. supply is connected via a respective pair (7, 8, 9) of thyristors to the impendances (1, 2,3) of the load and three substanially similar circuits produce control pulses in the secondary windings of transformers 12-14 for triggering each pair of thyristors. Each circuit includes a transistor 16 which controls the thyristor firing angle by controlling the charging rate of the condenser 15. A variable resistor 38 of a potential divider 37, 38 connected across a Zener diode 48 is adjusted to set the charging rate. A variable resistor 25 of a potential divider 23-25 sets the voltage between the bases of the injunction transistor 17 and therefore determines the voltage at which the condenser 15 discharges by switching on the transistor, thereby providing the control pulse to the thyristor as the condenser discharges through the primary winding of the respective transformer 12-14. The condenser 15 charges and discharges more than once during each half cycle but as the A. C. supply approaches zero and the condenser is being charged, the transistor 53 is switched on to discharge the condenser. At the beginning of each half cycle of the A.C. supply the condenser 15 is therefore discharged. In the event of the supply voltage increasing, the voltage at the base of the transistor 16 increases so that the charging rate of the condenser 15 decreases, the thyristors are therefore fixed later in each half cycle to reduce the load current. The power dissipated in the load therefore remains constant.