Patent GB-1300229-A (GB)
Title: Alternating voltage stabiliser
IPC: H03M1/00
Filed: 1969-11-20
Abstract: 1300229 Automatic voltage control WANDEL U GOLTERMANN 20 Nov 1969 [21 Nov 1968] 56876/69 Heading G3R An A.C. voltage stabilizer comprises transformer windings which are switched on or off by semi-conductor switches and which have respective numbers of turns corresponding to a predetermined digital code, these windings producing partial voltages which are added to form a supplementary voltage to compensate input voltage fluctuations sensed directly or as output voltage variations by an error sensing and control circuit having analogue to digital conversion for operating the switches at a zero instant of current following a period or instant during or at which the error is sensed. In Fig. 1 each partial voltage is produced in the secondary winding of a respective transformer 4 1 ---4 n  whose primary windings are controlled by pairs of Triacs 6, 12 and 7, 13 fired alternately by trigger circuits 14,15, the partial voltages being stepped so that each has double the value of its predecessor. A polarity reverser 10 provides "bucking" or "boosting" action and a discriminator 27 provides signals corresponding to zero current instants of the A.C. supply 1 for application to the error sensing and control circuit comprising members 16---26. When the output voltage at terminals 2 deviates from a set value, current flows to the input of an integrator 19 connected with summing resistors 18, 20, a constant voltage source 21 and members 16, 17 detecting the output voltage. The output of integrator 19 is passed to a bi-polar limit circuit 22 which produces, during a measuring period of one half cycle, a number of pulses corresponding to the magnitude and sense of the error voltage. The output of circuit 22 is passed, via a counting channel polarity reverser 23, to a binary forward and backward counter 24 connected to the steps of an intermediate register 26 driving the circuits 14, 15. Counter 24 also supplies a zero coincidence gate 25 which controls members 10, 23. At the end of each measuring period, the signal from discriminator 27 to register 26 produces operation of those circuits 14, 15 which have been set by the register. In a modification, Fig.2 (not shown) members 19, 22 are replaced by an analogue to digital conversion circuit comprising an integrator, a null comparator and a pulse generator and former all inter-connected by gate circuits. The integrator is discharged by constant pulses of the pulse former, the rate of discharge being chosen to provide a fast or slow speed of response to the error signal, e.g. discharge in part of a half cycle or in one or more half cycles following a measuring period. The embodiment of Fig. 3 derives, from the current transformer 41 feeding the zero current discriminator 71, a signal representative of input voltage variation, which signal is applied to a gate 57 together with a signal from a constant voltage source 59 and a signal from a voltage detecting circuit 54, 55. The analogue to digital conversion circuit comprises an integrator 62 containing an operational amplifier with capacitive feedback, a zero voltage comparator 63, a gate 64, a standard frequency generator 65, a gate 68, a constant current source 69 and a gate control circuit 70. Conduction of the gate 57 at each zero current instant initiates the error measuring operation, the gate 57 being cut off just before each such instant to allow adjustment of counter 66 and register 67 controlling the Triac trigger circuits 52, 53. In another embodiment Fig. 4 (not shown) a trasformer with a tapped secondary winding is interposed between the polarity reverser 10 and the transformers 4 i --- 4 n , whose primary windings are connected to the tappings through Triacs whose trigger circuits are controlled by the register. Further modifications comprise:- (1) providing the primary windings of transformers 4 1 ---4 n  with centre taps, connecting their ends to Triacs and omitting the polarity reverser and corresponding elements of the control circuit, Fig. 5 (not shown). (2) supplying the primary windings of transformers 4 1 ---4 n  through Triac bridges which provide polarity reversal, Fig. 6 (not shown). A voltage sensing element having a square wave voltage-current characteristic may be used in the error sensing circuit.