GB1309120A - Voltage stabilishing circuits - Google Patents

Abstract

1309120 Automatic voltage control SIEMENS AG 19 Oct 1970 [20 Oct 1969] 49515/70 Heading G3R An A.C. voltage stabilizer comprises a transformer TR1 whose primary winding has one end connected to an input terminal R of an A. C. supply and a plurality of taps AI--AN connected through repective bi-polar semi-conductor controlled rectifiers (TRIACS) SI--SN to the other terminal MP of the supply, the TRIACS SI--SN being controlled by an analyser circuit AW which is responsive to the supply voltage to vary the number of winding turns connected thereto to compensate for variations of the supply voltage and which operates the TRIACS to change the selected tap at an instant at which the TRIAC current passes through zero. The circuit AW comprises a measuring section M having a transformer TR2 whose primary winding is connected to terminals R, MP, a zero intersection indicator NI for producing a signal when the current in the winding TRI and TRIAC passes through zero, and a control section ST which supplies firing signals to the TRIACS SI--SN in accordance with signals from the sections M and NI. The section M includes a rectifier circuit whose output is taken through respective voltage dividers to the inputs of associated differential amplifiers which number one less than the number of TRIACS SI--SN and which have staggered threshold levels so that the number of amplifiers responding to each rise of the input voltage following its passage through zero depends on the peak amplitude of the input voltage. The control section ST selects one of the TRIACS SI--SN to be closed according to the signals appearing on output lines LI--LN determined by the number of differential amplifiers that have responded, each line LI--LN being associated with a respective bi-stable trigger stage in the section ST. These stages are connected to a logic circuit which determines, from the number of stages set up, the desired TRIAC to be fired and selects the trigger stage thereof. The firing signal is produced when a signal is supplied from the section NI indicating passage of the current through zero. The latter section comprises an inverted switching amplifier which produces a signal when the current passes through zero and has two inputs connected through resistors and diodes to a positive supply VL from the section M, to the terminal MP and to the centre-tapped secondary winding of a current transformer SW, Fig. 2 (not shown). The supply for the sections of the analyser AW may be obtained from a secondary winding of the transformer TRI, members R5, ZD1, ZD2 being provided for this modification to close the TRIAC SN when the stabilizer is switched on. Alternatively this secondary winding can replace the transformer TR2, with a compensating circuit provided in the section ST. Application of the invention to a three phase supply is envisaged.