Patent GB-711398-A (GB)
Title: Improvements in or relating to balanceable electric networks
IPC: H01F29/08
Filed: 1952-02-15
Abstract: 711,398. Electric indicatingsystems. BAILEY METERS & CONTROLS, Ltd. Feb. 15, 1952 [Feb. 16, 1951], No. 4085/52. 'Class 40 (1). In a balanceable electrical network including first and second transformers energized from a common A.C. source, having relatively movable cores, and twin secondary windings connected in opposition to give potential outputs of magnitude determined bythe core positions relative to the appropriate windings and of phase senses relative to the supply voltage determined by the senses of the core displacements, the first transformer establishes a first potential representative of a variable and the second transformer establishes a second potential adapted to balance the first potential, there being provided automatic balancing means associated with the second transformer and responsive to the vectorially combined transformer outputs to adjust the core of the second transformer to maintain balance between the output potentials in conjunction with phase shifting circuits for each transformer such as to vary the phase relatively to the supply voltage of each potential so that the relation between the position of each core and the phase of the corresponding potential is the same foieach transformer, thus reducing the residual voltage at balance; and in a modification the outer ends of the secondaries of the first transformer are connected to the ends of a potentiometer whose slider is connected to the secondary centre point of the transformer through automatic balancing means responsive to the vectorially combined outputs of the transformer and the potentiometer to adjust the slider of the latter to restore balance, with phase shifting circuits provided to ensure that the phases relatively to the supply voltage of the potentiometer balance voltage and the transformer secondary voltage vary in correspondence with each other and with the core and slider displacements so as to reduce the residual voltage at balance. An automatically balanced electrical indicating circuit (Fig. 1) comprises two transformers 1, 2 having movable cores 7, 5 and series opposed secondaries 10, 11; 12, 13 connected together through an amplifier 3 controlling a motor 4, so that when the transformer outputs are balanced the motor is at rest and any deflection of core 7 in response to a variable energises the amplifier and motor. with an unbalance voltage to readjust core 5, rebalance the transformer outputs and register the value of the variable on scale 6. The residual voltage applied to the amplifier when the cores are in their balanced positions is reduced, Fig. 3 (not shown), by providing the transformers with symmetrically disposed auxiliary secondary windings 8, 8A; 9, 9A; leaving open-circuited the windings associated with the secondary windings 10, 12 or 11, 13.having the smaller out-of-phase voltage component of output, and short-circuiting through suitable load resistances Rc, Rc<SP>1</SP> those windings associated with the secondary windings having the greater out-of-phase voltage component. A load resistance R 2  is also connected across the combined secondary 10, 11 or 12, 13 which has the larger out-of-phase voltage component and is adjusted at the maximum excursions of the transformer cores to equalise the phases of the resultant output voltages of the transformers. A modification (Fig. 4) comprises an A.C. energised transformer 1 having centre-tapped opposed secondaries 10, 11, a movable core 7 connected in bridge with a potentiometer 20; and an amplifier 3 connected between the potentiometer slider and the secondary centre point controlling a motor 4 driving the slider and the recorder 6. Deflection of the core unbalances the bridge which energises the amplifier and motor to rebalance it and indicate the core extension on the recorder 6. The residual voltage at balance is reduced, Fig. 5 (not shown), by providing the transformer with auxiliary secondary windings 8, 8A one of which is shunted by a load resistance Rc as described above and by shunting the potentiometer by a capacitor 21 if the core is movable symmetrically from its mid position, by connecting the capacitor 21A across the corresponding part of the potentiometer if the core is movable in one direction only, or by connecting the-.capacitor 21B in shunt across the potentiometer in series with a resistance between the potentiometer and the transformer if the core is movable symmetrically on either side of its mid position. Harmonics are removed from the residual voltage at'balance (not shown), by Figs. 6, 7, connecting the motor amplifier to the bridge through a filter tuned to the third harmonic of the supply and comprising a parallel resonant LC circuit in the live side of the amplifier input, which is also shunted by a capacitor resonating with the inductance at the supply frequency.