Patent GB-632454-A (GB)
Title: Improvements in or relating to electric control circuit arrangements
IPC: H02H7/00
Filed: 1947-10-20
Abstract: 632,454. Protective cut-out arrangements. REYROLLE & CO., Ltd., A., YOUNG, A. F. B., LACKEY, C. H., and HOWE, H. D. Oct. 20, 1947, No. 28022. [Class 38 (v)] [Also in Group XXXIX] In a control and protective circuit in which a main switch is controlled from a remote point, for example a mining installation having a gate-end switch controlled remotely by a switch on a coal-cutter, a Wheatstone bridge is used in conjunction with an electromagnetic amplifier so that current in the pilot circuit may be reduced to a minimum value. A circuit-breaker A controls the supply to a coal cutter D, the circuit-breaker being controlled by a remote switch E, which together with a resistance K and pilot wires C<SP>1</SP>, C<SP>2</SP> forms one arm of a Wheatstone bridge, the other three arms containing resistances L, L<SP>1</SP>, L2. The bridge is fed from the secondary winding B<SP>2</SP> of a step-down transformer B<SP>1</SP> and its unbalance current energizes a biassing winding H<SP>1</SP> of a transformer H through a full-wave rectifier M. The transformer H, which acts as an electromagnetic amplifier has its input and output windings arranged as an autotransformer winding H<SP>2</SP>, which is supplied from the secondary winding B2 and supplies a relay F through a full-wave rectifier G, the relay F controlling the operating winding A<SP>1</SP> of the circuit-breaker. With the switch E in the open position as shown, the bridge is unbalanced and the saturation of the core of transformer H due to the energization of winding H<SP>1</SP>, is such that the output from the winding H<SP>2</SP> is at a minimum and the relay F remains open. When the switch E is operated from the open to the running position K<SP>2</SP>, it passes through an intermediate closed position K<SP>1</SP> where sufficient resistance is inserted into the pilot circuit to balance the bridge. In this position the winding H<SP>1</SP> is de-energized and relay F receives the maximum output current of the transformer H and closes to energize winding A<SP>1</SP>, so closing the circuit-breaker A. In the running position K<SP>2</SP> further resistance is inserted to unbalance the bridge slightly so that the partial energization of winding H<SP>1</SP> causes a reduction in the relay current, not sufficient to open the relay, but sufficient to prevent the reclosure of the relay on the resumption of the supply after a supply failure. When the coal-cutter is running, any fault which alters the resistance of the pilot circuit, such as an earth fault, short-circuit, or open circuit, unbalances the bridge to cause the opening of the circuit-breaker. The characteristics of the transformer H are arranged so that a small alteration in the current in H<SP>1</SP> causes a substantial alteration in the output current. The pilot wire C<SP>1</SP> may serve as a safety earthing connection for the coal cutter. The transformer H has a short-circuited winding H<SP>3</SP> to smooth the magnetizing flux of winding H<SP>1</SP>, and in a modification (Fig. 2, not shown), a positive feed-back arrangement is provided by a further winding on the centre limb of the transformer which is fed with rectified current derived from the output current so providing increased sensitivity.