Patent GB-665571-A (GB)
Title: Improvements in the remote control and protection of electric motors, cables or other apparatus
IPC: H02H7/00
Filed: 1949-10-26
Abstract: 665,571. Protective cut-out arrangements. BROOKE, W., and BALDWIN, H. T. Oct. 26, 1949. No. 25670/49. Class 38 (v). In a remote control and protective arrangement for an electric motor M or other apparatus, a transductor Y is used in conjunction with a three position switch K to control the excitation of a relay coil or of the closing coil D of a contactor C, the coil being fully energised to close the relay of contactor, but only partly energised to maintain the relay or contactor closed, so that should the relay or contactor be opened under fault conditions, the part energisation is insufficient to reclose the relay or contactor involuntarily. The transductor Y has its D.C. control winding Y3 in series with the secondary winding G7 of a transformer G, pilot and earth conductors 5, 6, rectifier N and remote control switch K. When the switch K is in its "start" position K2, the winding Y3 is fully energised so that the AC windings Y1 and Y2 are of low impedance, but when the switch is moved to its "run" position K3, a resistance O reduces the current flowing in the winding Y3 so that the impedance of the windings Y1, Y2 is increased. In Fig. 1 the transductor windings Y1, Y2 are used to control the biasing of a gas filled valve F, which has its anode F1 connected in series with the coil D and energised by a full-wave rectified A.C. potential from secondary winding G6 and rectifier V, the anode voltage being indicated by curve a, b, c in Fig. 2. A transformer secondary winding G2 applies a primary alternating bias potential, curve e, Fig. 2, to the grid F2 through resistors k, l. A constant D.C. biassing potential is applied across the resistance k by a secondary G3 and rectifier W to provide a fixed negative bias, which depresses the datum level of alternating potential e to a level f where the valve F is in a non-firing condition. This is indicated in Fig. 2 which shows the curve e below the "hatched" firing zone of the valve. A D.C. potential is applied across resistor l in opposition to that across resistance k, by a circuit containing secondary winding G4, rectifier X and the transductor windings Y1, Y2. Thus when the switch K is moved to position K1, the reduction in the impedance of windings Y1, Y2 enables sufficient current to flow in the circuit to impress a potential across resistance l greater than that across resistance k. This raises the datum line of the grid bias from f to g so enabling the valve to fire on each half cycle to energise coil D to close the contactor. When the switch K is moved to its "on" or "running "position, the transductor Y reduces the voltage across resistance k lowering the datum line of the wave e to a level d where the curve e is outside the hatched area on alternate cycles a, c, etc. Thus the valve F continues to fire on alternate cycles b, etc. only and so reduces the average excitation of the coil D to a level sufficient to maintain the contactor closed but insufficient to reclose it if it has been opened by a fault condition. In a modification, Fig. 3 (not shown), having the same biasing arrangement, two valves are used, one valve being arranged to fire on half cycles a, c, etc. and the other on half cycles b etc. In a further modification, Fig. 4, two valves F, FF are used and the magnitudes of the applied A.C. grid bias potentials are varied to enable both valves to fire alternately to close the contactor C and one valve F only to fire on alternate half cycles to maintain the contactor closed. Secondary windings G2, G3 are arranged so that they supply negative grid biassing half waves e, f, Fig. 5, of different magnitudes to their associated valves F, FF respectively during the half cycles a, c and b, d respectively when the anodes of the valves are supplied with a positive potential from the centre tapped secondary winding G6. In this modification the transductor Y determines the magnitude of the A.C. potentials applied across resistors s, t in opposition to the primary grid biassing potentials. The potential applied across resistance s is equal to that across t. When the switch K is moved to "start" position K2, the net biassing voltages indicated as e, f in Fig. 5 are positive in respect to the anodes of the valves, so that both the valves fire to close the contactor C. When the switch K is moved to the "run" position the net biassing voltage e of valve F is positive whilst that f of valve FF is negative, so that valve F only continues to fire to maintain the contactor closed. In Fig. 6 (not shown) the transductor controls the excitation of a sensitive electromagnetically operated non-tilting type mercury relay controlling the contactor coil in a manner similar to that described in Specification 665,570. In a further arrangement the transductor controls the D.C. winding of a second transductor, which in turn control the excitation of the contactor coil. In each of these embodiments, an inductance P, transformer R, rectifier S, resistance L and secondary winding G8 are provided to give earth leakage protection and earth leakage "lockout" in a manner similar to that described in Specification 665,576. Specifications 605,927, 605,932, 623,911, 636,720, 665,570 and 665,572 are referred to.