Patent GB-665570-A (GB)
Title: Improvements in the remote control and protection of electric motors, cables or other apparatus
IPC: H02H11/00
Filed: 1949-10-26
Abstract: 665,570. Protective cut-out arrangements. BROOKE, W., and BALDWIN, H. T. Oct. 26, 1949. No. 25669/49. Class 38 (v). A remote control protective system for polyphase motors and other apparatus utilizes an electromagnetically-operated non-tilting type mercury relay in conjunction with a three position switch, a rectifier and a resistance for remote control purposes and to provide "lock-out" protection against a faulty pilot circuit, failure of the earth connection or failure of supply. Means are also provided to cause the opening of the relay under earth fault conditions and to prevent its reclosure until the fault has been removed. Remote control operation. In Fig. 1, a motor M is upplied from lines Ll ... L3 when the closing coil D of a contactor C is energized from a voltage stabilizing transformer F through a rectifier E when the contacts H1, H2 of the mercury relay H are closed. The operating coil H4 of the relay H is connected in a circuit containing the secondary winding G2 of an auxiliary transformer G supplied from the transformer F, the contact J of an overload relay, a resistance L, an earthing and pilot connection 6, a remote control switch K, a rectifier N and a pilot conductor 5. When the switch K is moved from its "off" position K1 to its "on" position K3, it passes through a "start" position K2 where the coil H4 of the relay is fully energized and the relay closes its contacts H1, H2 to start the motor. With the switch in its "on" or "running" position K3, the excitation of the coil H4 is reduced by a resistance O to a level sufficient to keep the relay closed, but insufficient to reclose it should it be opened. Protection against supply failure, opencircuited pilot circuit and failure of the earthing connection. In the event of the opening of the relay J, the failure of the supply or an opencircuit fault in the control circuit, the relay H is de-energised, causing the opening of the contactor C. Should the resistance of the earthing connection 6 increase as indicated at T, the reduction in the excitation of the coil H4 causes the relay to open. If the conductors 5, 6 become short-circuited, the rectifier N is no longer in circuit, so that the coil H4 is now energised by alternating current and as the coil H4 has a high impedance to A.C., it is no longer sufficiently energised to keep the contacts H1, H2 closed. In each case when the fault is removed, the motor does not start involuntarily if the switch is still in the "running" position K3, as the resistance O prevents the coil H4 from being sufficiently energized to reclose the contacts H1, H2. Thus to restart the motor the switch must be moved back into position K2 and then restored to position K3. Earth leakage and earth leakage "lock-out" protection. An inductance P having coils P1 ... P3 is connected to conductors 1, 2, 3 to provide an artificial neutral point Q, which when the contactor C is closed is connected to earth through contacts 21, C4 and the primary winding R1 of a transformer R. Should an earth fault occur as indicated at Z, fault current will flow through the fault to earth and back through the earth connection 6, winding R1 and inductance P, as the supply neutral is not earthed. The leakage current in winding R1 induces a current in secondary winding R2, which is rectified by a rectifier S and the resulting D.C. potential is applied across the resistance L in opposition to the voltage applied to the control circuit, so that current ceases to flow in the control circuit and the relay H is opened to open the contactor C. When the contactor is opened it connects at C4, 22 the artificial neutral point Q through rectifiers U, V to a secondary winding G3. Current now flows from this winding through the rectifiers U, V, switch C4, inductance P, conductor 3, fault Z, earth, earth connection 6, resistance L and conductor 13, thus maintaining a voltage across resistance L in opposition to the control voltage, to prevent reclosure of the relay H and contactor C until the fault has been removed, irrespective of any operation of the switch K. In a modification, Fig. 2 (not shown), a core-balance transformer is used to provide the leakage protection, so that the system may be used from a supply having an earthed neutral. In Fig. 3 (not shown), the relay H controls the grid bias of a gas filled triode, the anode current of which energises the contactor coil D. Specifications 605,927, 605,932, 623,911, 636,720 and 665,576 are referred to.