Patent GB-378180-A (GB)
Title: Improvements in or relating to control systems for electrical apparatus such as motors
IPC: H02H7/00
Filed: 1931-07-21
Abstract: 378,180. Protective cutout arrangements. GOWAN, S., 4, Wardle Road, Sale, Cheshire, FERGUSON, F. G., 11, Moreton Avenue, Stretford, Manchester, and ASSOCIATED ELECTRICAL INDUSTRIES, Ltd., Crown House, Aldwych, London. July 21, 1931, No. 20857. [Class 38 (v).] In a switchgear arrangement for controlling a motor 1, Fig. 1, driving coal-cutting machinery, in which a " gate end " switch 5 is controlled over pilot-andearth circuits P, E, by a switch 24 operated conjointly with the motor controller 9, and in which the switch 5 is operated by a main relay 14, rectifier means 50, 51 are provided to ensure that a secondary relay 52, associated with the relay 14, is supplied with sufficient current for operation only when the pilot wire P is connected to earth outside the switch 24, and even if the earth connection is of high impedance. The pilot-and-earth circuit is supplied from a step-down transformer 15 and as described in Specification 343,667, includes, in addition to the relay 14, and the switch 24, an auxiliary switch 23, for ensuring the proper operation of the controller 9. It also includes overload relays 21, 22. The relay 52, and the rectifier 31, are series connected in parallel with the coil 19 of the relay 14, and the rectifiers 50, 51 are connected in opposition, so that normally no current flows in the coil 52. If the pilot wire is earthed in the trailing cable 6, however, the rectifier 50 is cut out, wholly or in part, so that the coil 52 is energized, and opens its contacts 53 to de-energize the switch 5. If the coil 52 is so arranged that it responds only to alternating current, the rectifier 51 may be omitted. The coil 52 may be a secondary winding on the main relay 14, or the coil 52 and contacts 53 may be replaced by a heater element, and fusible member respectively. The switch 23 is operated by a coil 26, energized by the motor current, to insert a resistance 25 in the circuit of the coil of the switch 5, which is thus energized at a reduced voltage, necessitating the return of the controller 9 to the "off" position to close the switch 23 and shortcircuit the resistance 25 before the switch 5 can be reclosed after it has been opened. The coil 26 may alternatively be energized by the voltage across two of the motor phases. In Fig. 2, an alternative form of switch which may replace the switch 23 is shown. An arm 55 is moved from its " off " position to the position " start," and is then released, and moves under the action of a spring to the " run " position, an interlock being provided to prevent the motor controller 9 from being moved to its "off" position unless the switch arm 55 is in its "off" position. In Fig. 3, the invention is shown applied to an arrangement using direct control current at, say, 20 volts, supplied by a generator 58. The pilot-and-earth circuit containing the coil 19 of the main relay has a battery 57 of, say, 5 volts, opposing the voltage of the generator 58, leaving 15 volts for the operation of the relay 19. The secondary relay coil 52 is connected across the pilot-and-earth circuit in series with a rectifier 51, and a second 5-volt battery 59. The batteries 57, 59 are connected in opposition, and the rectifier 51 is connected to oppose flow of current from the generator 58 through the coil 52. Thus in the circuit of the coil 52, the batteries 57, 59 balance, and the coil is energized only when the battery 57 is wholly or in part short-circuited, by a connection between the pilot and earth wires in the trailing cable. A leaky resistance 60 is provided in shunt to the rectifier 51, to allow sufficient current to flow from the generator 58 to maintain the change of the battery 59.