Patent GB-180845-A (GB)
Title: Improvements in or relating to the control of electric lifting magnets, magnetic chucks, dynamo and motor field magnet circuits and the like
Filed: 1921-04-14
Abstract: 180,845. Holme & Co., Ltd., E., Brooke, W., Steel, Peech, & Tozer, Ltd., and Bowen, H. E. April 14, 1921. Sparking, preventing.-Relates to circuit control systems for electric lifting-magnets, magnetic chucks &c., and comprises a series of electrically-operated switches for effecting the several steps in the energization and de-energization of the magnet, de-energization being effected by first inserting a resistance in series with the winding and then short-circuiting the winding or discharging it through a low resistance. The discharge circuit includes the operative winding of a relay which maintains the circuit closed, and also prevents any further switching operations, until the current has reached a low limit. Means may also be provided for passing a small reverse current through the winding to effect complete de-energization. The connections are shown diagrammatically in Fig. 1, in which A represents the magnet winding, Band C starting and reversing resistances, D the relay winding, and 1 to 4 contactors. The sequence of operations is as follows. To energize the winding, the contactors 1 and 2 are first closed to pass current through A and B in series; the contactor 4 is then closed to cut out the resistance B. To discharge the mag-. net the contactor 4 is first opened to re-insert the resistance, and 3 is then closed to short-circuit the winding, the contactor 2 being simultaneously opened. When the discharge circuit is sufficiently reduced, the relay opens and opens the discharge circuit. The switching system is shown in Fig. 2, 1A to 4A being the contactor coils and 1B to 4B, 2C and 3C being auxiliary switches controlled by the contactors, these switches being shown in the positions they occupy when the corresponding contactors are open. The relay 5 controls a switch 5B which closes when the relay closes. On moving the drum of the controller E to position I, circuit is established through L<1>, 9, 11, 1A, L<2>, to close contactor 1. This closes switch 1B and establishes another circuit 11, 1B, 3B, 14, 3C, 2A, L<2>, to close contactor 2 and switches 2B, 2C. On moving the drum to position 2 an additional circuit is closed through L<1>, 9, 12, 2B, 4A, L<2> to close contactor 4 and cut out the resistance B. To de-energize the magnet, the controller is moved to the " off " position in which it is shown in Fig. 2, cutting off the main supply from coils 1A, 2A, 4A, and closing the switch 4B. Coils 1A, 2A, however, continue to be energized owing to the closure of switch 5B which is connected to the contact point 11. The movement of the controller to the " off " position closes the circuit L<1>, 9, 13, relay 5, 16, 3A, 4B, L<2> to close the contactor .3 and move the switches 3B, 3C to open the circuit through the coil 2A. The switch 3B is so arranged as not to be moved until after the contactor 3 is fully closed, and the switch 3C is short-circuited by the switch 2C so long as contactor 2 is closed, so that the latter contactor is not opened until after the contactor 3 is fully closed. So long as the relay 5 remains closed the contactors 1, 3 are maintained closed irrespective of any further movements of the controller, owing to the closure of the switch 5B, the circuits being L', 5B, 11, 1A, L<2>, and L', 5B, 11, 1B, 3B ,13, relays 5, 16, 3A, 4B, L<2>. If it is subsequently desired to pass a reverse current, the controller is moved to position 3 to close the circuit L<1>, 9, 16, 3A, 4B, and L<2>, to contactor 3 only and pass a reverse current through the resistances B, C, and winding A. No reversal can occur until discharge is complete and the contactor 1 opens. Premature movement of the controller to position 2 during discharge cannot close the contactor 2 owing to the current to the coil 2A being interrupted at the switch 3B. If the controller is left in this position the relay ultimatelv opens and interrupts the current through the coil 3A, the switches 3B, 3C being thereby moved and the circuits through the coil 2A completed. The switch 3C is, however, so arranged as not to close until the contactor 3 is fully open. The current-controlled relay winding D may be replaced by a potential-controlled winding connected either across a low resistance included in the discharge circuit, or across a portion of the magnet winding. An adjustable relay of the type shown in Fig. 8 of Specification 141,870 may be used.