Patent GB-880102-A (GB)
Title: Improvements relating to mining control systems for electric motors
Inventor: CHRISP GEORGE FRANCIS
IPC: H02P1/58
Filed: 1957-03-14
Abstract: 880,102. Control of two motors; protective cut-out systems. REYROLLE & CO. Ltd., A. March 14, 1958 [March 14, 1957], No. 8492/57. Classes 38 (3) and 38 (5). Electric motors 6, 7 operating in a mine, are respectively incorporated in a slicer unit and a haulage unit, a five-core trailing cable 8 being used to connect the motor 6 to a supply panel 10, whilst a three-core cable 9 carries the electrical connections between the motor 7 and a second supply panel 11, both panels being connected to a three-phase source 12. When a contact drum 49 is moved to connect contacts 50, 51, 52, a pilot-core 40 is connected in series with resistor 45 and rectifier 44 to earth-core 41, so that sufficient rectified current is passed to operate relay 24. Thus, contactor 13 is energized to start the slicer motor 6, and contact 19 opens to remove a short-circuit from the operating coil of a relay 25 which is connected in series with the coil of relay 24. The current in the pilot circuit is insufficient to operate the relay 25 at this time, however. The contact drum is now adjusted so that the pilot circuit comprises both resistors 45, 46, whereby the current is reduced to a value sufficient to hold the relay 24 only. In the next position, both resistors 45, 46 are short-circuited to close the relay 25, operate contactor 14, and start the motor 7. The controller has a final position in which the pilot circuit is completed through both resistors and the current is sufficient to hold both the relays 24, 25. The pilot circuit is supplied through transformer 34 having its secondary winding connected to resistor 37 and oppositelyconnected rectifiers 38 which serve to stabilize the voltage. The relays 24, 25, each have a copper sleeve surrounding the core so as to respond to half-wave rectified current but not to alternating-current. Thus, an earth fault on the pilot core cannot cause either of the contactors to close since the fault current is alternating in character. In a modification, the contactor 14 is replaced by two contactors for reversibly controlling the motor 7, and the relay 25 is replaced by forward and reverse relays each associated with one of the contactors, Fig. 3 (not shown). The operating coils of the relays are connected in parallel and each has one of two rectifiers of opposite polarity in series with it. The rectifier 44 now has an oppositelypoled rectifier and a two-way switch associated with it, so that the switch may be adjusted for selectively operating either the forward or reverse relay. Each of these relays has contacts which, when the relay is energized, prevents energization of the contactor associated with the other relay. In the event of an earth fault on the pilot core, the relay 24 drops-out to open the contactor 13 and to short-circuit the coils of the forward and reverse relays for the haulage motor. In a further embodiment, the contact drum 49 is replaced by a number of self-restoring, on-off, push-button switches, the haulage motor 7 being stopped as well as started from a remote position. The operation of a stop-switch for the haulage motor causes alternating current to flow in the pilot core as well as unidirectional current. The net current is sufficiently approximate to alternating current for the relay associated with the haulage motor to be released. However, the operating coil of the relay 24 is shunted by a rectifier and resistor at this time, and the relay remains operated, Fig. 4 (not shown). In this embodiment, if a fault occurs when both the relays 24, 25 are operated, the relays will be released if the fault current exceeds a predetermined value. Specification 521,729 is referred to.