GB568336A - Improvements in or relating to protective control systems for electric motors - Google Patents

Abstract

568,336. Protective cut-out arrangements. BROOKE, W., and BALDWIN, H. T. July 3, 1943, No. 10837. [Class 38 (v)] A remote-control and protective system, suitable for supplying mining apparatus over a trailing cable, and of the type wherein a main contactor is controlled by a remote switch (usually at the apparatus location) over a pilot circuit (usually incorporated in the trailing cable) and a relay device, employs as the relay a gas-filled triode the grid-potential of which is controlled by the remote switch. for operating the main contactor. The relay f controlling the latter is in the anode circuit of the valve F the grid of which is normally biassed against operation by an alternating-potential derived from the winding s<1> of a transformer H, the grid being connected to the mid-point of resistances g, k. A suitable bias-voltage is 15 volts, but by dispensing with k, 30 volts may be obtained. For connecting the grid n direct to the cathode, which is earthed at a to the gate-end box A, the pilot and earth-continuity conductors are connected to one another at will by a control switch G at the motor-location, so enabling anode current to flow, but if the pilot circuit becomes opened due to opening the switch G or mechanical fracture of the conductors, the grid-bias is re-applied and the motor stopped. When k is dispensed with, the earth-connection from a is taken to the left-hand end of s<1> and an inductance is connected by the switch G in parallel with the resistance g, either at the gate-end box, Fig. 2 (not shown), or at the motor, Fig. 3 (not shown), the latter arrangement additionally providing protection against short-circuiting of the pilot circuit. This last arrangement may be further modified to provide against automatic restarting after a supply failure, for which purpose the switch G has another contact u, Fig. 4, for short-circuiting the inductance t. When the supply is restored, and contact u is engaged, a reset relay J is energized from s<1> over leads 7, 8 and contacts v<3> of a no-volt relay K, the coil, y of which is energized, by closure of contacts x of relay J, over a circuit including conductors 11, 10 connected to s<1>. Relay K makes its own holding circuit at v<1> and the anode circuit at v<2> and opens at v<3> the energizing circuit of J. When switch G is changed over, the valve F is made conductive and the motor runs, but if the supply fails and is restored, the relay J cannot be re-energized to re-start the motor until switch G is moved over to contact u to cut out the inductance.