GB500472A - Improvements in and relating to high speed releasing devices suitable for electric circuit breakers - Google Patents

Abstract

500,472. Protective cut-out arrangements ; electrically - released switches. BRITISH THOMSON-HOUSTON CO., Ltd. Aug. 4, 1937, No. 21514. Convention date Aug. 4, 1936. [Class 38 (v)] A high-speed releasing device comprises a magnetic circuit including a relatively-movable armature, biassing means for causing relative movement between them, a source of M.M.F. normally holding them in closed magnetic circuit position against the bias, a trip coil related to the circuit so as to produce a flux in that part of it including the armature opposing that produced by the M.M.F., and a control circuit for energizing the trip coil including a non-inductive resistance in series with the trip coil, and a capacitance arranged to be connected across the trip coil when the coil is energized, for increasing the rate of building up of the tripping flux to the tripping value. The tripping circuit of the breaker 1, Fig. 1, comprises a relay 4 controlled by a current-transformer 5 in the line and arranged, when released, to close a switch S which makes, over contacts 25, 26, the circuit of the trip coil T of the trip magnet 3, whereby the armature 20 is released and spring 23 rotates latch 22 about pivot 22<1> to remove the latch 24 and allow the breaker to open. The relay and trip device are similar and of the type described in Specification 125,279, [Class 38 (v)]. The trip device comprises the permanent magnet (or an electromagnet) 17, soft-iron poles 18, 19 and armature 20, the action of the trip coil T being to weaken the holding flux on the armature 20. Movement of the latch 22 closes contacts 33 to energize a coil 32 which establishes over closed contacts 26 the circuit of relay resetting coil 30. A remagnetizing coil 34 for the permanent magnet 17 is energized by momentary closure of contacts 35 as the breaker 1 recloses, which latter operation may be effected by means (not shown) which may also reset latch 22. The permanent magnet 17 may consist of a precipitation-hardened product such as a nickel-iron-aluminium and cobalt alloy of the character set out in Specifications 423,897 and 424,746, [both in Group II]. The poles and core of the trip coil are of soft-iron laminations, and the core is spaced from the poles by non- magnetic members. Since the permeance of the precipitation-hardened magnetic alloy is low as compared with that of the poles and armature, the " bucking " flux in the latter will be relatively large and build up to the tripping value more quickly. A further increase in speed of tripping is obtained by reducing the number of turns in, and hence the inductance of, the trip coil T, and placing a non-inductive resistance R in series therewith for fixing the ultimate trip current, curves being given in the Specification in illustration of this. The effect is further increased by placing a capacitance C in shunt across the trip coil, the voltage across which capacitance is that of the source 12 when the contacts 26 are closed, and is reduced when switch S closes, the capacitance thus discharging through the trip coil and increasing the rate of building- up of the trip current. Specifications 455,860 and 600,214 also are referred to.