Patent GB-345966-A (GB)
Title: Improvements relating to protection gear for electrical apparatus and particularly for polyphase electric motors
IPC: H01H83/22
Filed: 1929-12-31
Abstract: 345,966. Protective cut-out arrangements. ELECTRICAL APPARATUS CO., Ltd. and BARBOUR, R. H., Vauxhall Works, South Lambeth Road, London. Dec. 31, 1929, Nos. 39893/29 and 28819/30. [Class 38 (v).] In the protection of A.C. apparatus such as polyphase motors a relay is employed having a member which is normally balanced mechanically and is arranged to move as a whole under the action of three or more windings carrying the load current or a current proportional thereto on the occurrence of general overload conditions but can tilt in one or other of a number of planes if the current in one circuit differs considerably from those in the other circuits. Latches may also be provided to prevent the tilting of the member during predetermined conditions as during a motor starting operation. As shown in Fig. 1 the magnetic plate a, Fig. 3, is normally balanced on a pivot h and is adapted to be acted upon by electromagnets c<1>, c<2>, c<3> connected in the phases of a 3-phase circuit. The pivot h is formed at the upper end of a rod supported by a compressed spring k so that with balanced overload the plate a is drawn down bodily towards the poles of the electromagnets, withdrawing the projection e from a contact f supported by a horizontal blade g in the circuitbreaker maintaining circuit. A disturbance of balance between the phases however causes a tilting movement of the plate on its supporting axis and such movement similarly causes a breaking of the circuit-breaker maintaining circuit. With, however, extremely heavy overloads of short duration such as may occur during the starting up of a protected motor, armatures v of pivoted latches r<1>, r<2>, r<3> are attracted laterally to the poles of the electromagnets and while permitting the downward bodily movement of the armature their upper arms prevent the armature from any appreciable tilting which might occur in response to temporary lack of balance during starting conditions. The armature is biased against its tilting movement by weights q, Fig. 2, which are normally supported on a fixed plate r, one of the weights being picked up by the member a when tilting in one or other direction. These weights may be replaced by suitably formed blade springs. To render the relay responsive to persistent overloads of moderate value the load current is caused to act directly or indirectly on a bimetallic strip p bearing on the end of a plunger l and so regulating the pressure of the spring k. Thus on bending of the strip the spring pressure is released somewhat and attraction of the plate a takes place at a reduced current value. Any number of circuits may be arranged to act upon the member a and in the case of a 2-phase circuit four electromagnets are conveniently provided. The device need not be arranged with the member a in a horizontal plane but could be constructed so that this member is balanced at any angle, or, alternatively it could be inverted in which case the weight of the parts could replace the spring k. Specifications 261,646 and 262,342. [both in Class 38 (iii), Electric motor control systems &c.], are referred to.