Patent GB-840840-A (GB)
Title: Overload protective system for electric circuits
Inventor: PRINCE ARTHUR ENNIS
IPC: H02H7/085
Filed: 1958-07-09
Abstract: 840,840. Electrically-tripped (or released) Switches; electromagnetic switches; protective cut-out systems; control of motors. OTIS ELEVATOR CO. July 9, 1958, No. 21994/58. Classes 38 (3) and 38 (5). An overload protective system includes a device with one or more operating coils and a restraining coil arranged for an inverse time-lag delay in the restraining action. As shown in Figs. 1 ... 3 the operating coils 40 ... 42 attract plungers 46 which are provided with adjustable bolts 54 each passing through one arm of a three-armed plate 37 so that the bolt heads 56 engage its underside. Resilient bumpers 57 on the bolt heads 56 engage stops 58 which act as pivots when only one or two of the plungers 46 are operated. A pivoted lever 30 is biased by a spring 33 to engage a bumper 34 carried by a sleeve 36 which rests at the centre of the plate 37. On overload, initial movement of the plate lifts the sleeve 36 until it engages the head 74 of a stem 72 attached to the armature 70 of the restraining electromagnet 60. The lever 30 carries contacts 85 which during the initial movement disengage fixed contacts 83 to interrupt the D.C. supply to the coil 60 which is in parallel with a capacitor 86 and resistor 90 so that the restraining coil voltage falls as the capacitor discharges. When the pull of the operating coil or coils on the plate 37 overcomes the pull of the coil 60, the lever 30 is moved up to strike a plunger 26 carrying a contact bridge 15 and a permanent magnet 23. The magnet is thus disengaged from a magnetizable bushing 24 and the bridge 15 disengages fixed contacts 16 to interrupt the supply. On de-energization the spring 33 tends to restore the lever 30 but the armature 70 engages one or more of three non-magnetic clips 64, 65, 66 since during operation the plate 37 moves the armature toward the particular coil or coils 40, 41, or 42 which causes operation. When the armature is restored to its central position the contacts 83, 85 are reclosed. The contacts 15, 16 are biased open by a spring 20 and are relatched by depressing a button 18 so that the magnet 23 engages the bushing 24. Only one or two operating coils may be used but the corresponding projections 58 are retained as pivots. Alternatively (Fig. 5) when only one operating coil is required, the plate 37 may be replaced by a bar 99 pivoted at 100. In a further arrangement (Fig. 6) the contacts 83, 85 are replaced by series-connected bridge contacts 105, one on an extension of each bolt 54, with co-operating fixed contacts 106. The lostmotion between the plate 111 and the armature 70 is effected by a sleeve 115 on the plate and a washer 114 on a stud 110 fixed to the armature. The Specification also includes arrangements in which the system is used for the protection of motors particularly for lifts. In Fig. 7 the armature MA of a D.C. hoisting motor is supplied from a generator with an armature GA and series field winding GSF. The generator armature is driven by a three-phase induction motor having stator windings DM1, DM2, DM3. Two of the coils of the protective device PM1, PM2 are connected in series with stator windings and the third coil PM3 is in the D.C. circuit between the generator and motor. Initial operation of the device opens contacts PC2 to allow the capacitor 86 to discharge through the restraining coil RM, and the contacts PC1 open after the time lag. In Fig. 8 a two-speed A.C. hoisting motor has operating coils PM1, PM2 of the device in series with two of its three fastspeed windings FS1, FS2, FS3 and the third coil PM3 in parallel with the operating coil G of the switch for connecting the slow-speed windings SS1, SS2, SS3 to the supply. When the switch G is operated the coil PM3 causes the contacts PC2 to open and if the switch G remains in operation for more than a predetermined time the capacitor 86 discharges sufficiently to permit the separation of the contacts PC1.