GB712824A - Improvements in electrical load controlling devices - Google Patents

Abstract

712,824. Load limiting arrangements. ELECTRICAL APPARATUS CO., Ltd. Jan. 7, 1952 [Jan. 17, 1951; May 9, 1951; June 16, 1951], Nos. 1244/51, 10860/51 and 14308/51. Class 38(4) A load limiting arrangement, adapted to initiate repeated opening and closing of the connection between a supply terminal P and a load terminal P1 under conditions of excessive load, and which is also responsive to the supply voltage or frequency, comprises a normally closed thermal switch 14-17 interconnecting the terminals P, P1 (through the primary winding 18 of a current transformer 19) and having its bimetal elements 14, 15 influenced directly by the load current and by a supplementary heater 20 which is connected across the normallyopen contacts of a second thermal switch 25-29. Assuming a plug switch 43 closed, heater 29 of the second thermal switch is energized from terminal P and neutral N over a series circuit including primary and secondary windings of transformer 19, heater 29, resistor 36 and rheostate 38, so that contacts 27; 28 close and short-circuit heater 20 of the first thermal switch. Reduction of the supply voltage during a peak load period, to a prearranged value determined by the setting of rheostat 38, allows contacts 27, 28 to open, so that heater 20 (in series with a normally short-circuited resistor 37) is now energized from the secondary winding of the current transformer 19 and will cause periodic opening and closing of the first thermal switch if the load is excessive. With the plug switch closing contacts 44, a circuit 34, 35, tuned to slightly below normal supply frequency, replaces resistor 36, so that opening of contacts 27, 28 is now effected by a reduction in supply frequency. Removal of a plug switch 45 renders resistor 37 effective and so makes the heater 29 partly responsive to load. In a modified arrangement, a tuned-reed switching device, operated by a transformer in the supply line, completes a heating circuit for opening contacts 27, 28 upon a predetermined reduction in supply frequency. The thermal switches may have magnetic or toggle devices for ensuring snap-action. Specification 712,850, [Group XXXVII], is referred to.