GB511977A - Improvements relating to thermal control devices for electrical apparatus - Google Patents

Abstract

511,977. Thermal switches and protective systems. MICRO SWITCH CORPORATION. Feb. 28, 1938, No. 6311. Convention date, Feb. 27, 1937. [Class 38 (v)] A thermal switch has two bimetallic strips one operated by the temperature of an electric device and the other by heat due directly to the current flowing. In Fig. 3, two bimetallic strips 6 and 7 are both fixed at one end in an insulating support 8 on a base plate 13 covered with heat insulating material 15 which is fixed to a motor or other device. Contact 29 which moves between a fixed contact 26 and a stop 22 is carried at the junction of two bowed compression members 28 which rest at one end in a notch 31 in an insulating block 16 carried by a bent-over end of the bimetallic strip 6, with a tension member 27 held at one end by an upturned lug 21 on bimetallic strip 7. The load current passes from terminal 23 through the two legs of element 6, the closed contacts 26 and 29, the tension member 27 and element 7 to a terminal lug 24. The heated motor cooling air passes over the switch to cause element 7 to bend down until the switch opens with a snap action in the manner described in Specification 419,917, the materials and dimensions of element 6 being such that it bends but little due to changes in air temperature, but with a heavy overload, bending of strip 6 operates the switch, the resistance of element 7 being small so that little heating by the current occurs in it. Modifications are described wherein a pressbutton with optional 'latching needs to be operated before the switch recloses with decrease in temperature, or in which the bimetallic element has four legs, two possible seriesparallel arrangements of which make the switch suitable for use with two values of normal current, or in which the current flows not through the elements but through a separate heater. In Fig. 12 two switches are shown connected in two phases to protect a three-phase motor, the main current flowing through strips 6a and 6b only, while the current passing through the elements 7a and 7b and the switch contacts 29a and 29b energises the trip coil 50 of a circuit-breaker 51. The Specification as open to inspection under Sect. 91 also describes a switch which as shown in Fig. 16 (Cancelled) has a similar snap-action mechanism; the two bimetallic strips 60 and 61 are in contact and tend to bend in opposite directions, the switch opens due to the predominant effect of element 60 when the surrounding temperature changes, or due to a sudden rise in the current passing through the heater 62, the heat from which affects only the strip 60. No current passes through either bimetallic element. This subject-matter does not appear in the Specification as accepted. Reference has been directed by the Comptroller to Specification 481,982.