Patent GB-587947-A (GB)
Title: Improvements in electrical energy regulators for electric cookers, ovens and the like
IPC: H01H73/28
Filed: 1944-08-02
Abstract: 587,947. Thermal switches. WOOLNOUGH, G. L., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Aug. 2, 1944, Nos. 14767/44 and 4374/45. [Class 38 (v)] A bimetallic thermostatic regulator switch for hotplates &c. comprises a base carrying an operating knob and spindle on which is a cam member adapted to displace a bimetallic strip relatively to fixed and movable contacts in a direction perpendicular to terminal members which carry the movable and fixed contacts respectively and which extend parallel to the axis of the cam member Fig. 4 shows a regulator employing a snap-action switch of the kind described in Specification 419,917. The fixed contact 5 is carried by the terminal strip 3 which extends in a direction parallel to the axis of the cam member 36. The movable contact 6 is carried by a member comprising three blades 8, 12, 13, the centre blade 8 being secured at its lower end to the terminal strip 2 which has a vertically extending portion 10 having grooves 11 at its lower end to receive the ends of the blades 12, 13. The part 10 carries an adjustable screw stop 15. The switch is actuated by a bimetal device as described in Specification 517,538 and which comprises two bimetal elements 17, 18, the element 17 carrying a heating winding 19 and the element 18 compensating for changes in the ambient temperature. Both elements are carried by a block 20 through which passes a bolt 23 pivotally mounted in the base 1 and the free end of the element 18 is formed with a projecting portion 18a which abuts against the blade 8 so that when the element 18 moves outwardly the contacts 5, 6 are separated. Pivotally mounted on the bolt 23 and independently of the block 20 is a lever 26, 27, 28 carrying an adjusting screw 29 bearing against the strip 17 and also a bent extension 37 co-operating with the cam 36. When the operating knob and cam 36 are in the off position the extension 37 holds the lever 26, 27, 28 and screw 29 in such a position that the projection 18a separates the contacts 5, 6 whereby the current to the heating element of the hotplate is interrupted. Fig. 5 shows the connections from the supply mains L1, L2 to the heating element H by way of the terminal 3, contacts 5, 6 and terminals 2, 4. The winding 19 is shunted across the heater H between the terminals 2, 4 and in series with the contacts 5, 6 The surface of the cam 36 is of such a shape that as the operating knob is rotated in a clockwise direction the extension 37 allows the screw 29 to move away from the strip 17 thus withdrawing the projection 18a from the blade 8 and allowing the contacts 5, 6 to close. The hotplate is thereby energized and the winding 19 heats the strip 17 which at a certain temperature is bent sufficiently to cause the projection 18a again to separate the contacts 5, 6, the opening and closing cycle being continued indefinitely. Further rotation of the knob and cam 36 brings a progressively smaller radius of the cam into contact with the extension 37 thus removing the screw 29 further from the strip 17 and requiring a higher temperature of the strip to open the contacts 5, 6. When the extension 37 abuts against the stop 38a on the cam the deflection of the strip 17 is insufficient to open the contacts and the hotplate is energized continuously. The setting of the switch contacts may be varied by the adjusting screw 15 or by the peg 9 by which the blade 8 is secured to the terminal 2, the screw 29 serving as an adjustment for the bimetal strip assembly. Fig. 7 shows an end view of a regulator employing two similar snap-action switches for controlling a two-element hotplate by means of a single knob and cam 36. The contacts 50, 51 of the second switch are connected respectively to the terminal 4 and a fourth terminal 52. The lever 26, 27, 28 shown in Fig. 4 is replaced by a lever carrying in addition to the stop 29 an adjustable stop 59 bearing against the blade 8a of the second switch. The stop 59 is carried by a cross member extending between parallel links forming a part of a secondary adjusting lever 54 pivoted to lugs 53 extending from the main lever 28. The cam 36 is formed with an eccentric projection or boss 61 which bears against the side of one of the links supporting the cross member carrying the stop 59. Rotation of the cam 36 causes the boss 61 in one position of the cam to displace the lever 54 and close the contacts 50, 51. The second element of the hotplate is then energized continuously and is brought into action after the cam has been turned to a position to cause the first heating element to be energized continuously. A thermostatic device may be incorporated in the circuit of the second heating element. In another modification in which two snap-action switches are employed the contacts 50, 51 shown in Fig. 7 are normally closed and are opened positively by the stop 59 when the boss 61 attains a certain position as the cam is rotated. Specification 587,953, [Group XXXV], also is referred to.