Patent GB-818371-A (GB)
Title: Improvements relating to means for controlling the power dissipation in electric heaters
Inventor: PANISSET DENIS SYDNEY
IPC: H05B1/02
Filed: 1954-08-16
Abstract: 818,371. Electric resistance heaters. SIMPLEX ELECTRIC CO. Ltd. Aug. 16, 1955 [Aug. 16, 1954; July 1, 1955], Nos. 23737/54 and 19031/55. Class 39(3 [Also in Group XXXV] An electrically heated device 10 having a substantial mass e.g. a cast iron hot plate or a tubular sheathed resistance element, which does not rapidly reach working temperature from cold with normal current supply, is rapidly heated to working temperature by an overload current which does not involve a series-parallel conversion of the heating resistor 11. The current in the resistor 11 both during the starting overload period and the running period is controlled by a regulator effecting a cyclic variation in the current to determine the average value thereof (e.g. a resistance heated bimetal 15) and means are provided to change over to normal load when the device reaches working temperature. In series with the resistor 11 is a resistor 12, which may be in thermal association with the device. Current is supplied from terminals 14 to the resistors 11, 12 through contacts 13 in series therewith operated by a thermally sensitive responder, which may be a bimetallic strip 15 heated by passage of current therethrough or by an adjacent winding of resistance wire or may be a sensitive element of magnetic material undergoing a useful change of permeability with temperature variation with an electric heating resistance adjacent thereto. In order to render the temperature sensitive element 15 immune to changes in ambient temperature, it is mounted in a housing 16 preferably with lagged walls containing a secondary heating resistor 17 connected in parallel with the resistor 11 or 12. Adjacent the resistor 17 is a thermally sensitive element 19, which comprises a bimetal strip preferably responsive only to the temperature of the housing, although it may be heated by passage of current therethrough as shown or by an additional local heating element preferably a series resistor. The bimetal 19 maintains the housing 16 at a desired temperature above the ambient. Thus the bimetal 15 is primarily heated at 17 to a temperature at which a cycling overload current is passed to the resistor 11, and later is heated in addition by secondary heat from the housing 16 as it warms up, and then a normal cycling load current passes to the resistor 11. Transmission of heat from resistor 17 to the bimetal 15 may be delayed by interposing a thermal barrier such as an asbestos sheet. The controlled temperature of the housing may be adjusted by displacing a contact 18 not mounted on the bimetal 19 by a cam or other mechanical device e.g. to 80‹-120‹C. or up to 300‹ C. where the bimetal 15 is arranged to open the contacts 13 above this temperature. The knob for adjusting the contact 18 may carry the main switch. The housing may take 2-4 minutes to reach its preset temperature, and to ensure delay the heater 17 may be wound on a former of large thermal capacity. The normal overload is 25-50 per cent above the normal load of the device but may be 200 per cent, in which case the starting period may be 30-45 seconds. The housing 16 and its components should have a cooling characteristic corresponding closely to that of the device 10, so that if the latter is still warm after use, the starting up time is shortened. Current regulators other than those operated by thermally sensitive responders may be used e.g. motor driven cams operated to give an overload on starting by other means. Specifications 802,372 and 818,372 are referred to.