Patent GB-778122-A (GB)
Title: Improved control system for a heater element
IPC: H05B1/02
Filed: 1955-10-13
Abstract: 778,122. Automatic temperature-control systems for resistance heaters. GENERAL MOTORS CORPORATION. Oct. 13, 1955 [Nov. 1, 1954; Jan. 14, 1955], No. 29177/55. Class 38 (4). [Also in Group XXXVII] The invention relates to a supply system for an electric heating element, whereby the temperature of a cooking vessel placed on the element may be maintained at one of a number of predetermined values. Thus, in Fig. 2, the movement of a knob 44 to set a desired cooking condition and temperature causes the rotation of a cam 58 to position a thermostat bellows 46 responsive, through a liquid capsule 34 and tube 38, to the temperature of a vessel placed on a heater 22, the latter being energized from a three line source L1, N, L2 and comprising sections of unequal resistance. The setting of knob 44 to a " warm " or " low boil " condition causes the rotation of a cam 72 to permit a switch arm 75 to make with contact 77 and thereby energize a lamp 80 at low voltage from line L1 and the tap on an autotransformer 82 connected between lines L1, N, the lamp remaining energized until the knob is returned to the off position. Cams 74, 76 are also rotated to permit their respective switch arms 121, 131 to make with contacts 94, 125 respectively, whereby the control wires 135, 169 of thermal relays comprising pivoted spring-biased conducting members 139, 167 are energized over thermostat switches 50, 48 respectively from line L1 and the tapping on transformer 82. Wires 135, 169 expand to permit closure of the respective relay contacts by means of switch arms 145, 165 coacting with their respective members 139, 167, whereby the high resistance section 159 of heater 22 is energized across lines L1, L2 and the low resistance section 161 across lines L1, N. At a first predetermined temperature, movement of bellows 46 causes thermostat switch 48 to open, whereupon relay 167 is de-energized to cause the series connection of both heater sections across lines L1, N and thereby reduce the heating effect. If the vessel temperature rises above a second predetermined value, thermostat switch 50 is opened to de-energize relay 135 and hence the heaters, the temperature differential being set by a screw 54. If knob 44 is set to a " medium boil " position, cams 74, 76 rotate sufficiently to cause their switch members 121, 131 to make with contacts 125, 96 respectively, whereby the energizing circuit for relay control wires 135, 169 are reversed and heater section 161 only remains energized between lines L1, N-when switch 48 opens at the first predetermined temperature. For a " high boil " or " fry " setting of knob 44, a cam 78 rotates sufficiently to cause its switch arm 129 to make with contact 98 and thereby complete a shunt path for switch 48 so that full energization continues until switch 50 opens to de-energize the heater. The contour of cam 58 has a portion of constant radius to give the same temperature setting of the thermostat for the setting of knob 44 to any of the " boil " positions. In modifications, Figs. 3, 4, respectively (not shown), (1) a third thermal relay permits reduced energization in the " high boil " and " fry " settings after the first predetermined temperature is reached, the heater section 161 being de-energized when switch 48 is opened, (2) the " medium boil " setting is dispensed with and, in the " high boil " and " fry " settings, heater section 159 is de-energized when thermostat switch 48 is opened as described for the " medium boil " setting in the arrangement of Fig. 2. In another arrangement, Fig. 5, the heater comprises sections 424, 426 of equal resistance and three thermal relays 436, 454, 462 are provided, two of which have double throw switch arms making with one or other of two contacts. The setting of knob 44 to a " warm or " low boil position causes the rotation of cams 474, 476, 478, whereby their respective switch members 480, 486, 490 make with respective contacts 482, 484, 494 to energize the control wires 466, 444 of relays 462, 436 over thermostat switch 48 from line L1 and the tap on autotransformer 82, control wire 456 of relay 454 being also energized over thermostat switch 50. The control wires expand to cause their respective switch arms to make with respective contacts 452, 446, 470, whereby both heater sections are connected across lines L1, L2. When thermostat switch 48 opens, relay wires 444 and 466 are de-energized to cause their respective switch arms to make with respective, contacts 440, 468, whereby heater section 424 only is energized between lines L1, N to give reduced heating.. Opening of thermostat switch 50 de-energizes relay 454 to disconnect the heater. For a setting of knob 44 to a " medium boil " position, cam 478 is rotated sufficiently to cause its arm 490 to make with contact 492 so that control wire 444 of relay 436 is energized over switch 50 rather than switch 48. Thus, when switch 48 opens, only relay 462 is de-energized and both heater sections are thereby connected across lines L1, N. For a " high boil " or " fry " setting of the knob 44, cams 474, 476 are rotated to cause their respective switch arms 480, 486 to make with contacts 484, 488 respectively, switch arm 490 being still made with contact 492. The control wire 456 is now energized through switch 48, and control wire 466 through switch 50, whereby the opening of switch 48 de-energizes relay 454 only to deenergize heater section 424 and leave section 426 connected between lines L1, L2. In a further arrangement, Fig. 6, the heater comprises three Y-connected sections 787, 791, 793 and a composite thermal relay (see Group XXXVII) shown divided for convenience, replaces the relays described in the other arrangements. The relay basically comprises bimetallic elements 739a, 739b, energized between a common contact 743 and respective contacts 725, 749, and a bimetal element 755 energized between a contact 759 and contact 743. Elements 739a, 739b coact when energized with respective switch arms 775, 773 connected to a terminal 769 to cause them to make with respective contacts 781, 777, and element 755 coacts when energized with a double throw switch arm 799 connected to a terminal 797 to cause it to break with contact 1200 and make with contact 1202. If knob 44 is turned to a " warm " position, a cam 684 is rotated to cause its switch arm 686 to make with contact 690, whereby the relay element 739a, 739b and 755 are energized over the appropriate thermostat switch 48 or 50 from line L1 and the tap on autotransformer 82. Switch arms 775, 773, 779 thereby make with respective contacts 781, 777, 1200 to connect the free ends of heater sections 787, 791 to line L1 and the free end of section 793 to line L2, whereby sections 787, 791 are in parallel with each other and in series with section 793 across lines L1, L2. When switch 48 opens, bimetal elements 739b and 755 are deenergized, whereby switch arm 773 breaks with contact 777 to disconnect the end of heater section 787 from line L1 and switch arm 799 re-makes with contact 1202 to connect the end of section 793 to line N, the sections 791 and 793 being now connected in series between lines L1 and N to give reduced heating. Opening of switch 50 de-energizes the bimetal element 739a to disconnect the heater. For a setting of knob 44 to a " boil " or " fry " position cam 684 moves its arm 686 to make with contact 688, so that bimetal 755 is connected through switch 50 rather than switch 48. When the latter opens, therefore, only bimetal 739b is de-energized and heater sections 791, 793 remain in series across lines L1, L2. In a modification, Fig. 7 (not shown), additional cam-operated switches allow a greater variety of heater connections for the following knob settings:-' warm, low boil, medium boil, high boil, and fry.'