Patent GB-787145-A (GB)
Title: Improved control system for a heater element
IPC: H05B1/02
Filed: 1955-10-13
Abstract: 787,145. Automatic temperature control systems for resistance heaters. GENERAL MOTORS CORPORATION. Oct. 13, 1955 [Nov. 1, 1954; Nov. 1, 1954], No. 29176/55. Class 38 (4). In a temperature control system for a cooking vessel placed on an electric heating element, the initial continuous energization of the heater is reduced, if a first predetermined temperature is exceeded, by supplying the element through an intermittently-closed thermal switch; and, if a second higher temperature is exceeded, the energization is either cut-off or further reduced by supplying the element through a second thermal switch of lower " on-off " ratio than the first. Thus, in Fig. 1 the setting of a knob 94 to a desired temperature causes the movement of a shaft 96 to close contacts 86-88, this movement being also communicated through a coupling 125 to a thermostat bellows 30 responsive to the vessel temperature through a tube 32 and element 34, whereby a member 42 attached to the bellows is displaced sufficiently to cause the closure of thermostat contact pairs 44-46 and 54-56. Heater elements 24, 26 are then energized over contact pairs 86-88 and 44-46 from lines L1, L2, elements 28, 26 being energized from the same lines over contact pairs 86-88, 54-56 and the intermittentlyclosed contacts 66, 68 and control wire 76 of a cycling thermal switch 62. The latter comprises a pivoted member 70 coacting with the pivoted contact arm 68, whereby expansion of currentcarrying wire 76 allows a bias spring 80 to turn member 70 and thereby pivot contact arm 68 against the action of its bias spring 74 to break contacts 66-68, the cycle being completed by the subsequent closure of the contacts when wire 76 contracts on cooling. At the selected temperature, the bellows 30 has expanded sufficiently to open contacts 44-46, whereafter heating is maintained at a reduced rate by the continued energization of heater elements 26, 28 only through the cycling switch 62. If a somewhat higher temperature is exceeded the further expansion of bellows 30 causes contacts 54-56 to open and thereby de-energize the heating elements. For a high temperature setting by knob 94, the rotation of a cam 129 on shaft 96 is sufficient to cause a contact arm 131 to close with a contact 135, whereby a shunt path is complete across the cycling switch 62. The turning of knob 94 to an " on " position also causes the closure of contacts 141-142, whereby a lamp 145 is energized over line L2 and a neutral line N. In a second arrangement, Fig. 4, the heater 210 is energized over lines L1, L2, through cam-operated line switches 225, 226 and four series-connected switches 240, 215, 217, 241. Switches 215, 217 are operated by a movable arm 216 in response to the expansion of a thermostat bellows 212, and switches 240, 241 are cycled on and off by respective heater windings 246, 249 for their respective bi-metallic elements 245, 250, the windings 246, 249 being connected in shunt across switches 215, 217 respectively. In operation, movement of a knob 229 to set a desired temperature closes line switches 225, 226 to energize heater 210 over closed series switches 215, 217, 240, 241, the cycling switches 240, 241 being permanently closed since their windings are short-circuited by closed switches 215, 217. At the desired temperature, switch 215 is opened to energize the winding 246 of cycling switch 240, whereby the latter is intermittently closed and the heater 210 is energized at a reduced rate, over the shunt path for open switch 215 provided by winding 246. If a somewhat higher temperature is exceeded, switch 216 is opened to energize the winding 249 of cycling switch 241, whereby the latter is intermittently closed and the heater is energized at a still further reduced rate over the shunt path for open switch 217 provided by winding 249, the cycling switch 240 being now permanently closed since its cycling is prevented by the shorter " off time of switch 241. In a third arrangement, Fig. 6 (not shown), cycling switches of the type described in the arrangement of Fig. 1 replace the bi-metallic cycling switches in the arrangement of Fig. 4.