Patent GB-794391-A (GB)
Title: Improved control system for electric heater elements
IPC: H01H37/50
Filed: 1956-04-18
Abstract: 794,391. Automatic temperature control systems; switching systems for consumption apparatus. GENERAL MOTORS CORPORATION. April 18, 1956 [May 16, 1955], No. 11823/56. Class 38 (4). [Also in Group XXXVII] In an automatic temperature control system for an electric hot-plate, of the kind in which a thermistor is resiliently held against the base of a vessel on the hot-plate, the thermistor is connected in series with a hot-wire relay across an auxiliary supply, and the relay controls the current to the electric heating element. In one embodiment, Fig. 1, the thermistor 42 is held by a spring 30 against a vessel 38 placed on the hot-plate 20. The thermistor 42 is in series with a resistance wire 56 across the secondary of a step-down transformer 74. When the wire 56 is cold it pulls contacts 57 and 62 down into contact with an arm 66 against the influence of blade springs 58, 68. This connects the wires 88, 131 of two further hot-wire relays across the secondary of transformer 74 and expansion of these wires closes contacts 92, 98 and 137, 139 so that the heater element 20 is connected between outer lines L1, L2 of a single-phase 3 wire supply to provide maximum heating. As the temperature of the thermistor rises, the current through wire 56 increases, causing it to expand and allow the contact assembly 57, 62, 66 to rise until contact arm 66 meets a temperature selecting cam 147 and contacts 66, 62 separate. This de-energizes wire 88 to change contact 92 from 98 to 96 and so connect the heater element 20 between the neutral supply conductor N and line L2 to reduce the temperature. If, however, this reduction is insufficient, further expansion of wire 52 opens contacts 57, 62, when contact 62 is held by an abutment 70. This de-energizes wire 131 to open contacts 137, 139 and open-circuit the heater 20. In a second embodiment, Fig. 4, the hot-wire 224 in series with the thermistor 24 is compensated against supply voltage changes by a second wire 228 connected across the secondary of the step-down transformer 232 to adjust the position of a spring support 226. When cold the wire 224 holds the contact stack 246, 250, 254 closed against the pressure of a spring 244. As the thermistor is heated, two-stage control is again effected. Contacts 250, 254 open first to disconnect one half of the electric heater element 20 from the supply L1, L2, leaving the other half of the element 20 energized from the supply L1, L2 via an interrupter 325 which provides intermittent heating. Further temperature rise opens contacts 246, 250 to disconnect the intermittent heating. The temperature selecting cam 262 has projections 266, 272 which open-circuit all supplies in the " off" position, and a cam 298 ganged with cam 262 closes contacts 296, 321 in the high temperature range so as to short-circuit the interrupter 325 and apply full voltage to the second part of the heater 20. In another embodiment, Fig. 7 (not shown), a compensated hot-wire relay similar to that in Fig. 4 controls the heater through a further double hot-wire relay as in Fig. 1. In a modification, Fig. 8 (not shown), an alternative form of voltage compensation for the relay is disclosed.