Patent GB-815408-A (GB)
Title: Control apparatus for heating units
IPC: H05B1/02
Filed: 1957-03-15
Abstract: 815,408. Automatic control systems for electric cookers. PROCTOR ELECTRIC CO. March 15, 1957 [March 27, 1956], No. 8583/57. Class 38 (4). An automatic cooker control apparatus comprises a first bi-metal arm sensitive to the cooking temperature and a second bi-metal arm remote from the cooker head and, in a pilot circuit, resistance heaters for both bimetal arms, one arm operating a switch controlling the cooker heat and the other operating a switch controlling the pilot circuit. At the centre of radiant ring heater 12, Fig. 2, is a detector and transmitter unit 13, in which is mounted a bi-metal arm 66 carrying a resistance heater 65 and a metal disc 14, which bears on a heated vessel. The heater control is governed by a knob, on the spindle of which are annular arms for operating switch arms bearing contacts 22, 26, 28, 29 and 33 and a further cam-operating latch 30. Fig. 5 shows these contacts in position after initially switching on. For warming or boiling operations control is in two steps-a continuous preheating followed by a thermostatically controlled approach and subsequent maintenance of the desired temperature. For frying, the thermostatically controlled phase is accompanied by further intermittent switching on and off to limit the heat supplied at the high temperature, but low load conditions pertaining to the frying operation. The preheating step is further subdivided; initially heater 12 is energized via the circuit L1, contacts 26, 27, 36, 34, heater 12, resistance heater 38, contacts 32, 28, lead 42, L2. When heater 38 has bent bi-metal 43 the contacts 32, 28 open and the current passes through the parallel, but higher resistance circuit 43, 44, 45, contacts 33, 29, arm 46, latch prop 35, L2. The latch prop being bi-metal, expands and bends releasing the latch 30 to complete the preheat cycle. The duration of the cycle depends on the initial knob setting which, by cam and follower action, overcloses arm 39 by a corresponding amount. During the preheat cycle the pilot circuit has been operating; this comprises L1, contacts 26, 27, lead 60, resistance heater 57, lead 59, contacts 62, 63, resistance heater 65, N. The bi-metal 66 operates contacts 62, 63 by means of button 84, Fig. 2. Heater 65 operates these contacts, and with them heater 57, cyclically during preheat, the ratio of off to on periods increasing as bi-metal 66 bends more under the simultaneous effect of heat from the vessel. Heater 57 bends contact 23 towards contact 22, the position of the latter depending on the knob setting, as described for contact 28. For most heat settings contacts 22, 23 have a degree of overclose. When latch 30 releases and makes contacts 49, 52 the heater 12 circuit is established through main contacts 22, 23 and it remains closed until the vessel heat is such that the off to on ratio of the pilot circuit gives heater 57 sufficient time to cool in the off cycle for contacts 23, 22 to open. Thereafter the main heater circuit cycles with the pilot circuit the off to on ratios increasing as the desired temperature is approached asymptotically. For frying a further intermittent switching is introduced. When latch 30 releases, arm 45 tends to open contacts 33, 29, but is prevented by its cam and follower from doing so except in the frying heat range. With these open heater 12 circuit has to be completed through contacts 28, 32 which are themselves cycled intermittently by heater 38. In an alternative embodiment, heater 12 is energized across terminals L1, L2 only, during preheat, its energy consumption being then four times that for normal connection across L2, N. U.S.A. Specification 2,666,124 is referred to.