GB642408A - Improvements in or relating to electric ovens - Google Patents

Abstract

642,408. Automatic temperature control of heating resistances. BAKOVENBOUWVOORHEEN H. P. DEN BOER N.V. May 2, 1947, No. 11914. Convention date, March 7, 1946. [Class 38 (iv)] In an electric oven 4 two separate thermostats 16,20 are provided which control a magnet switch 6 for switching on and off the entire heating device, and a star-delta switch 28 is provided for disconnecting a portion of the heating device, or connecting it in series or parallel with the remaining portion. A three-phase alternating current network 1 supplies the oven through conductors 5 and the magnet switch 6, controlled by an energizing coil 10, the circuit of which is completed from a point 12 through contacts 15 of the thermostat 16, set at the required temperature of opera. tion, to one side of the coil 10 and through a switch 18 and contacts 19 of the thermostat 20, set at a lower temperature, or through the switch 18 and holding contacts 11, 17 of the magnet switch 6 to the other side of the coil 10. During baking a byepass switch 24 in parallel with the thermostat 20 is closed so that operation about the required temperature is maintained by the thermostat 16. After baking the switch 24 is opened and the oven remains switched on through the holding contacts 11, 17 and switch 18. When, however, the contacts 15 open, the holding circuit is broken and the oven cools down irrespective of the operation of thermostat 16 until it reaches the lower temperature at which thermostat 20 operates. It will then heat up again to maximum temperature despite the opening of contacts 19 of the thermostat 20, due to the action of the holding circuit which is completed when the thermostat 20 operates. The supply of energy to the top portion of the oven 26 and the bottom portion 27 is controlled by astar-delta switch 28.