GB1473582A - Electrical heating system - Google Patents

Abstract

1473582 Induction heating SACHS SYSTEMTECHNIK GmbH 14 May 1974 [25 May 1973 28 March 1974] 21185/74 Heading H5H [Also in Division H2] An induction heating system for warming plates or containers comprises an induction heating coil 5 mounted beneath or within a rigid, non-magnetic electrically insulating panel 1 and a co-operating secondary means mounted on or associated with the plates or containers, the secondary means being in the form of a magnetic or electrically conductive layer 8. As shown in Fig. 1, a table-top panel 1 has mounted below its surface, a primary coil and core arrangement 5, 4 to receive current from a device 6 in the frequency range 5-20 kHz. The coil 5 is preferably of band-like conductor spirally wound. A food-dish 3 having a ferromagnetic or electrically conductive base covering 8 as secondary means is placed over the primary coil and this base is heated by the inductive energy transfer. The covering 8 may be of perforated foil mounted with epoxy resin adhesive, and possibly having a protective external layer of this adhesive or ceramic glaze. Alternatively an electrically and thermal insulating pad having a first face covered in a ribbed foil may be used and a vessel to be heated placed on the foil. A suitable driving circuit is shown in Fig. 5 and comprises push-pull transistors 24, 24<SP>1</SP> supplying a centre-tapped coil 20 which with capacitor 38 forms a weakly damped resonance circuit. Transformer 26 provides feed-back to render the generator self-oscillating. The transistors are controlled by resistor 27 which is in parallel with capacitor 21. Under no load the transistors oscillate with a small power consumption, and when a load, e.g. of foil 8 is coupled with the field, the resonant properties of the oscillator are changed with an increase in oscillatory frequency, and the circuit assumes the characteristics of a highly damped leakage reactance transformer. In a modification, Fig. 6 (not shown), resistor 27 is replaced with a further oscillator circuit of a coil and a capacitor together with a further transistor shunted by a resistor. This oscillator is tuned to the frequency of circuit 20, 38. Under no load, very little power is dissipated by the transistors 24, 24<SP>1</SP> owing to the high impedance of the second oscillator circuit. When a load is introduced this oscillator circuit goes off resonance and causes its associated transistor to conduct further to apply a greater base current to transistors 24, 24<SP>1</SP>.