Patent GB-1389574-A (GB)
Title: Solid state induction cooking appliance
IPC: H05B6/12
Filed: 1972-06-12
Abstract: 1389574 Control systems for high frequency heating apparatus; controlling inverters GENERAL ELECTRIC CO 12 June 1972 [19 Nov 1971] 27352/72 Headings H2H and H2F [Also in Division H5] A cooking apparatus comprises an induction heating coil 15 mounted beneath a untensil support plate and a static power conversion unit supply oscillating electrical energy to the coil 15 at ultrasonic frequency. As shown a pancake coil 15 comprising two series sections 15a, 15b is fed from A.C. 21, 22 via a full wave rectifier 13 with a filter capacitor 24 and an ultra-sonic frequency inverter 14. Inverter 14 comprises thyristors 27, 28 connected in series between the D.C. supply terminals from rectifier 13. The junction 15c of coil sections 15a, 15b is connected to the junction 29 between commutating capacitors 30, 31 across the terminals 25, 26. Thyristors 27, 28 may be replaced by diacs, triacs or transistors. As shown the heating coil 15 provides the commutating inductance, although a separate commutating inductance (Fig. 6, not shown) may be provided. The gating circuits for thyristors 27, 28 have reference-slave relationship. The reference gating circuit for thyristor 28 is controlled by switch 33 and includes a Zener diode 36 providing a regulated voltage of R.C. charging circuit 37, 38, 39. Adjustable resistor 38 is manually adjustable to vary the repetition rate of the generation of firing pulse. A Shockley diode 40 or unijunction transistor connected to the junction of resistor 38 and capacitor 39 breaks down when capacitor 39 reaches a predetermined voltage and applies a firing pulse to thyristor 28. A biasing resistor 41 is provided in the cathode gate circuit of the thyristor and an overvoltage protecting "thyristor" selenium Zener diode 42 is provided across the thyristor 28. The slave gating circuit for thyristor 27 is similar except for a by-pass resistor 43 in parallel with Zener 36<SP>1</SP>. On closure of switch 33 a gating signal is applied to thyristor 28 as timing capacitor 39 discharges through diode 40 and the cathode-gate circuit of the thyristor,thereby also resetting the timing capacitor 28 begins to conduct and causes the voltage of commutating capacitor 31 across coil section 15 which causes a strong reverse voltage pulse to be applied to thyristor 27 by autotransformer action. Capacitor 30 charges whilst capacitor 31 discharges so that at the end of the half cycle junction 29 becomes negative relative to terminal 26. The slave gating circuit associated with thyristor 27 operates in the same manner as the reference gating circuit from the source voltage provided by capacitor 30. In an alternative embodiment (Fig. 7, not shown), the inverter is supplied with rectified current from a three phase supply. Capacitors can be switching in parallel with the commutating capacitors to control the current in induction coil. Similarly, the current in the coil can be varied by switching in a different coil section. The gating circuits are controlled by a temperature sensor detecting the utensil temperature, e.g. a thermostat. The gating circuits comprise a single phase source feeding a full wave rectifier connected to a unijunction relaxation oscillator feeding a bistable multivibrator connected to the firing electrodes of the thyristors.