Electric heating – Inductive heating – With heat exchange
Patent
1999-07-19
2000-11-28
Leung, Philip H.
Electric heating
Inductive heating
With heat exchange
219670, 219665, 219666, H05B 608, H05B 612
Patent
active
061538639
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to induction heating apparatus.
Cooking hobs are known which comprise one or more large induction coils, on which pans having an electrically inductive base can be stood. In use, a high frequency signal (in excess of 20 kHz) is applied to the coil, which generates a magnetic field that induces eddy currents in the pan base. The base of the pan is not an ideal conductor, and thus the electrical energy is dissipated as heat as current flows through the pan base. Thus, the heating effect is proportional to I.sup.2 R, where I is the current in the base of the pan and R is the electrical resistance of the pan.
The resistivity of the pan base depends on the material that it is made from. Thus, it will be appreciated that the temperature which the pan base reaches will be dependent on the material of the pan, with the obvious disadvantage that discrepancies will occur between the heat setting, which has been selected by the user, and the actual heat developed.
SUMMARY OF THE INVENTION
We have now devised an inductive heating apparatus which alleviates the above-mentioned problem.
In accordance with this invention, there is provided an induction heating apparatus comprising an inductive heating coil, means for sensing the current and/or voltage value of an a.c. supply applied to the heating coil, means for selecting the desired heat output of the apparatus, means for comparing the sensed current and/or voltage value with an output of the selecting means, and means for varying a parameter of the supply to the heating coil in accordance with the value of an error signal output from the comparing means.
In use, we have found that the heating coil of an inductive heating apparatus acts rather like the primary winding of a transformer with the pan acting as a single shorted turn secondary winding. The heating effect in the base of the pan is proportional to I.sup.2 R, where I is the current in the base of the pan and R is the electrical resistance. The heating effect in the base of the pan is also dependent on the depth of penetration of the magnetic field into the base, and this depth of penetration is inversely proportional to the coil frequency. Thus, it will be appreciated that the heating effect at a given frequency can be determined by measuring the current and/or voltage at the coil primary, and that the heating effect can thus be varied by varying the current, voltage or frequency value applied to the coil.
In one embodiment, the varying means is arranged to vary the drive frequency which is applied to the coil, in order to vary the depth of the penetration of the magnetic field into the pan base, so that the heating effect is correspondingly varied.
Preferably the heating coil forms part of a tuned circuit, which is preferably arranged to oscillate at its resonant frequency, in order to maximise the voltage across the coil, and hence maximise its efficiency.
Preferably the resonant frequency of the heating coil is varied as the drive frequency is varied, by varying the impedance of the coil and/or by varying the capacitance of the tuned circuit.
Preferably the impedance and/or capacitance is varied by respectively switching inductors and capacitors into or out of the tuned circuit.
A disadvantage of varying the operating frequency of the coil is that beating or heterodyning can occur if there is more than one heating coil in an inductive heating apparatus. This beating or heterodyning occurs when the coils operate at a different frequency, thereby causing a third frequency of a value which is equal to the difference in the coil frequencies. Often, this frequency will be less than 16 kHz, with the result that it is audible and annoying to users.
Thus, it is preferable that the coil operates at a fixed frequency in order to avoid the problems of heterodyning.
Thus, in an alternative embodiment the varying means is arranged to vary the value of the current applied to the coil by varying its number of turns, and from this it will be appreciated that a differen
REFERENCES:
patent: 4426564 (1984-01-01), Steigerwald et al.
patent: 4820891 (1989-04-01), Tanaka et al.
patent: 4886952 (1989-12-01), Horiuchi
patent: 5191302 (1993-03-01), Rossinick
Induced Energy Limited
Leung Philip H.
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