Electric heating – Inductive heating – With heat exchange
Reexamination Certificate
2000-04-07
2002-12-24
Leung, Philip H. (Department: 3742)
Electric heating
Inductive heating
With heat exchange
C219S675000, C219S673000, C219S627000, C336S225000, C336S232000
Reexamination Certificate
active
06498325
ABSTRACT:
BACKGROUND OF THE INVENTION
Induction heating consists in generating what are referred to as eddy currents in an electrically-conducting element by means of a magnetic field. The magnetic field is generated by an inductor, adapted to match the region of the item to be heated, through which an alternating current is passed. The alternating current is itself produced by a generator, which adapts the frequency and amplitude of the current to produce the desired heating.
When using induction heating for cooking purposes, the object to be heated is an electricity-conducting receptacle. Although the invention described below may be applied to non-magnetic materials of the copper or aluminum type, heating magnetic materials is more specifically of interest (low-carbon steels, cast irons, magnetic inoxidizable materials). In the applications sold to the general public, the receptacle is generally of a diameter of between 120 and 280 mm and is between 1 and 4 mm thick. These diameters may be as much as 450 mm and the thickness 10 mm in professional applications.
The formula of skin thickness divided by the relative magnetic permeability &mgr;
r
and electric conductivity &sgr; of the recipient is applied:
&dgr;={square root over (2/&mgr;
o
·&mgr;
y
·&sgr;·&ohgr;)}
which gives the frequency:
f=&ohgr;/2&pgr;
being in the order of 10-50 kHz, used to produce effective heating in ferritic bases of a minimum thickness of 1 mm. The supply voltage of 50 or 60 Hz distribution network is generally rectified and filtered; the excitation frequency is produced by means of a generator, generally by resonance. This generator is connected to a generally flat inductor (referred to as a “pancake”) placed facing the base of the pan to be heated underneath an electrical insulation material and acting as a support for a plate, usually glass ceramic.
One of the difficulties of this known system is that of being able to heat receptacles of different materials, shapes and diameters, which are not known in advance, uniformly and in the best adapted manner. The designer of such a product has to strike a compromise by opting for an inductor of a diameter somewhere between the smallest and the biggest receptacle likely to be heated, generally about 180 mm. As cooking appliances of this type are more widely sold, manufacturers are now starting to sell specialised hobs of different diameters selected for a specific type of receptacle: hobs of 140 mm for small receptacles, hobs of 220 mm for large receptacles and even hobs of 280 mm for very large receptacles. The bigger the hob is, the higher the power must be. Each hob is therefore supplied by a different generator, a solution which is not industrially or economically attractive. Having studied the inductor, it is possible, by altering the number of turns and the space between two successive turns, to provide an arrangement in which the complex impedance Z=R+j·L·&phgr; in inductors of different diameters under load is more or less identical, which means that the same generators can be used on hobs of different dimensions. The power rating which is then defined for the hob of the largest diameter is limited when applied to hobs of small diameters. This solution is not economically attractive either because it means using high-power generators to transmit low power.
Some manufacturers use a high-power generator to supply different hobs by incorporating an electromechanical switch. Although this solution allows the power to be regulated from one hob to another by varying the cyclical ratio, it is not satisfactory in terms of cooking and operating noise. It should be pointed out that the impedance (L, R) of an inductor increases with the number of turns. Conventional inductors are made up of a strand of several wires of a small section wound in a spiral, either in turns one around the other or, if the space between the turn must be variable, on a matrix. An inductor of a small diameter will therefore have a lower impedance than a larger inductor and the generator will transmit, as a first approximation, a higher power P=R·I
2
in the small inductors, which is the opposite of the desired objective.
Finally, in the conventional application where the inductor is a simple coil, the centre of the load which corresponds to the centre of the coil is relatively large and is not heated by induction which means that the heat distribution may not be acceptable unless the receptable has good heat diffusion properties.
A second difficulty of the known system is that of avoiding irradiation of a magnetic field in the immediate area around the inductor where the user is likely to be. In practice, the system must be able to work with small receptacles, even with receptacles positioned off-centre. If the ratio of the size of the inductor to the size of the receptacle is high, then coupling will be mediocre, affecting the energy output. The effect of this is to generate a large leakage field in the immediate vicinity of the inductor. In the past, the standards governing electromagnetic compatibility have made it compulsory to limit this field. More recently, there has been a further sharp cut in the permissible levels due to the possibility of the appliance user being exposed to these leakage fields and the potential hazards it might pose to health.
Adapting the size of the inductor system to suit the load as proposed in document EP-92 400 362.2 is one way of reducing the leakage field. However, this method is not sufficient, even assuming the system were adapted turn by turn.
Another known approach is to place inductors in pairs in phase and in phase opposition, these inductors being in series so that an identical current can be passed through them. This latter concept, which has long been known (document EP-86 17 273), if difficult to apply because if a simple inductor is divided into inductor pieces, the effect of mutual inductance between one turn and the other turns in its vicinity will be considerably reduced, which will lead to a drop in the impedance of the conductor. Consequently, the current remains high even though the designer has opted for a generator system suited to low impedance levels.
In addition, because the total current from the generator passes through the turns, they must have a large section, which presents an added difficulty in terms of fitting in adequate number of turns (ampere turns) underneath the load to be heated. Accordingly, it is necessary to try and reduce the section of the conductor and still allow the inductor to heat, it also being possible for the inductor to be cooled by the load (document FR-96 05 978). This solution is of interest but is difficult to implement and implies a significant increase to the temperature of the interface, cancelling out one of the interesting features of induction, heating, whereby a “cold” medium can be used for cooking, and hence increasing the risk of accidents due to burning.
SUMMARY OF THE INVENTION
The objective of the present invention is to propose an inductor for cooking by induction heating that is simple in design and exhibits a high impedance so that it can be used in conjunction with other similar inductors to form an induction-heated cooking hob and in which combinations in parallel or in parallel series and in phase opposition will make it possible to produce a full range of hobs whilst keeping the level of magnetic disturbance to a very low level, regardless of the type of receptacle to be heated on the hob or more generally the heating surface.
To this end, the invention relates to an induction-heated cooking hob of the type defined above, characterised by:
at least one inductor made up of a large number of turns with at least one conductor,
the conductor is rectangular in section and of a small thickness, the large face of the section being parallel with the winding axis of the conductor forming the turns,
a layer of insulation of a minimum small thickness between adjacent turns,
the winding is substantially rectangular in shape when seen in a plan view.
This high-i
Akel Dominique
Schlumberger Henri
Drinker Biddle & Reath LLP
Jaeger Regulation
Leung Philip H.
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