Electric heating – Inductive heating – With heat exchange
Reexamination Certificate
2000-04-07
2003-03-04
Leung, Philip H. (Department: 3742)
Electric heating
Inductive heating
With heat exchange
C219S626000, C219S662000, C219S665000, C363S021030, C363S097000
Reexamination Certificate
active
06528770
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns an induction cooking hob comprising induction heaters fed by generators.
Background of the Invention
Induction cooking, or more generally induction heating, uses eddy currents induced in the part to be heated by a high frequency magnetic field, the part being of an electrically conducting material. This part is, for example, a saucepan. The magnetic field is generated by an inductor supplied with a high frequency alternating current by a generator which sets the frequency and amplitude of the current as a function of the heating required. The frequency used for heating depends on a certain number of parameters and in particular the relative magnetic permeability &mgr;
x
of the receptacle and its electrical conductivity &sgr;. Starting from the skin thickness, which one takes for example to be equal to half the thickness of the bottom of the receptacle to be heated, one then determined the angular frequency &ohgr; by using the formula:
δ
=
2
μ
0
·
μ
r
·
σ
·
ω
from which one deduces the frequency by the formula:
f
=
ω
2
π
One thus obtains an optimum frequency to be used, of the order of 10 to 50 kHz.
The generator is fed from an electrical supply whose voltage is rectified and filtered. The generator supplied with this rectified voltage U is generally a resonance generator. In effect, the inductors are typically implemented by winding an electrical conductor in a spiral so that, at the operating frequency, the applied load presents this inductor with a resistance R compatible with the power P=U
2
/R to be transmitted to the load. These same inductors are generally isolated mechanically, electrically and thermally from the load to be heated, which entails an air gap of several millimetres between the load and the inductor. At this distance and in this range of frequencies, the impedance Z=R+j&ohgr;L of the loaded inductor is strongly reactive, which entails an inductor quality factor Q=L&ohgr;R>>1. It then suffices to add one or more capacitors C to the inductor, whose inductance is L, to form a circuit resonant at a frequency:
f
=
1
2
π
L
·
C
For this reason, the generators are mainly resonance inverters. The impedance Z and in particular the inductance L of the inductor depend on the characteristics of the load. The operating frequencies for a cooking hob with several heaters are in general not identical but close to each other. This phenomenon is on the other hand accentuated by the fact that to retain soft switching modes, power adjustments are generally made by adjusting the operating frequency and therefore two heaters intended to heat identical loads at different powers will use different frequencies. It must be noted that this method of adjustment has the disadvantage of forcing the inverter to operate at frequencies far from its natural resonant frequency, which causes high losses. The best compromise is to have dual thyristor operation while working for maximum power as close as possible to the resonance which is the lowest working frequency and to increase the operating frequency to reduce this power.
These neighbouring frequencies produce beat frequencies which are transmitted to the receptacle being heated and which, owing to their small difference, are in the audible range (a few Hz to a few kHz). These beat frequencies, apart from the noise that they generate in the loads, cause difficulties in the control of independent generators.
To avoid this phenomenon which, because of its amplitude, can make using the product very inconvenient, it is necessary to maintain a good separation between the different generator—induction heater pairs, which is a major drawback for product modularity; for the same reason, it is for example impossible to heat a large saucepan on several neighbouring heaters fed by different generators.
One known solution consists of supplying neighbouring heaters cyclically for a period varying from a second, for mechanical switching devices, to ten or so milliseconds, for completely electronic solutions. In both cases, the generators must be designed with excess power capacity since power is not transmitted continuously to the heater but is alternating, with a duty cycle which varies according to the power levels demanded from each heater connected to the generator. Furthermore, this cyclic supply can be felt to be a nuisance in use of the device because of the harsh power variations in the load if the period is of the order of a second, or because of the noise related to switching if this period is of the order of a few milliseconds, which corresponds to frequencies of a few hundred hertz.
Another known solution in the field of control and power electronics is to supply the inductor at the same frequency by using generators with hard switching, for example a chopper whose power adjustment method can then be at a fixed frequency in pulse width modulation (PWM) mode. It is however not prudent to use this type of generator to supply standard inductors, in particular because of the high quality-factor of the coils at the operating frequency. In effect this leads to difficulty in making current flow in inductive coils (saw-tooth currents) and major losses when the current in these coils is cut, which requires very large over-capacity in the power generator.
SUMMARY OF THE INVENTION
This invention aims to solve these problems and sets out to develop an induction cooking hob having low and high power, and in general, an induction heating device operating at a single frequency or at multiple frequencies to avoid beat frequencies and above all allowing the use of low power generators and in particular modular generators.
To this end, the invention concerns a cooking hob of the type defined above, characterised in that the inductors that are neighbouring or constituting the same heater are fed at the same frequency or at multiple frequencies and in that it includes at least one high power heater comprising at least two inductors having a quasi identical on-load impedance returned to these inductors whatever the load put on this heater. A single controller then governs the generators which operate in resonant mode with soft switching.
Advantageously, this cooking hob includes two induction heaters equipped with inductors, at least one of the heaters (first heater) being high power with at lest two inductors having a quasi identical on-load impedance at maximum load whatever the nature, shape and position of the load placed on this heater. An inverter generator is associated with each heater and operates with soft switching, a single controller governing the two generators. A switching device is associated with the generator of the second heater and has two states:
a normal state for which the switching device connects the generator to the inductor of the second heater,
a power state in which the switching device connects the generator of the second heater to the second inductor of the first heater.
The resonance inverter generators, when they are synchronised in frequency, allow implementation of a high power heater with particularly economical low power generators since they are operating continuously in soft switching mode. A switching device allows the power from two or more generators to be routed to different heaters but it is quite possible to not use this switching device and to permanently connect several generators to one heater, thus increasing its power.
Thanks to the switching device, the cooking hob allows advantage to be taken of the fact that in everyday use of the equipment, it is not necessary for the user to continuously have high power available as much as with induction systems where the power is transmitted directly to the load. The efficiency is particularly high. These power levels are useful during special, short duration preparation sessions (boiling water, heating large quantities of liquid, getting a large grill up to temperature). In continuous use, the power
Akel Dominique
Schlumberger Henri
Drinker Biddle & Reath LLP
Jaeger Regulation
Leung Philip H.
LandOfFree
Induction cooking hob with induction heaters having power... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Induction cooking hob with induction heaters having power..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Induction cooking hob with induction heaters having power... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3008881