DOMESTIC COOKERS WITH OPTIMISATION OF THE POWER SUPPLIED"

* * * * * * TECHNICAL FIELD

The present invention relates to a procedure and a system for the induction heating of industrial and domestic cookers.

In particular the present invention relates to a procedure and an induction heating system which makes it possible to identify the type and dimensions of the container or pan placed on top of an induction heating plate. The invention then creates a mathematical model equivalent to the container or pan and on the basis of this supplies the optimum heating power suitably adapted to the type and dimensions of the container or pan used.

The invention can be applied in the sector for industrial electronics and in particular to the sector for induction heating systems for industrial and domestic cookers.

BACKGROUND ART

Induction heating systems are frequently used for the low and medium power systems applied to cookers for use in industry and the home. In general, these systems use resonant or semi-resonant converters on the power supply to the induction element.

One of the most widely used types is the half-bridge resonant converter which operates on the basis of an equivalent circuit of the converter-inductor-pan system which is treated like a RLC (resistance, inductance, capacitance) system to produce an equivalent electrical model. The following reference parameters are customarily used to make the equivalent electrical model:

a) The capacitance element is contained in the power converter.

b) The inductance element is contained in the inductor system and in the coupling that this produces in the pan.

c) The resistance element, net of the intrinsic ohmic circuit of the circuit, is mainly that obtained by the transfer of power to the material.

This series RLC equivalent model is excited by a bridge which is able to vary the amplitude of the power source applied by operating on the pulse-width modulation (P M) and at the same time varying the excitation frequency of the system.

The document EP-A1-2334142 describes an induction heating device equipped with differential control means which supplies different currents depending on whether the container or pan to be heated is made from aluminium or steel. However this device is not equipped with means for detecting if the container or pan is made from aluminium or steel or any other material.

DESCRIPTION OF THE INVENTION

This invention overcomes the typical drawbacks and disadvantages of the prior art by providing a new induction heating procedure and system which is able to identify the material type of pan to be heated and to automatically adapt the heating performance to the optimum for the material identified and thereby optimise the use of the system and the running costs of the same.

This is achieved by means of an induction heating procedure for industrial and domestic cookers having the characteristics described in the main claim.

The dependent claims describe particularly advantageous embodiments of the system according to this invention .

In addition, claims 7 and 8 describe a heating system for application to industrial and domestic cookers which makes it possible to recognise the type of pan to be heated and optimise the supply of heating power to match the pan recognised.

Experiments conducted by the applicant have shown that the performance of an induction heating machine is strongly dependent on identifying the optimum working frequency range.

In general, the efficient transfer of power is obtained only in conditions where the working frequency used approximates to the resonance frequency of the LC circuit where, however, the inductance element L depends not only on the geometry of the heating coil but also on the shape, dimensions and material comprising the container or pan to be heated.

This is a particularly critical element of induction heating systems because using the same frequencies or some fixed frequencies creates the risk of not obtaining an efficient use of energy and causing high energy consumption leading to relatively high running costs for the system concerned.

The present invention has been developed starting out primarily from the observation that the resonance frequency of the system at which the current (and therefore the power) delivered reaches its maximum peak depends on the shape, dimensions and material of the container or the pan to be heated.

According to the invention, when the container or - A - pan is placed on the heating plate the system control electronics performs a step-wise scanning of the working frequency of the machine between two preset limit values looking for the maximum peak and therefore the resonance frequency of the system.

At this stage any container type placed on the heating plate will be detected and identified by the system control electronics as an equivalent resistance.

The control electronics is able to identify, for each type of container placed on the plate, the system resonance frequency which corresponds to the point at which the plate delivers the maximum power to the container .

Tests performed by the applicant have also shown that the optimum working point of the heating system is not situated exactly at the resonance frequency value but is rather slightly higher than this resonance frequency.

In these tests the best results were recorded for frequency values approximately 5% higher than the corresponding resonance frequency values. These values correspond to the Zero Voltage Switching (ZVS) condition which permits an increase in the energy efficiency of the electronics and a consequent reduction in consumption.

In a particularly advantageous embodiment of the invention, the control electronics are programmed to fix the optimum working frequency at a value which is approximately 5% higher than the resonance frequency detected as being the maximum power supply peak for a specific type of container to be heated.

DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become clear on reading the description given below of one embodiment, provided as a non-binding example, with the help of the accompanying drawings, in which:

Figure 1 shows a circuit diagram of an induction heating system according to the present invention;

- Figure 2 shows a circuit diagram of the equivalent electrical circuit used in the present invention; and

- Figure 3 is a diagram showing the method for optimising performance according to the present invention.

DESCRIPTION OF ONE EMBODIMENT OF THE INVENTION

In Figure 1, the numeral 10 indicates in its entirety a class D inverter used in an induction heating system for application to industrial and domestic cookers .

The inverter 10 is powered by a suitable power supply connection 11 and is used to power an inductance element or coil 12 where a container or pan 13 to be heated can be placed.

The inverter 10 also comprises a pair of electronic switches 14, 14' which interrupt and/or allow the passage of current towards the coil 12 in accordance with the control instructions received from a microprocessor 17.

Typically this resonant bridge inverter configuration works on an equivalent circuit of the converter-inductance-container (pan) system which for the purposes of processing an equivalent electrical model can be likened to a series RLC system. This is clearly shown in Figure 2 where the capacitance element C is contained in the power converter, the inductance element L is contained in the inductor system and the coupling which this produces on the pan and the resistance element R, net of the system intrinsic ohmic circuit is mainly that obtained by the transfer of power to the material comprising the pan.

The transfer of power in a similar system takes place where the working frequency used approximates to the resonance frequency of the LC circuit where, however, the inductance element L depends not only on the geometry of the heating coil but also on the shape, dimensions and material comprising the container or pan.

Clearly, as the material and geometry of the pan varies and as the inductor parameters vary, the system resonance frequency will vary by a factor Q (a factor indicating network losses) for the system. This can lead to a decay, in some cases quite substantial, in system performance and as a consequence can lead to high consumption levels.

In addition, it is not usually possible to foresee the shape, dimensions and constituent material of the pan 13 which will be placed in contact with the coil 12.

On the basis of this observation, the present invention is designed, when the pan 13 is placed on a coil 12, to perform a scan of the machine's working frequency in order to identify the power transfer maximum peak which corresponds to the system resonance frequency and the Q factor indicating the losses.

This operation is performed by a circuit 15, connected to the microprocessor 17 measuring the current running through the coil 12, and another circuit 16 also connected to the microprocessor 17 measuring the voltage at the terminals of the coil 12. This operation also involves a voltage controlled oscillator (VCO) which outputs a variable frequency starting out from a variable input voltage. The VCO circuit can be of the external type but preferably can be integrated directly into the microprocessor 17.

In a way which is absolutely innovative with respect to known technical solutions, the system provided by the present invention performs a series of successive frequency measurements, at preset intervals, using the VCO (voltage controlled oscillator) circuit, in order to identify the resonance frequency of the system comprising that particular pan 13.

The frequency value which corresponds to the maximum current value flowing through the coil 12 is recorded and identified. Next, according to the invention, the optimum working frequency is set for that particular pan 13 at a value which is slightly higher, by approximately 5%, than the resonance frequency recorded. This value has been established by empirical testing as being the value which permits the best results in terms of the system heating performance .

This mode of operation is clearly illustrated in Figure 3 which shows how the system electronics modifies the frequency by searching for the voltage pulse (Vpulse) whose presence indicates that the resonance frequency has been reached. At this point the system is calibrated so as to maintain the zero voltage switching (ZVS) condition and this optimises the energy performance of the electronics thereby reducing consumption.

The scanning frequency can be set, for example, starting from a value of 25 kHz and ranging up to a. value of 17 kHz, in steps of 100 Hz. The maximum current value peak (or, in the equivalent mode, the power transferred to the pan 13) which corresponds to the system resonance frequency is identified and recorded within this range.

It should be noted that in this way the system according to the present invention makes it possible to measure the active power supplied to the load (in other words, the kW transferred to the pan) . According to the invention it does this in real time by calculating the product of the instantaneous voltage applied to the RLC circuit and the circulating instantaneous current and then extracting, using a low-pass filter, the average content representing the effective power, measured in Watt, transferred to the pan 13.

The measurement of these properties makes it possible, using suitable software managed by the microprocessor 17, to calculate the performance of the pan 13 in terms of efficiency and consumption. In this way it is possible to establish an indicator . of the efficiency of the shape, dimensions and materials of a particular pan 13 in comparison with another pan and to know whether a pan 13 is more or less suitable for the induction heating process, all other factors being equal. It is therefore possible to compare the ratio between the apparent power used by the converter and the actual power transferred.

The invention as described above refers to a preferred embodiment.

It is clear that the invention is susceptible to numerous variations which lie within the scope of its disclosure as defined in the attached claims.

In the example described above, the VCO (voltage controlled oscillator) circuit used for frequency scanning can be external or can be incorporated in the system containing the system control microprocessor. Inverter and power converters other than the types described above may also be used. The voltages, currents, frequencies and the related electrical powers used can vary widely depending on the type of application in which the system is used. Standard industrial applications use a 380-400V power supply rating while domestic applications customarily use a 220-230V power supply.

These and other variants within electrical technology are possible.