ABSTRACT

This paper entails the development of an industrial induction cooker. The induction cookers output power is to be controllable over the full desired operating range. The cooker is also to function with all ferrous vessels. The cooker consists of three main components, namely the load, the inverter stage and the control stage. This paper also briefly discusses the efficiency of the cooker.  It also looks at the difference between the simulated output power versus the actual output power.

1.  INTRODUCTION

Cookers can broadly be divided into two types, namely gas cookers and electric cookers.  The main function of the cooker is to heat the vessel in which the food is.  This vessel in turn distributes the heat to the food and cooks it.  Such cooking can amount to as much as 10% of all energy used in a household. [1,2] By utilizing this energy efficiently the energy used for cooking can be substantially reduced.  Most cookers expend wasted energy in the form of heat.

Cookers have always been designed to generate heat in a manner that will most efficiently transfer this heat to the cooking vessel.  Induction cookers are a relatively new application of induction heating.  Induction cookers do not work on the same principal as other cookers.  They do not generate heat, which is then transferred to the cooking vessel.  Induction cookers work on the principal that they generate the heat in the cooking vessel itself.  This improves the transfer of energy and reduces the amount of wasted energy normally associated with conventional cookers.

The main disadvantage of induction cookers is that their initial cost is more than that of a conventional cooker.  Another disadvantage of induction cookers is that only ferrous vessels will operate on them.

2.  INDUCTION HEATING

Induction heating involves the varying of the magnetic fields in the primary, thus inducing circulating eddy currents, caused by electromagnetic induction, in the secondary.  In the case of the induction cooker the cooking vessel acts as a short-circuited secondary, and the induction coil acts as the primary.

Induction heating can be defined in the following manner: By varying the current in the primary circuit an alternating magnetic field is setup.  When a conductive material (the cooking vessel in the case of the induction cooker) is placed within this alternating magnetic field, eddy currents are induced in the conductive material.  Due to the resistivity of the material, the eddy currents will cause conduction losses in the material, leading to heat being generated within the material [3,4,5].

Induction heating employs three main effects, namely electromagnetic induction, skin effect and heat transfer.  Induction heating allows a defined section of the vessel to be heated accurately.  The skin effect allows the penetration depth of the magnetic field to be controlled thus ensuring maximum heat transfer [4].

3.  INDUCTION COOKING

Conventional cookers such as gas and ‘electric resistance’ cookers loose a lot of energy to the environment in the form of   heat, as the heat must be transferred form the ‘element’ to the cooking vessel.  This heat loss results in poor thermal efficiency.  Due to the nature of induction heating all the heat that is generated by the induction cooker is generated within the cooking vessel, thus the thermal efficiency of the induction cooker is far greater than conventional cookers.

The power of an induction cooker is instantly controllable.  This results in quick rise in temperature, which in turn results in reduced cooking times.  If the cooking vessel is removed from the cooker the power is instantly reduced to a minimum. As the heat is generated in the cooking vessel and not the element, the surface of the cooker remains relatively cool.  The only heat is that which is transferred from the cooking vessel to the cooker.

4. CONVENTIONAL COOKERS

As mentioned conventional cookers can be divided into two main types, namely gas cookers and electric cookers.  Electric cookers can also however also be sub-divided into three types, namely radiant coil cookers, solid disc cookers and ceramic top cookers.

Gas cookers power is easy to adjust and response is instant, thus providing good temperature control.  Consumption of gas varies depending on the size of the burners.  This type of cooker is the least efficient of all the cookers.  The main advantage of a gas cooker is that gas is relatively inexpensive.  The main disadvantage is that these types of cookers are the least safe out of all the cookers.

Radiant coil cookers are the most common of all the cookers.  They are also the cheapest cookers to purchase initially.  These types of cookers are heated by electrical resistance.  These cookers are more efficient than solid disc cookers and ceramic top cookers.

Solid disc cookers are similar to radiant coil cookers.  They are just solid and fixed to the cooker.  They do not need to be removed for cleaning.  These elements have a greater thermal mass than radiant coil elements and are thus less efficient and have a slower response time.

Ceramic top cookers have their elements below heat resistant ceramic glass.  This improves appearance and improves cleaning.  These types of cookers take longer to heat up than radiant coil and solid disc cookers.  Their response is also slower than the above-mentioned cookers.

5. CONVENTIONAL COOKERS VERSUS INDUCTION COOKERS

Figure 1 bellow shows a graph illustrating the efficiency of the various cookers mentioned thus far.

Figure 1. Efficiency of various cookers[2,6,7]

As can be seen from figure 1, induction cookers are far more efficient than any of the other cookers.  This is due to the fact that the heat is generated within the cooking vessel for induction cookers and thus there is maximum heat transfer.

A study by the commission of the European Community on ‘An experimental investigation of cooking appliances, domestic cold appliances and clothes dryers in 100 households showed that the annual consumption of an electric cooker was 457kWh/year, while induction cookers only used 337kWh/year [Electrical cookers use 35% more power] [1].  This study also mentioned that the cookers were responsible for approximately 7% of the total consumption of a household.

Induction cookers have a great many advantages over conventional cookers.  These include efficiency, safety and controllability.  The cooker remains cool and heat is instant.

6.  SYSTEM DEVELOPMENT

A prototype induction cooker was designed and manufactured with the following criteria:

• High efficiency
• Low initial cost
• Is to operate with all kinds of vessels namely, steel, stainless steel, aluminium, alloys and combinations
• Output power to be controllable over the whole desired temperature range
• Minimum component count
• Robust
• Reliable

The design methodology adopted for the induction cooker was to first design the load.  Once the load’s characteristics had been defined an inverter topology could be selected and implemented.  Once the inverter topology had been defined a control method could be implemented and safety circuits could be incorporated.

Figure 2 shows the system layout of the induction cooker.

Figure 2. Block diagram of the Induction cooker

The induction cooker is to be fed from 220V AC 50Hz single phase.  This supply is rectified and the unregulated DC is fed to the inverter.  The inverter’s output is the fed to the load circuit.  The output of the inverter is controlled by the control circuit.

A pancake litz coil was constructed from 26 strands of 0,4mm litz wire.  The coil consisted of 35 turns.  It had an inner diameter of approximately 40mm and an outer diameter of approximately 185mm.  The coil was designed to carry 25A rms.  Figure 3 shows a cross section of a load circuit for an induction cooker, it also shows a cross section of the pancake coil.

Figure3. Cross section of a load circuit for an induction cooker

A step response test was carried out to determine the characteristics of the load circuits.  Figure 4 shows the resultant waveform of a typical step response test.

Figure4. A resultant waveform from a typical step response test.

This test was carried out for the no load condition, a 16cm steel vessel, an 18cm steel vessel and a cast iron vessel.  From this test the resonant frequency, the inductance, the resistance and the quality factor of each load circuit could be determined.

From this information it could be seen that the various vessels resonant frequencies were all around 30kHz.  Simulations of the various load circuits and converter topologies were carried out on Pspice to gain a better understanding of the load.  A series resonant, half bridge, Voltage fed inverter topology was selected as the most suited for the application according to the simulations.  Figure 5 shows the inverter topology that was employed for the induction cooker.

½ resonant capacitor
			Switch 1
			Switch 2

Figure 5. Series resonant, Voltage fed, half bridge inverter topology selected for induction cooker.

During the design of the inverter the following problems were encountered, dv/dt, thermal runaway and over current.  The dv/dt was overcome with a snubber capacitor.  The thermal runaway was occurring due to the fact that the MOSFETs could not cool down as the insulators used to mount them had poor thermal conductivity.  The silpads were replaced with mica-washers and this solved the thermal runaway.  It was found that the resonant current was close to the maximum current rating of the MOSFETs being used, thus it was decided to parallel MOSFETs.  The inverter consists of 2 parallel IRFP460’s per leg.

Control of the induction cooker was achieved by means of a swept frequency power control topology.  Figure 6 shows the power curves for the various loads between 38kHz and 80kHz.

Figure6. Power curves for the various loads between 38kHz and 80kHz.

It can be noted that at around 40 kHz all the loads outputs are around 2 kW and that at around 80kHz the outputs are all around 500W.  Thus it was decided to implement a swept frequency power control topology.

Figure 6 shows that the inverter operates on the inductive side of the resonant curve.  This mode of operation is preferred as it protects the MOSFETs by soft switching them.

During the design it was established that certain safety circuits would be required for the protection of the induction cooker as well as the operator.  It was decided to incorporate the following safety circuits in the design:

·         Over-current protection

·         Auto-detection of incorrect cooking vessel

Over current protection is achieved by means of a current transformer that senses the current in the coil.  When the current in the coil exceeds the maximum desired current, the safety circuit shuts down the MOSFET driver, thus effectively disabling the circuit.  When a cooking vessel of the incorrect material is placed on the cooker, the current drawn will exceed the maximum desired current, thus invoking the safety circuit and once again effectively disabling the circuit.

7.       RESULTS

The prototype induction cooker was successfully designed and manufactured.  A swept frequency control topology was successfully implemented.  The control topology was designed to have ten preset positions, similar to a conventional cooker.  Figure 7 shows the input power versus the output power for the induction cooker.

Figure 7. Input power versus output power for the induction cooker

All of the above mentioned data was recorded using the 18cm steel vessel with 1 liter of water at room temperature.  From Figure 7 it can be seen that the control topology was a success.  It was possible to control the output of the induction cooker over its entire power range.

Figure 8.  Efficiency of induction cooker over its entire power range

Figure 8 shows the efficiency of the induction cooker.  As the frequency increase so the efficiency decreases.  This is due to two main reasons, namely the dead time is not a fixed percentage of the duty cycle in this control topology.  Thus at higher frequencies the dead time is significantly greater in proportion to the trigger signal than at lower frequencies.  This in turn creates a ‘pulse width modulation’ effect.  The other cause of the poorer efficiency at high frequencies is the fact that as the frequency shifts away from the resonant frequency so the power factor worsens.

Figure 9. Simulated output power versus actual output power

Figure 9 illustrates the difference between the output power that was simulated on Pspice and that that was achieved in practice.  Figure 9 shows that the actual power achieved was very close to the simulated output power.  The main difference is due to losses in the circuit and the poorer power factor at higher frequencies as the circuit moves away from the resonant frequency.

1 liter of water was boiled in a time of 2 ½ minutes.  750ml of oil was heated to a temperature of 165°C and kept at this temperature with the power set to 500W.  2,8 kW constant power was achieved with the prototype induction cooker.

8.        CONCLUSIONS

The prototype 2 kW induction cooker was successfully developed.  All of the original design criteria laid out at the start of the project were met.  The induction cooker is very efficient at high power ratings, approximately 98% at full power.  The output power of the induction cooker was fully controllable from 130W to 2,8 kW.

9.  REFERENCES

[1]           Cabint Olivier Sidler, PW Consulting. “ An experimental investigation of cooking appliances, domestic cold appliances and clothes dryers in 100 households”.  Commission of the European Community, Electricity demand management. June 1999

[2]           Home energy saving, Energy smart cooking. http://www.sea.viv.gov.au/energy_smart/home

[3]           Davies, E.J.: “Conduction and induction heating” IEE, First Edition, 1993

[4]           Davies, J., Simpson, P.: “Induction heating handbook” Mc Graw Hill, 1979

[5]           Koertzen, H.W.E.: “Variable load induction heating by medium frequency power electronic converters” Rand Afrikaans University, 1994

[6]           About Induction cooking. http://owlcroft.com/garden/induction.html

[ 7 ]         Induction cooking.  http://www.ripples.co.uk/induct.html

10. AUTHORS

Principal Author:  Jimmy Schedel holds a BTech degree in Electrical Engineering from the Cape Technikon.  At present he is a Chief Industrial Technician at Naval Engineering Services. His address is:

1 Falcon Crescent

Parklands

Blouberg

7441

Co-author:  Irshad Khan holds a MTech degree in Electrical Engineering from the Cape Technikon.  At present he is a lecturer at the Cape Technikon and is involved in the Centre for Instrumentation Research.

School of Electrical Engineering

Cape Technikon

PO Box 652

Cape Town 8000

Presenter:  The paper is presented by Irshad Khan from the Cape Technikon.  At present he is a lecturer at the Cape Technikon and is involved in the Centre for Instrumentation Research.

School of Electrical Engineering

Cape Technikon

PO Box 652

Cape Town 8000