INDUCTION HEATING RICE COOKER

BUREAU OF RESEARCH & CONSULTANCYINSTITUT TEKNOLOGIMARA40450 SHAH ALAM, SELANGOR

MALAYSIA

DR. NABIL MOHMOUD ABDUL KADIRAHMAD MALIKI BIN OMAR

DECEMBER 1996

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INDUCTION HEATING RICE COOKER

DR. NABIL MOHMOUD ABDUL KADIRAHMAD MALIKI BIN OMAR

DECEMBER 1996

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Date: 10th December 1996File Project No: 600 - BRC(5/3/136)

HeadBureau of Research and ConsultancyITM,ShahAIamSELANGOR

INDUCTION HEATING RICE COOKER

Our above research project has completed. Here enclosed 3 final project reports to BRC asreferences. Thank you.

Your Sincerely

w/AhmacKMaliki Bin OmarProject member.

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THE MEMBER OF THE RESEARCH TEAM

DR. NABIL MOHMOUD ABDUL KADIR(LEADER)

(Signature)

AHMAD MALIKI BIN OMAR(MEMBER)

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ACKNOWLEDGEMENTS

IN THE NAME OF ALLAH THE MOST HIGH AND MERCIFUL

We thank Allah (S.W.T) for offering us the strength to finish this project.

Our main thanks go to Biro Penyelidikan Perundingan in MARA Institute OfTechnology (BRC/ITM) for providing us with enough budget to do this piece ofresearch on the Induction Heating phenomena and its application on Rice Cookers.

The success of this project can be related mainly to the hard work of the finalyear project students in Electrical Engineering Department Peto Galim, Zulkarnain,Norafidah and Ferry Syafrizal.

We hope that this report will be considered as a good start for researchers in thefield of electromagnetics and its application on devices.

Dr Nabil M Abdul Kadir Ahmad Maliki OmarJune 1996

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ABSTRACT

Induction Heating is an alternative method to heat up metallic vessels compared

to the conventional hot plates. Induction heating as a phenomena depends on the

eddy current losses produced in a ferromagnetic materials. These provides ohmic

power loss in the form of heat. The heat is caused by a high frequency flux produced

from a controller. The later produce high frequency currents fed to an exciting coil.

The cooking vessel is usually placed on the top of a flat or inside a solenoidal

exciting coil. Eddy currents will flow on the surfaces of the ferromagnetic vessel

which can cause tremendous increase in the temperature of the vessel.

Two types of the high frequency H-bridge inverters are used to produce

magnetic field at 15 kHz and 38.4 kHz. A Resonant Inverter is also used to produced

magnetic fields at 50 kHz. Power MOSFET is used as a fast switching transistor in

the three controllers. The output power of the three types of inverters with the

presence of different exciting coils (load ) ranges from 200 - 1000 watts.

Three types of exciting coils are used. The first type is a solenoidal, the second

is flat single concentric coil, while the third consists of small flat coils arranged in

different shapes to match the bottom of the vessel. Good comparison has been

achieved between the performance and efficiency of the three controllers.

IV

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CONTENTS Page No

Chapter 1

Introduction .....................................................:..................., 1

1.1 Inverters ...................................................................... 7

1.2 Power MOSFET ................................................................ 26

1.3 Induction Heating Technique .......................................... 28

Chapter 2

Electric Cooker ........................................................................ 32

2.1 Conventional Rice Cooker .............................................. 32

2.2 Conventional Induction Cooker ...................................... 36

2.2.1 General Description .............................................. 36

2.2.2 Control Circuit ...................................................... 38

2.2.3 The Inverter Circuit .............................................. 49

2.2.4 Proposed Controlling Circuit for Induction Heating 54

2.2.4.1 Comparator Circuit ................................... 55

2.2.4.2 Timer ....................................................... 56

2.3 Experimental Results ....................................................... 59

Chapter 3

Resonant Inverter Design ........................................................ 67

3.1 MOSFET Triggering Controller and Operation ............. 68

3.1.1 Opto-Isolator(HCPL2601 ) ............................... 69

3.1.2 Hex Inverter (CD4069) .................................... 71

3.1.3 MOSFET Driver (ICL7667) ................................. -71

3.1.4 Hex Buffer ( CD4050 ) .......................................... 72

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3.2 The Resonant Inverter ...................................................... 73

3.2.1 Protection Element ................................................ 73

3.2.1.1 Freewheeling Circuit ................................ 73

3.2.1.2 Snubber Circuit ........................................ 74

3.2.1.3 The use of a Diode ................................... 75

3.2.2 Operation of Resonant Inverter Circuit .............. 76

3.3 The Magnetic Circuit ...................................................... 77

3.4 Results and Discussion ................................................... 82

Chapter 4

High Frequency H-Bridge Inverters ....................................... 92

4.1 High Frequency H-Bridge Inverter ( 15kHz) .............. 92

4.1.1 Bridge Rectifier ................................................. 93

4.1.2 MOSFET Triggering Controller and Operation 94

4.1.2.1 Triangular Wave Signal Generator ...... 96

4.1.2.2 Comparator Operational Amplifier ..... 99

4.1.2.3 Opto-Isolator ( 4N27 ) ......................... 100

4.1.2.4 Constant Gain Operational Amplifier 101

4.1.3 Inverter Circuit and Operation ......................... 103

4.1.4 Results and Discussion .................................... 105

4.2 High Frequency H-Bridge Inverter(38.4 kHz) ......... 110

4.2.1 MOSFET Triggering Controller and Operation 111

' 4.2.1.1 High Frequency Square Wave Signal Generator 111

4.2.2 Inverter Circuit and Operation ............................... 113

4.2.3 Results and Discussion .......................................... 115

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Chapter 5

General Discussions, Future Work and Recommendations 122

References .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

Appendix

Hardware Development ......................................................... 128

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CHAPTER 1

INTRODUCTION

Induction Heating is an alternative method to heat a metal vessel.

Conventionally hot plate is used to heat up the metal vessel. Induction heating

phenomena depends on the losses produced in a ferromagnetic materials known as

eddy current loss. These losses provide ohmic power loss and cause local heating.

The previous work was developed by using High Frequency H-Bridge

(15 kHz) and (38.4 kHz) Inverters [1,2]. In this project, the inverter circuit is used in

Resonant Inverter producing 50 kHz. The resonant frequency operates at 50 kHz

which is a recommended frequency for induction heating purposes. The main

advantage of using this frequency is to prevent the stress on the power device during

switching. The technique also reduces the number of power switching devices used

which in this case eliminate the possibility of short circuit in any part of the

inverter circuit.

Many types of vessels are used as load with the exciting coil.

Ferromagnetic vessels such as stainless steel is the most suitable vessel material for

induction heating purpose [2]. Different kinds of coils are used to study the

performance of the resonant inverter circuit. Rasmussen, C.B.; Alvsten, B.; Dahl, J. (

1994 ) described a converter where both series and parallel resonant circuits are used.

In order to minimise losses switching is made to zero current ( ZCS ). The converter

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described consists of four transistors H-Bridge connected and it is used itr~an

application for an induction cooking stove with two hot zones. The switching

frequency is 50 kHz.

Garcia, J.R.; Burdio, J.M.; Martinez, A.; Sancho, J.9 { 1994 )

described a new simple method to calculate AC magnetic hysteresis loss in RF power

transformers is proposed. As the calculation of eddy current loss is well-known, with

this new method is possible to obtain a complete solution of magnetic losses, their

spatial distribution and sensitivity analysis in induction heating, transformer cores or

other applications. Results from an experimental test in a commercial induction

cooking unit are provided for verification.

Leisten, J.M.; Hobson, L. ( 1990 ) designed a parallel resonant inverter is

described which is suitable for commercial and domestic use as an induction cooking

power supply. A gate turn off thyristor ( GTO ) operating a gate assisted zero current

switched mode ( ZCSM ) in this circuit provides several advantages including high

operating frequency ( >40 kHz ), and good efficiency ( >95% ). The circuit makes

good use of the high GTO current and voltage ratings, and with a small change in

operating frequency can cope with a wide range of output powers and loading

conditions. Design and construction details are given for a 3.2 kW prototype inverter

based on the Philips TO3P packaged BTR59 GTO operating at around 40kHz.

Tanaka, T. ( 1989 ) The author describes the development of a novel

induction heating not only special iron vessels but also non-magnetic and low

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resistivity metal vessels such as aluminium pans, which cannot be heated by

conventional induction ranges. Input resistance of exciting coils together with the

various metal vessels were investigated, and the optimum characteristic of a high

frequency inverter were determined. The inverter has double-layer coils and two

resonance circuit capacitors. Proper coils and frequencies are automatically selected

through the detection of the vessel material. The induction range can also prevent

undesirable levitation of light aluminium vessels on the top plate by monitoring the

variation of the resonance frequency.

Conception, analysis and design of a soft switching series resonant converter

for induction heating applications are studied [4]. Analysis of switching and losses

processes in the converter and optimal control strategy are explained. This is in

addition to the design of a lossless turn-off snubber for the inverter.

Hybrid Resonant a converter [5] describes both series and parallel resonant

circuits are used. In order to minimise losses, switching is made at zero current

(ZCS). The converter described consists of four transistors connected as H-Bridgeand is used for the applications of induction cooking stove with two hot zones. The

switching frequency is 50 kHz.

The concept and analysis of HF series resonant power converters for

induction heating applications is explained [6]. A study of optimum and clamped

drivers is presented. The losses in the inverter are calculated and the concept of series

resonant power converters for induction heating applications is presented.

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A series inverter for induction heating applications working with~MOSFETs

is presented [7]. Two different operation modes of a MOSFET inverter are discussed.

The presented concepts are verified with measurements resulting from a prototype

inverter running at 10 kW/400 kHz. This operation is important to protect the

switches from overvoltage stress. Identification of the cause of the stress and works

out options to tackle the overvoltage amplitude is discussed. It is found that the

parasitic of the MOSFET with a suitable gate drive may be utilised to aid lossless

turn off. When the operation is below resonance, the high switching speed imposes

heavy stress on the MOSFETs.

A new quasi-resonant inverter for an induction heating apparatus including a

matching transformer is proposed [8]. In this inverter the power source is separated

from the resonant circuit during the resonant capacitor. This is due to the discharged

capacitor and a DC blocking capacitor which is connected in series to the load.

Furthermore, the operating frequency and the output current are controlled by means

of varying the oscillation cycle and the initial current to the resonant inductor.

Current-fed parallel resonant converters with insulated gate bipolar

transistors (IGBTs) working at frequencies up to 100 kHz for induction heating

applications are presented [9]. The power range of the described generators is 3 kW

to 600 kW. The influence in practical generators of the parasitic inductance between

the output of the inverter and the resonant tank is explained clearly.

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A starting scheme for thyristor-based parallel resonant current source

inverters, with particular reference to induction heating applications, is presented

[10]. The input stage in this paper to the DC-link is a three-phase phase controlled

rectifier. It is shown that instead of using the usual start-up circuits providing forced

commutation, it is possible to start the system by using a single additional thyristor

across the DC-link inductor, together with a special timed gating at the input and

output converter. The scheme provides a start-up for a wide range of loads. The paper

explains the principle stating the concept and derives a simplified model to quantify

the important parameters that govern the start-up. The theoretical concept is verified

by experimental data demonstrating the start of such inverters.

The partial series-resonant power converter [11], previously used for DC-DC

converters, is applied to induction heating. The circuit is a half-abridge,

series-resonant circuit with clamping diodes that constrain the voltage of the resonant

capacitors to values between zero and the supply voltage. Three types of control are

investigated: constant switching frequency; constant turn-off" current; and a

combination thereof. The latter seemed to be the best. This circuit is only found

suitable for-loads in which the Q value is low while the temperature, of the workpiece

is below the Curie temperature.

The feasibility of the application of a PWM scheme to the current-fed

inverter in mis'particular field is investigated [12]. The steady-state performance of

the system i& evaluated by a simulation program which is based on_the state-space

method of analysis. The analytical results by simulation are verified experimentally

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on a laboratory set-up of a power level of 100" W at about 14 kHz. The current-fed

PWM inverter is designed to achieve the goal of using a simpler control scheme so

that to reduce the power losses and overall maintenance cost. It does not use a stage

of controlled rectifier for the output regulation, which has the drawback of injectinglarge harmonics into the utility system. The overall control strategy is relatively

simple, because the control loop does not include the input circuit of the system. The

current-fed PWM inverter presented in this paper [12] shows the possibility of

achieving output power regulation by means of both the swept-frequency method and

the PWM scheme. A simple IC-based triggering circuit has been developed which

can provide the required stable PWM signals in a range wide enough to achieve the

goal of control.

The authors in [13] described a control scheme incorporated in the

voltage-fed full-bridge series resonant high-frequency inverter using static induction

high power transistors (SITs), which is based on a load-adaptive variable frequency

modulated phase-shift PWM (pulse-width modulation) control strategy. The

operating principle of the load-adaptive variable frequency PWM series resonant

inverter system with a new control scheme is described along with its operating

characteristics in steady state. The 20 kW - 200 kHz prototype inverter system

suitable for induction heating in industry is demonstrated, including a specially

designed power transformer. SIT stacks with a water cooling system, and a building

- block assembly for high - power use. Experimental and simulation results are

presented. Finally, an improved variable frequency PWM series resonant inverter

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topology incorporating partially inserted capacitive lossless snubber is proposed for

soft switching and compared with the inverter mentioned above.

A phase-shift-control ted series-resonant [14] inverter is used as power supply

for a 10 kW, 500 kHz induction heating system. Analysis of the system for power

regulation with zero voltage switching is presented, including the effect of MOSFET

output capacitance. A control scheme is proposed to ensure switch turn-on with

zero-voltage for all load conditions. The switching frequency is kept as close as

possible to resonance to maintain near-minimal circulating energy. A prototype

power supply was built and tested at 10 kW.

1.1 Inverters

Inverters [15] are static circuits that convert power from a dc source to ac

power at a specified output voltage and frequency. Inverters are used in many

industrial applications. The following are some of their important application.

1. Variable - speed ac motor drives2. Induction Heating3. Aircraft power supply4. Uninterruptible power supplies ( UPS ) for computers.

In general, there are two types of inverters : voltage source inverter (VSI) and

current source inverter (CSI). Inverter is a converter which changes d.c input to a.c.

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output. The d.c. may be derived from a battery, in which case forced commutation

components are required for the power switches if they are thyristor, or it may be the

d.c. energy from the load being fed back into an a.c. supply, as when a.c. to d.c.i

converters are operating in an inversion mode. The operating mode is now natural

commutation. In the voltage source inverter, the input is a dc voltage supply and the

inverter converts the input dc voltage into a square-wave ac output voltage source as

shown in Figure 1 .la. In the current source inverter the input is a dc current source

and the inverter converts the input dc current into square-wave ac output current as

shown in Figure l.lb.

+1

V vsi

t J

+

v

* CSI

(a) (b)

Figure 1.1: Inverter Configuration(a) Voltage Source Inverter(b) Current Source Inverter

Voltage Source Inverters (VSI) - The input of a voltage source inverter is

a stiff dc voltage supply, which can be a battery or the output of a controller rectifier.

Both single-phase and three-phase voltage source inverters are used in industry and

will be discussed here. The switching device can be a conventional thyristor ( with its

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commutation circuit), a GTO thyristor, or a power transistor. Here a thyristor symbol

enclose in a circle is used to represent the On/Off switch.

Single-Phase VSI - The half bridge configuration of the single phase

voltage source inverter is shown schematically in Figure 1.2a. The dc supply is

center-tapped. Switches.S, and S3 are ON/Off solid-state switches ( SCRs or GTO

thyristors, BJTs or MOSFETs ). Diodes D, and D2 are known as feedback diodes

because they can feedback load reactive energy.

During the positive half cycle of the output voltage, the switch S, is turned

On, which makes V0~ +V/2 . During the negative half-cycle, the switch S2 is turned

On, which makes V0 = -V/2 . Waveforms of gate pulse ( iBt and ig3 ) and output

voltage V0 are shown in Figure 1.2b. Note that prior to turning On a switch, the

other one must be turned Off, otherwise both switches will conduct and short-circuit

the dc supply. If the load is reactive, for example, a lagging power factor load, the

output current i0 lags the output voltage V0 , as shown in Figure 1.2c. Note that

during 0 < t < T/2 , V0 is positive; that is, either S, or D, is conducting during

this interval. However, i0 is negative during 0 < t< t, therefore D, must be

conducting during this interval. The load current i0 is positive during t, < t < T/2 and

therefore S, must be conducting during this interval. The devices conducting during

various intervals of time are shown in Figure 1.2c. The feedback diodes conduct

when the voltage and current are of opposite polarities.

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'gl

'82

D

D.

V/2V

S, D,T/2 T

S2 D2

S, D,

Figure 1.2 : Half-Bridge Voltage Source Inverter

(a) Circuit, (b) and (c) Waveform

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The fall bridge configuration of the single phase voltage source inverter is

shown in Figure 1.3a. Switches S, and S2 are fired during the first half-cycle and

switches S3 and S4 are fired during the second half-cycle of the output voltage. The

output voltage is a square-wave of amplitude V, as shown in Figure 1.3b. Note that

the frequency of the firing pulses decides the output frequency of the inverter.

(a)

T/2

-V

(b)

Figure 1.3 : Full-Bridge VS1

(a) Circuit, (b) Waveform

D

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tV

A

'"'

. 4- D.C I L

R

FW

(a)

D,

ieD

(b)

'2 A

Figure 1.4 : (a) Commutation Circuit To Turn Off The Thyristor S(b) McMurray Commutation Circuit

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Commutation Circuits - If the switch used is a conventional thyristor

(SCR), commutation circuits are required to turn it Off. Many form of

commutation circuits are used to force-commutate a thyristor. One type of

commutation circuit is shown in Figure 1.4a. Another commutation circuit that has

been extensively used in inverters is shown in Figure 1.4b. This circuit is known as

the McMurray Inverter. The element S)A , S2A , L and C form the commutation

circuit, and these can be operated to turn off the main thyristors S, and S2 . For

example, to turn Off the main thyristor S, at instant j ( prior to turning On theother main thyristor S2), the auxiliary thyristor S,A is fired. As a result, an

oscillatory current impulse ic flow in the circuit consisting of L , C , S, and

S,A . The commutation current ic flows opposite to the load current i0 already

flowing through the S,. When ic - i0 , the net current through S, is zero and S, turns

Off.

Three- Phase Bridge Inverter - Using single-phase half bridge inverter as a

building block, a three-phase inverter can be constructed, as shown in Figure 1.5a.

The load is shown as connected, in star. The firings ( and hence the operation ) of the

three half - bridges are phase - shifted by 120. The pole voltages VAO , VBO and

Vco are shown in Figure 1.5b. When S, is fired at cof 0 pole A is connected to

the positive bus of the dc supply, making VAO=V/2.

When S4 is tired at 0)f = 71 , pole A is connected to the negative bus of thedc supply, making VAO = -V/2. Waveforms of VBQ and Vco are exactly the same as

those of VAO , except that they are shifted by 120 . The line voltages are related to

the pole voltage as follows :

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(a)

Figure 1 .5(a) : Three Phase Inverter - Circuit

[1.1]

[1.2]

[1.3]

The line voltages are graphically constructed as shown in Figure 1 .5b. These

voltages are quasi-square waves with 120 pulse width. They have a characteristic

six-tapped wave shape. The pole voltages can be written as :

VAO VAN + VNO

VBO = VBN + VNO

VCo= VCN

[1.4]

[1.5]

[1.6]

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AOA

V/2-V/2

-> wt

BO*

VCO

wt

wt

ABA

'BC

-Vw t

-Vw t

CA,

'NO

V/6

-Vwl

wt

AN

wt

Figure 1.5(b): Three Phase Inverter - Waveform

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