Practical tests on a novel series-connected CSIprototype for controlled induction heating

applicationMolay Roy

Research ScholarDept. of Electrical Engineering

Bengal Engineering and Science UniversityShibpur, Howrah - 711103, W.B., India

Email: molay.roy@gmail.com

Mainak SenguptaAssociate Professor

Dept. of Electrical EngineeringBengal Engineering and Science University

Shibpur, Howrah - 711103, W.B., IndiaEmail: mainak.sengupta@gmail.com,

msg@ee.becs.ac.in

Abstract—Induction heating is commonly used in the indus-try for the metal heating, melting, hardening etc. The powerconverter used is a current source inverter (CSI). High powermedium frequency application like melting use thyristor basedCSIs, while in medium power and high frequency inductionheating application (like surface heat treatment) IGBT basedCSIs are increasingly preferred. In this paper simulated andexperimental performance of an SCR-based CSI and IGBT basedCSI in series cascade (with common dc-link) have been presented.Two laboratory prototypes of rating 2kW, 10kHz each havebeen fabricated and tested, both individually and then in seriescascade. Cascade mode operation of CSIs may find importantapplication in multi-stage surface heat treatment of metals.

Index Terms—Induction heating, SCCSI, CSI design andfabrication, Phase-shift method, SEQUEL based simulation.

I. INTRODUCTION

Induction heating (IH) uses high frequency electricity, toheat materials that are electrically conductive, using the prin-ciple of eddy current losses. It is very efficient since localisedheat is actually generated inside the “work-piece”. Inductionheating lends itself to some unique applications in the industry.Current source inverter is used as a power converter and a ca-pacitor is connected in parallel with the induction coil (parallelresonant circuit[1][2]) in the IH applications. Thyristor-basedCSI is most commonly used for (i) the low cost and (ii) easyavailability of higher rated thyristors. The major drawback ofa thyristor is that it is a load commutated device. Hence theseinverters have to be made to operate at leading power factor[3].On the other hand IGBT-based CSI can be operated at almostunity power factor[4].

Series-connected high frequency CSIs are expected to findincreased application in multi-stage surface heat treatment.Two or more CSIs can be operated at the same frequency or atdifferent frequencies depending on the application. OperatingCSIs connected to a common dc-link has its challenges. Anovel phase-shifting method has been proposed in this paperto control the power each inverter separately.

Fig. 1. Schematic diagram of induction heating process.

II. BASIC PRINCIPLES OF INDUCTION HEATING

Induction heating is the process of heating an electricallyconducting object (usually a metal) by electromagnetic in-duction. The elementary mechanism behind this is the flowof induced currents in a conductive material, due to emfinduced in it, when it is placed in an alternating magneticfield (Faraday’s law). These currents, which are treated asunwanted eddy currents in most applications, ultimately heatup the material obeying, Joule’s law.

The conversion process of electrical power for a typicalinduction heating unit is shown in Fig. 1[7]. It is obvious thathigher the frequency, higher is the induced emf and hence themagnitude of the eddy currents. However higher frequencyhas the problem of lesser flux penetrating into the ‘job’ tobe heated and hence heating effects getting concentrated onthe ‘skin’ of the ‘job’. This is good for surface heat treatmentapplications but is not so for melting applications. Typicalfrequencies for melting applications are around 500Hz, whilefor surface hardening/heat treatment applications it is around5kHz or above[10].

III. POWER SUPPLIES FOR INDUCTION HEATINGAPPLICATIONS

An induction heating power supply/converter is either basedon current source inverter or voltage source inverter configu-ration. For reasons well known[9], CSI configuration with aparallel capacitor across the load is preferred.

2012 IEEE International Conference on Power Electronics, Drives and Energy Systems December16-19, 2012, Bengaluru, India

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The present work proposes multiple CSI-fed induction heat-ing units (IHUs). The operation of a single induction heatingunit from a current source inverter has been discussed in thenext section. Thereafter, operation and need of multiple CSIsin series cascade (SC-CSI) configuration in induction heatingapplication have been discussed.

A. Operating Principle of CSI

A current source inverter consisting of thyristors is shownin Fig. 2, where the induction heating coil is representedas equivalent series R-L load with a suitable capacitor con-nected across it, to set up parallel LC resonance (naturalresonance frequency of ωr = 1√

LC). The thyristors being

Fig. 2. Circuit diagram of CSI, R-L load with parallel capacitor.

load commutated devices, have to be operated at leadingpower factor[4]. The fundamental frequency component ofvoltage is in phase with the fundamental frequency componentof current if switching frequency (ωs) is equal to naturalresonance frequency (ωr). However, to get desired leadingpower factor angle (Fig. 3), the inverter must be operated atfrequencies above the resonance frequency (ωs > ωr). A large

Fig. 3. Output voltage, current and voltage across the thyristor of parallelresonant circuit consisting of parallel branches of series R-L with C.

value of inductor (Ld) is connected in the DC-link (20mH)and hence DC-link current is almost constant (id = Id). Inthis inverter after every 180o duration any diagonal pair ofthyristors commutate together. The inverter output current i′Lis ideally of square-wave shape or trapezoidal. Let, at someinstant T1 and T2 be operating. At the negative going zero

crossing instant of the load terminal current the other twothyristors (T3, T4) are to be turned on, when the load voltageitself falls across the previously conducting thyristors (T1,T2). This helps in commutation of these outgoing thyristors(T1, T2). The reverse voltage appears across them for a timeinterval of γ/ωs, which should be sufficiently large than theturn-off time (tq) of the thyristor that is being used[8]. IGBT-based CSI may be used to achieve operation at almost unitypower factor[4].

B. Series-connected current source inverter (SC-CSI) fed in-duction heating units

This work is primarily aimed at developing series-connectedcurrent source inverter fed induction heating units operatingfrom a common DC-link (circuit diagram shown in the Fig.4). Let us suppose that the first current source inverter is CSI-1

Fig. 4. Circuit diagram for a single power source fed two series-connectedcurrent source inverter (CSI) fed induction heating units.

and corresponding heating coil is IHU-1 and similarly, secondcurrent source inverter is CSI-2 and corresponding heatingcoil is IHU-2. In case of surface heat treatment, some time isneeded for the heat treatment of metal samples upto differentskin-thickness. This type of series connected converter findsapplication in those cases, where two inverters can be operatedat two different frequencies. The skin depth of heat treatmentdepends on the frequency of the inverter. If two invertersoperate at different frequencies then it is possible to have heattreatment upto different skin depths of the metal.

In the above scheme separate power control for each inverteris also needed to control the heating temperature and the “no-power” mode operation. There are different ways for that. Themost common ways of power control are by use of controlledrectifier (thyristor-based controlled rectifier) or dc-link currentcontrol (using buck chopper). In both cases net power canalso be controlled. To control each inverter separately, onemay consider controlling the frequency of each separately.This has its obvious challenges. Here a phase-shifted controlmethod has been suggested and demonstrated, which cancontrol power by shifting (or delaying) phase of the controlsignals. This of course is not possible with an SCR-based CSI.

Fig. 5. Circuit diagram of IGBT-based CSI, R-L load with parallel capacitor.

C. Phase-shifted power control method

The single phase inverter shown in Fig. 5, will generate asquare current waveform (i′L) when gate-pulse T1 = T2 andT3 = T4, and the duty ratios are 50%. The output current canbe controlled by adjusting the phase shift between pulse T1

and T2. The phase-shifted pulse shown in the Fig. 6, whereT2 gate-pulse is phase-shifted from T1 gate-pulse by an angleα. T3 and T4 gate-pulse are inverted versions of T1 and T2

respectively. It also serves to provide protection against open-

Fig. 6. T1 to T4 are the gate pulse of corresponding device and i′L (loadcurrent) in that condition.

circuiting of the converter. Now, in this condition, currentflows through the load only during the high gate-pulse of T1

and T2 (during α to π in Fig. 6). Otherwise current free-wheelsthrough either leg (T1, T4 or T2, T3). Consequently, no powertransfer takes place during the ‘zero’ intervals of i′L, whichnow appears quasi-square.

So, in the phase-shifted method, power can be varied fromfull power to zero power by varying α from 0 to π. For α = π,no average power will be transferred to the load. The abovecircuit with its control has been simulated using SEQUELusing the configuration as shown in Fig. 7. The hardwareimplementation of the same is presented in the next section.

Fig. 7. SEQUEL circuit for two series-connected CSIs fed from one DC-link.

IV. SIMULATION OF CONVERTER PERFORMANCE

The SEQUEL[11][12] based circuit is shown in the Fig. 7.Each of the inverters operate with logic pulses as shown inthe Fig. 6. The device T2 starts to operate after α degree ofthe start of device T1. The phase angle α shifted from 0 toπ. The circuit performance has been simulated at α = 0 forCSI-2 to get full power operation and α = π/2 for CSI-1 to getlow power in the unit. The waveforms are presented in Fig. 8and Fig. 10. Series R-L loads represent equivalent resistanceand inductance of the induction coils. Suitable capacitors are

Fig. 8. Simulated waveform of vL (blue trace) and i′L (green trace) (Groundline at -60) of CSI-2 at 0o phase shift.

connected in parallel also as discussed earlier. The parametersused in the simulation are taken from the actual values usedin the experiments (the design of which was done earlier[5])and are enlisted in Table I. Fig. 8, shows simulated waveformsof voltage across the induction coil (blue trace) and net loadcurrent (green trace). Gate pulses for the CSI-1 are shown inthe Fig. 9, where α is 90o. The corresponding coil voltage (vL:blue trace) and net load current (i′L: green trace) waveformsare shown in Fig. 10.

Fig. 9. Simulated waveform of gate pulses of CSI-1 at 90o phase shift.

Fig. 10. Simulated waveform of vL (blue trace) and i′L (green trace) (Groundline at -40) of CSI-1 at 90o phase shift.

V. LOGIC DEVELOPMENT AND HARDWAREIMPLEMENTATION

In the present work the SC-CSI has been fabricated byputting (i) an IGBT-based CSI-1 and (ii) an SCR-based CSI(which is operationally identical to an IGBT-CSI having α = 0,CSI-2) in series cascade. At front end a 3-phase diode bridgerectifier followed by a buck chopper and appropriate DC-linkinductor (to get constant DC-link current) are connected. ThisDC-link is common to both the IHUs (Fig. 4). The circuitparameter and devices are enlisted below. The control logic of

Sl. Quantity Value or Ratingno.1 DC link inductor(Ld) 20mH2 Coil inductance(Lcoil) 12.6 µH3 Coil resistance(Rcoil) 54 mΩ4 Capacitor across coil(C) 21µF5 Resonance frequency 9.78kHz6 IGBT SKM75GB123D7 DIODE DSEI60-12A8 Thyristor SPSI 359 Inverter operating frequency 9.8kHz

TABLE ITABLE OF IMPORTANT PARAMETERS AND LIST OF DEVICE USED.

the circuit has been developed in the FPGA platform (Alteramake Cyclone EPIC12Q240C processor[13]). Experimentalwaveforms have been included in Fig. 11 which show loadvoltage and current of CSI-1 (which is IGBT-based) and Fig.12 shows the load voltage and current waveform for the series-connected CSI-2 (SCR-based).

Fig. 11. Experimental waveform of vL (upper trace) and i′L (lower trace)for CSI-1.

Fig. 12. Experimental waveform of vL (upper trace) and i′L (lower trace)for CSI-2.

VI. CONCLUSIONS

In this paper performance of a high frequency series-connected current source inverter fed induction heating unit,one fed from a IGBT-based CSI other one fed from thyris-torised CSI, has been presented in details. Initially bothinverter operate at the same frequency without any power con-trol (phase shifting). The power control using phase-shiftingmethod, was first simulated in SEQUEL. Thereafter, the twoconverters were fabricated and tested with load. The controlpulse have been generated from a FPGA platform (Alteramake Cyclone EPIC12Q240C processor). The experimentalresults are in excellent agrement with the simulated ones.

VII. ACKNOWLEDGEMENTS

The authors wish to thank Mr. S. B. Chanda, Chairmanand Managing Director at Megatherm Electronics Pvt. Ltd.,Kolkata and his colleagues Mr. A. K. Kolay and Mr. B.Roy Chowdhury for the material and fund support in makingthe induction heating coils. Very special mention must bemade of Prof. V. Ramanarayanan for his technical discus-sions and motivation. Funds support received from NaMPETinitiative, DIT, Govt. of India towards circuit fabrication isalso gratefully acknowledged. The authors finally acknowledgethe support received from the Dept. of EE, BESU, Shibpur,Howrah towards this work.

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