CHAPTER ONE
.                                                INTRODUCTION
- Background to the Work
Induction Heating is the process of contactless and direct heating of a work piece by a varying magnetic field. This technique has become state of the art and is diversifying to numerous heating operations thereby replacing older and more traditional techniques of heating, ranging from industrial to domestic needs.
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Its attraction everywhere and especially in industry is based on fast start up, fast heating, energy saving and environmental friendliness, contactless and direct heating of work piece. Induction heating today has found its way into processes like;
- Pre-Heating of metal work pieces: as in forging, extrusion, and rolling.
- Heat Treatment operations: as in hardening, annealing and tempering.
- Others such as: melting, welding, brazing, semi-conductor growth, non-metal to metal boding, liquid heating, etc.
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A pulse width modulated/frequency modulated Voltage Source Inverter (VSI) is built to power a crucible type Induction Furnace developed based on a new per-unitized model. The Induction Furnace is one type of diverse induction heating loads.
Since the early days of induction heating, the need for improved power supply equipment presented the challenge against its rapid development. The basic task has always been the development of large power source for low, medium and high frequency applications with improved efficiency.
Supply frequency systems, motor-generators, spark-gap generators and vacuum oscillators belong to the early power supplies in use [1]. The growth of semiconductor technology began to introduce static power switches dominated by the thyristor for induction heating [2]. New device optimization has continued to improve the operating frequency, power level, simplicity and efficiency of induction heating power supplies. The introduction of the Insulated Gate Bipolar Transistor (IGBT) in mid 1980s began to oust other members of the thyristor family due to its ease of control and switching speed [3]. These very important features which conduce to induction heating together with continued improvements in the power handling capability of the IGBT has made it a device of choice.
Various designs of saturable core reactors operating at mains frequency have their harmonics harnessed to constitute source of low (supply) frequency induction heating systems.
Much earlier, motor-alternator sets were used to provide the medium frequency power needed for induction heating works. It has the drawbacks of fixed frequency operation, bulkiness, high noise level and high fault infestation resulting in long downtimes. A number of such units imported into this country have broken down and become abandoned due to lack of maintenance expertise.
One such unit for a 50kg crucible induction furnace is installed for the Federal Government Science Equipment Development Institute Enugu (SEDI-E). SEDI-E needs a replacement of the power supply unit of its induction furnace which broke down more than seven years ago and is still in that state for want of capability to resuscitate or replace the power unit.
Spark gap and Vacuum Tube Oscillators were also used to generate high frequency induction heating power. Vacuum tubes have low efficiency and very high tube voltage. They are continually declining in popularity. Spark gap oscillators are auto-tuned to resonance but have high peak voltages than would appear in the motor-generator set or vacuum tube generator. They have however faded out.
A solid state powered unit is a modern and preferred system. It has the advantages of compactness, flexibility, noiselessness, ruggedness, versatility and environment friendliness. Solid state power units which have been used for this purpose include:
Cycloinverter/Cycloconverter: This is a direct alternating current (a.c) to a.c converter with the drawbacks of complex control and fixed or variable frequency operation.
Resonant Inverter: A direct current (d.c) power source is needed to feed the resonant inverter. The d.c source can be a controlled thyristor rectifier providing power control or uncontrolled diode bridge in which case power control is effected at the inverter stage. It has Current source inverter (CSI) or Voltage source inverter (VSI) versions. It is for medium and high frequencies with series or parallel compensated tank load but it is load commutated.
Various modifications of the resonant inverter are currently being pursued with new generations of power semiconductor devices targeting improvements in ease of commutation, compactness, power conversion efficiency, frequency limits, power handling capability and other improved performances. This work is one such effort.
1.2Â Â Â Â Â Intended Problem to be solved by this Work/Proposal
A pulse width modulated voltage source resonant inverter is to be developed, built and tested based on a novel generalized model of induction heating load is expected to supply controlled power to the load at unity power factor while at the same time optimize power consumption by heating the dynamic tank load, with high ‘quality factor’ (Q), at resonance and minimize current through the inverter stage with reduced power supply I2R losses. The inverter is to feature performance enhancing control simplifications which avoids traditional thyristor based a.c/d.c. converter used for power control and drops the usual commutation circuits associated with thyristor d.c/a.c converters. The normally short-circuit prone VSI is also to be equipped against damaging current using a current limit control technique and improved topology which confers short circuit immunity of the VSI. This high performance VSI and the novel generalized model can provide the needed know how for developing high performance  and cost effective power supply units with rapid deployment features in a variety of new as well as defective resonant induction heating equipment.
It will operate the load at unity power factor from two stage power conversion in which an uncontrolled 3-phase full bridge diode rectifier provides a fixed d.c supply to a full bridge single phase PWM VSI using IGBT with variable output voltage and frequency.
The basic block diagram of a single phase inverter is shown in figure 1.1.
Figure 1.1: The basic block diagram of single phase inverter
1.3Â Â Â Chapter Arrangement
Chapter 2:Â Here the basic phenomena of induction heating are outlined. The development of eddy current in a work piece, the electromagnetic interaction between the coil, work piece and the resultant forces in an induction heating system are illustrated.
       Chapter 3: Various types of converters (power supplies) for induction heating are overviewed here with comparative look.
 Chapter 4: This proposed scheme is analysed in this chapter and the power circuits are simulated. The trigger pulse generation is developed, control requirements are determined and the scheme is illustrated with the design of a 500kW unit.
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Chapter 5:Â The implementation of a laboratory prototype is presented and tested.
The results are obtained, displayed and analysed.
Chapter 6:Â Summary of work done.
References         Â
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