Projec Final Induction

7/30/2019 Projec Final Induction

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CHAPTER: 1 INTRODUCTION

1.1 TYPES OF ELECTRICAL PROCESS HEATING

Prior to describing induction heating, some types of electric process heating are

explained below to help you understand normally used heat sources.

The types of electric heating are as follows:

Resistance Heating

Conduction Heating

Infrared Radiation Heating

Induction Heating

Dielectric Hysteresis Heating

Electric Arc Heating

Plasma Heating

Electron Beam Heating

Laser Heating

Resistance heating is the most common type of electric process heating. It uses the

relationship between the voltage and current of resistance in Joules Law.

Conduction heating exploits the heat energy generated when an object is placed

between two electric poles, which is another application of Joules Law. In this case,

however, a different relationship exists between voltage and current, especially when the

circuit current is high, because the object itself contains both resistance and inductance

features.

The main topic of this document is induction heating, which is a combination of

electromagnetic induction, the skin effect, and the principle of heat transfer. In short,

induction heating refers to the generation of heat energy by the current and eddy currentcreated on the surface of a conductive object (according to Faradays Law and the skin effect)


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when it is placed in the magnetic field, formed around a coil, where the AC current flows

through (Amperes Law). Detailed descriptions of induction heating are presented in the

following sections of the document.

1.2 BASIC OF INDUCTION HEATING

Induction heating is comprised of three basic factors: electromagnetic induction, the

skin effect, and heat transfer. The fundamental theory of INDUCTION HEATING, however,

is similar to that of a transformer. Electromagnetic induction and the skin effect are described

in this section.

Figure1-1 Basic of induction heating


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1.3 ELECTROMAGNETIC INDUCTION

As shown in Figure 1-1, when the AC current enters a coil, a magnetic field is

formed around the coil according to Amperes Law.

Hdl = Ni = F

= HA (Formula 1-1)

An object put into the magnetic field causes a change in the velocity of the magnetic

movement. The density of the magnetic field wanes as the object gets closer to the center

from the surface.

According to Faradays Law, the current generated on the surface of a conductive

object has an inverse relationship with the current on the inducting circuit as described in

Formula 1-2. The current on the surface of the object generates an eddy current.

E = = N

(formula 1-2)

As a result, the electric energy caused by the induced current and eddy current is

converted to heat energy as shown in Formula 1-3.

P = E2/R = i

2R (formula 1-3)

Here, resistance is determined by the resistivity () and permeability () of the

conductive object. Current is determined by the intensity of the magnetic field. Heat energy is

in an inverse relationship with skin depth which is described in Section 1-4.

If an object has conductive properties like iron, additional heat energy is generated due

to magnetic hysteresis. The amount of heat energy created by hysteresis is in proportion to the

size of the hysteresis. In this document, this additional energy is ignored because it is far

smaller (less than 10%) than the energy generated by induction current.


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1-4 SKIN EFFECT

The higher the frequency of the current administered to the coil, the more intensive is

the induced current flowing around the surface of the load. The density of the induced current

diminishes when flowing closer to the center as shown in Formula 1-4 and 1-5 below. This is

called the skin effect or kelvin effect. From this effect, one can easily infer that the heat

energy converted from electric energy is concentrated on the skin depth (surface of the

object).

Ix = i0 (Formula 1- 4)

where, ix = distance from the skin (surface) of the object, current density at x.

Io = current density on skin depth (x=0)

Do = a constant determined by the frequency (current penetration depth or skin

depth)

d0 = (Formula 1- 5)

where,

= resistivity.

= permeability of the object.

= Frequency of the current flowing through the object.

Formula 1-5 states that the skin thickness is determined by the resistivity,

permeability, and frequency of the object. Figure 1-2 below is the distribution chart of current

density in relation to skin thickness.


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Figure 1-3 Flow of high frequency electricity

The energy transfer of induction heating is affected by the distance between the coil

and the work piece . The induction coil is usually made of copper tubing and fluid cooled.

Diameter, shape, and number of turns influence the efficiency and field pattern.

1.6 WHAT IS INDUCTION HEATING USED FOR?

Induction heating can be used for any application where we want to heat an

electrically conductive material in a clean, efficient and controlled manner. One of the most

common applications is for sealing the anti-tamper seals that are stuck to the top of medicine

and drinks bottles. A foil seal coated with "hot-melt glue" is inserted into the plastic cap and

screwed onto the top of each bottle during manufacture. These foil seals are then rapidly

heated as the bottles pass under an induction heater on the production line. The heat

generated melts the glue and seals the foil onto the top of the bottle. When the cap is

removed, the foil remains providing an airtight seal and preventing any tampering or

contamination of the bottle's contents until the customer pierces the foil.

Another common application is "getter firing" to remove contamination from

evacuated tubes such as TV picture tubes, vacuum tubes, and various gas discharge lamps. A


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ring of conductive material called a "getter" is placed inside the evacuated glass vessel. Since

induction heating is a non-contact process it can be used to heat the getter that is already

sealed inside a vessel. An induction work coil is located close to the getter on the outside of

the vacuum tube and the AC source is turned on. Within seconds of starting the induction

heater, the getter is heated white hot, and chemicals in its coating react with any gasses in the

vacuum. The result is that the getter absorbs any last remaining traces of gas inside the

vacuum tube and increases the purity of the vacuum.

Yet another common application for induction heating is a process called Zone

purification used in the semiconductor manufacturing industry. This is a process in which

silicon is purified by means of a moving zone of molten material. An Internet Search is sure

to turn up more details on this process that I know little about.

Other applications include melting, welding and brazing or metals. Induction

cooking hobs and rice cookers. Metal hardening of ammunition, gear teeth, saw blades and

drive shafts, etc are also common applications because the induction process heats the surface

of the metal very rapidly. Therefore it can be used for surface hardening, and hardening of

localised areas of metallic parts by "outrunning" the thermal conduction of heat deeper into

the part or to surrounding areas. The non contact nature of induction heating also means that

it can be used to heat materials in analytical applications without risk of contaminating the

specimen.

Similiarly, metal medical instruments may be sterilised by heating them to high

temperatures whilst they are still sealed inside a known sterile environment, in order to kill

germs.

1.7 BASIC REQUIREMENT FOR INDUCTION HEATING:

A source of High Frequency electrical power.

A work coil to generate the alternating magnetic field.

An electrically conductive workpiece to be heated.


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FIGURE 1-4 Basic Block Diagram Of Induction Heating.


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CHAPTER 2 RECTIFIER

2-1 INTRODUCTION

Although in our daily life we use A.C. current devices. But rectifier is a Electronic

device which converts A.C. power into D.C. power.

The study of the junction diode characteristics reveals that the junction diode offers a

low resistance path, when forward biased, and a high resistance path, when reverse biased.

This feature of the junction diode enables it to be used as a rectifier.

The alternating signals provides opposite kind of biased voltage at the junction after

each half-cycle. If the junction is forward biased in the first half-cycle, its gets reverse biased

in the second half. It results in the flow of forward current in one direction only and thus the

signal gets rectified.

In other words, we can say, when an alternating e.m.f. signal is applied across a

junction diode, it will conduct only during those alternate half cycles, which biased it in

forward direction.

In most power electronic applications, the power input is in the form of a 50 or 60hz

sine wave ac voltage provided by the electric utility, that is first converted to a dc voltage.

Increasingly, the trend is to use the inexpensive rectifiers with diodes to convert the input ac

into dc in an uncontrolled manner, using rectifier with diodes, as illustrated by the block

diagram of fig.2-1. In such diodes rectifiers,the power flow can only be form the utility ac

side to the dc side. A majority of the power electronics applications such as switching dc

power supplies, ac motor drives, dc servo drives, and so on, use such uncontrolled rectifier. Inmost of the applications, the rectifier are supplied directly from the utility source without a

60-hz transformer. The avoidance of this costly and bulky 60-hz transformer is important in

most morden power electronic systems.


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Figure 2-1 Circuit diagram of rectifier

The dc output voltage of a rectifier should be as ripple free as possible. Therefore, a

large capacitor is connected as a filter on the dc side. this capacitor gets charged to a value

close to the peak of the ac input voltage. As a consequence, the current through the rectifier is

very large near the peak of the 60hz of ac input voltage and it does not flow continuously;

that is, it becomes zero for finite durations during each half cycle of the line frequency. These

rectifiers draw highly disorted current from the utility. Now and even the future harmonics

standards and guidelines will limit the amount of the current distortion allowed in the utility,

and the simple diode rectifiers may not be allowed. Circuits to achieve a nearly sinusoidal

current rectification at a unity power factor for many application.

2-2 TYPES OF RECTIFIER:

Half bridge

Full bridge

D1

D2

D3

D4

CVs Vo

+

-

+

-

Is

Id


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2-3 APPLICATION OF RECTIFIER:

The primary application of rectifiers is to derive DC power from an AC supply.Virtually all electronic devices require DC, so rectifiers are used inside the power supplies of

virtually all electronic equipment.

Converting DC power from one voltage to another is much more complicated. One

method of DC-to-DC conversion first converts power to AC (using a device called

an inverter), then use a transformer to change the voltage, and finally rectifies power back to

DC. A frequency of typically several tens of kilohertz is used, as this requires much smaller

inductance than at lower frequencies and obviates the use of heavy, bulky, and expensive

iron-cored units.

Rectifiers are also used for detection of amplitude modulated radio signals. The signal

may be amplified before detection. If not, a very low voltage drop diode or a diode biased

with a fixed voltage must be used. When using a rectifier for demodulation the capacitor and

load resistance must be carefully matched: too low a capacitance will result in the high

frequency carrier passing to the output, and too high will result in the capacitor just charging

and staying charged.

Rectifiers are used to supply polarised voltage for welding. In such circuits control of

the output current is required; this is sometimes achieved by replacing some of the diodes in

a bridge rectifier with thyristors, effectively diodes whose voltage output can be regulated by

switching on and off withphase fired controllers.


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CHAPTER: 3 INVERTER

3-1 INTRODUCTION OF INVERTER

A device that converts dc power into ac power into ac power at desired output voltage

and frequency is called an inverter. Some industrial applications of inverters are for induction

heating, adjustable-speed ac drives, stand by air-craft power supplies, UPS(uninterruptible

power supplies) for computers, hvdc transmission lines etc. phase-controlled converters, when

operated in the inverter mode, are called line-commutated inverters. But line-commutated

inverter require at the output terminals an existing ac supply which is used for their

commutation. This means that line-commutated inverters cant function as isolated ac voltagesources or as variable frequency generators with dc power at input. Therefore, voltage level,

frequency and waveform on the ac side of line-commutated inverters cannot be changed. On

the other hand, force commutated inverters provide an independent ac output voltage of

adjustable voltage and adjustable frequency and have there much wider application.

3-2 TYPE OF INVERTERS

Inverters can be broadly classified into two types;

Voltage source inverter

Current source inverter

A voltage-fed inverter(VFI) or voltage source inverter(VSI) is one in which the dc source

has small or negligible impedance.

Voltage Source Inverters is one in which the DC source has small or negligible

impedance. In other words VSI has stiff DC voltage source at its input terminals. A current

source inverter is fed with adjustable current from a DC source of high impedance, i.e; from a

stiff DC current source. In a CSI fed with stiff current source, output current waves are not

affected by the load.


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From view point of connections of semiconductor devices, inverters are classified as

under:

Bridge Inverters

Series Inverters

Parallel Inverters

3-3 SWITCHING DEVICES:

We can use three types of switches in inverter as listed bellow:

BJT(Bipolar Junction Transistor)

POWER MOSFET(Metal-oxide-semiconductor field effect transistor)

IGBT(Insulated Gate Bipolar Transistor)

3-3-1 BJT:

A bipolar transistor is a three-layer, two junction npn or pnp semiconductor device.

With one p-region sandwiching by two n-regions npn transistor is obtained. With two p-

regions sandwiching one n-region pnp transistor is obtained. The term bipolar denotes that

the current flow in the device is due to the movement of both holes and electrons. A BJT has

three terminals named collector(C), emitter (E),and base(B). Power transistors of npn type

are easy to manufacture and are cheaper also. Therefore, use of power npn transistor is very

wide in high voltage and high current application.

3-3-2 POWER MOSFET:

A power MOSFET has three terminals called drain(D), source(S) and gate(G) in place

of the corresponding three terminals collector, emitter and base for BJT. A BJT is a current

controlled device where as a power MOSFET is a voltage-controlled device. As its operation

depends upon the flow of majority carriers only, MOSFET is a unipolar device. The control

signal, or base current in BJT is much larger than the control signal(or gate current) required


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in a MOSFET. This is because of the fact that gate circuit impedence in MOSFET is

extremely high, of the order of 109

ohm. This large impedence permits the MOSFET gate to

be driven directlt from microelectronic circuits. BJT suffers from second breakdown voltage

whereas MOSFET is free fron this problem. Power MOSFETs are now finding increasing

applications in low-power high frequency converters.

3-3-3 IGBT:

IGBT has been developed by combining into it the best qualities of both BJT and

PMOSFET. Thus an IGBT posseses high input impedence like a power MOSFET and has

low on-state power loss as in a BJT. FurtherIGBT is free from second breakdown problem

present in BJT. All these metal oxide insulated gate transistor (MOSIGT),

conductively/modulated field effect transistor(COMFET) or gain/modulated FET(GEMFET).

It was also initially called insulated gate transistor IGT.

FEATURES OF IGBT:

Large current devices

High voltage devices

High speed switching device

Self turn off device

Voltage driving device

ADVANTAGES :

Optimize consistency

Maximize productivity

Improved product quality

Extended fixture life

Reduce energy consumption


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3-5 BASIC CIRCUIT DIAGRAM OF INVERTER:

Figure 3-1 Series-parallel resonant inverter

3-6 PULSE MODULATION SCHEMES:

3-6-1 PULSE-AMPLITUDE MODULATION:

In PAM the successive sample values of the analog signal s(t) are used to effect the

amplitudes of a corresponding sequence of pulses of constant duration occurring at the

sampling rate. No quantization of the samples normally occurs (Fig. 3-2a, b). In principle the

pulses may occupy the entire time between samples, but in most practical systems the pulse

duration, known as the duty cycle, is limited to a fraction of the sampling interval. Such a

restriction creates the possibility of interleaving during one sample interval one or more

pulses derived from other PAM systems in a process known as time-division multiplexing

(TDM).


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Figure 3-2(a)Analog signal, s(t). (b) Pulse-amplitude modulation. (c) Pulse-width

modulation. (d) Pulse position modulation

3-6-2 PULSE-WIDTH MODULATION:

In PWM the pulses representing successive sample values of s(t) have constant

amplitudes but vary in time duration in direct proportion to the sample value. The pulse

duration can be changed relative to fixed leading or trailing time edges or a fixed pulse

center. To allow for time-division multiplexing, the maximum pulse duration may be limited

to a fraction of the time between samples (Fig. 3-2c).

3-6-3 PULSE-POSITION MODULATION:

PPM encodes the sample values ofs(t) by varying the position of a pulse of constant

duration relative to its nominal time of occurrence. As in PAM and PWM, the duration of the


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pulses is typically a fraction of the sampling interval. In addition, the maximum time

excursion of the pulses may be limited (Fig. 3-2d).

3-6-4 PULSE-CODE MODULATION:

Many modern communication systems are designed to transmit and receive only

pulses of two distinct amplitudes. In these so-called binary digital systems, the analog-to-

digital conversion process is extended by the additional step of coding, in which the

amplitude of each pulse representing a quantized sample of s(t) is converted into a unique

sequence of one or more pulses with just two possible amplitudes. The complete conversion

process is known as pulse-code modulation. Figure 2-3a shows the example of three

successive quantized samples of an analog signal s(t), in which sampling occurs every T

seconds and the pulse representing the sample is limited to T/2 seconds. Assuming that the

number of quantization levels is limited to 8, each level can be represented by a unique

sequence of three two-valued pulses.

Figure3-3(a) Three successive quantized samples of an analog

signal. (b) With pulses of amplitude V or 0. (c) With pulses of amplitude V orV


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IV. The output voltage control can be obtained without any additional components.

V. With this method, lower order harmonics can be eliminated or minimized Along with its

output voltage control.

VI. As higher order harmonics can be filtered easily the higher order harmonics can be

minimized.

3-9 APPLICATIONS OF INVERTER:

DC POWER SOURCE UTILIZATION:

Inverter designed to provide 115 VAC from the 12 VDC source provided in an automobile.

The unit provides up to 1.2 Amps of alternating current, or just enough to power two sixty

watt light bulbs.

An inverter converts the DC electricity from sources such as batteries, solar panels, or fuel

cells to AC electricity. The electricity can be at any required voltage; in particular it canoperate AC equipment designed for mains operation, or rectified to produce DC at any

desired voltage.

Grid tie inverters can feed energy back into the distribution network because they produce

alternating current with the same wave shape and frequency as supplied by the distribution

system. They can also switch off automatically in the event of a blackout.

Micro-inverters convert direct current from individual solar panels into alternating current

for the electric grid.

UNINTERRUPTIBLE POWER SUPPLIES:

An uninterruptible power supply is a device which supplies the stored electrical power to

the load in case of raw power cut-off or blackout. One type of UPS uses batteries to store

power and an inverter to supply AC power from the batteries when main power is not


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available. When main power is restored, a rectifier is used to supply DC power to recharge

the batteries.

It is widely used at domestic and commercial level in countries facing Power outages.

INDUCTION HEATING:

Inverters convert low frequency main AC power to a higher frequency for use in induction

heating. To do this, AC power is first rectified to provide DC power. The inverter then

changes the DC power to high frequency AC power.

HVDC POWER TRANSMISSION:

With HVDC power transmission, AC power is rectified and high voltage DC power is

transmitted to another location. At the receiving location, an inverter in a static inverter plant

converts the power back to AC.

VARIABLE-FREQUENCY DRIVES:

A variable-frequency drive controls the operating speed of an AC motor by controlling the

frequency and voltage of the power supplied to the motor. An inverter provides the controlled

power. In most cases, the variable-frequency drive includes a rectifier so that DC power for

the inverter can be provided from main AC power. Since an inverter is the key component,

variable frequency drives are sometimes called inverter drives or just inverters.

ELECTRIC VEHICLE DRIVES:

Adjustable speed motor control inverters are currently used to power the traction motor in

some electric locomotives and diesel-electric locomotives as well as some battery electric

vehicles and hybrid electric highway vehicles such as the Toyota Prius. Various

improvements in inverter technology are being developed specifically for electric vehicle


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applications.[2] In vehicles with regenerative braking, the inverter also takes power from the

motor (now acting as a generator) and stores it in the batteries.

THE GENERAL CASE:

A transformer allows AC power to be converted to any desired voltage, but at the same

frequency. Inverters, plus rectifiers for DC, can be designed to convert from any voltage, AC

or DC, to any other voltage, also AC or DC, at any desired frequency. The output power can

never exceed the input power, but efficiencies can be high, with a small proportion of the

power dissipated as waste heat.

CHAPTER 4 RESONANT TANK CIRCUIT:

4-1 SERIES RESONANT TANK CIRCUIT:

The work coil is made to resonate at the intended operating frequency by means of a

capacitor placed in series with it. This causes the current through the work coil to besinusoidal. The series resonance also magnifies the voltage across the work coil, far higher

than the output voltage of the inverter alone. The inverter sees a sinusoidal load current but it

must carry the full current that flows in the work coil. For this reason the work coil often

consists of many turns of wire with only a few amps or tens of amps flowing. Significant

heating power is achieved by allowing resonant voltage rise across the work coil in the series-

resonant arrangement whilst keeping the current through the coil (and the inverter) to a

sensible level.

This arrangement is commonly used in things like rice cookers where the power level is

low, and the inverter is located next to the object to be heated. The main drawbacks of the

series resonant arrangement are that the inverter must carry the same current that flows in the

work coil. In addition to this the voltage rise due to series resonance can become very

pronounced if there is not a significantly sized workpiece present in the work coil to damp the

circuit. This is not a problem in applications like rice cookers where the workpiece is always


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the same cooking vessel, and its properties are well known at the time of designing the

system.

The tank capacitor is typically rated for a high voltage because of the resonant voltage rise

experienced in the series tuned resonant circuit. It must also carry the full current carried by

the work coil, although this is typically not a problem in low power applications.

4-2 PARALLEL RESONANT TANK CIRCUIT:

The work coil is made to resonate at the intended operating frequency by means of acapacitor placed in parallel with it.

This causes the current through the work coil to be sinusoidal. The parallel resonance also

magnifies the current throughthe work coil, far higher than the output current capability of

the inverter alone. The inverter sees a sinusoidal load current.

However, in this case it only has to carry the part of the load current that actually does real

work. The inverter does not have to carry the full circulating current in the work coil. This is

very significant since power factors in induction heating applications are typically low. This

property of the parallel resonant circuit can make a tenfold reduction in the current that must

be supported by the inverter and the wires connecting it to the work coil. Conduction losses

are typically proportional to current squared, so a tenfold reduction in load current represents

a significant saving in conduction losses in the inverter and associated wiring. This means

that the work coil can be placed at a location remote from the inverter without incurring

massive losses in the feed wires.

Work coils using this technique often consist of only a few turns of a thick copper conductor

but with large currents of many hundreds or thousands of amps flowing. (This is necessary to

get the required Ampere turns to do the induction heating.) Water cooling is common for all

but the smallest of systems. This is needed to remove excess heat generated by the passage of

the large high frequency current through the work coil and its associated tank capacitor.


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Figure 4-1 Parallel resonant

In the parallel resonant tank circuit the work coil can be thought of as an inductive

load with a "power factor correction" capacitor connected across it. The PFC capacitor

provides reactive current flow equal and opposite to the large inductive current drawn by the

work coil. The key thing to remember is that this huge current is localised to the work coil

and itscapacitor, and merely represents reactive power sloshing back-and-forth between the

two. Therefore the only real current flow from the inverter is the relatively small amount

required to overcome losses in the "PFC" capacitor and the work coil.

There is always some loss in this tank circuit due to dielectric loss in the capacitor and skin

effect causing resistive losses in the capacitor and work coil. Therefore a small current is

always drawn from the inverter even with no workpiece present.When a lossy workpiece is

inserted into the work coil, this damps the parallel resonant circuit by introducing a further

loss into the system. Therefore the current drawn by the parallel resonant tank circuitincreases when a work piece is entered into the coil.


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CHAPTER 5 IMPEDANCE MATCHING

This refers to the electronics that sits between the source of high frequency power and

the work coil we are using for heating. In order to heat a solid piece of metal via induction

heating we need to cause a TREMENDOUS current to flow in the surface of the metal.

However this can be contrasted with the inverter that generates the high frequency power.

The inverter generally works better (and the design is somewhat easier) if it operatesat fairly

high voltage but a low current. (Typically problems are encountered in power electronics

when we try to switch large currents on and off in very short times.) Increasing the voltage

and decreasing the current allows common switch mode MOSFETs (or fast IGBTs) to be

used.

The comparatively low currents make the inverter less sensitive to layout issues and stray

inductance. It is the job of the matching network and the work coil itself to transform the

high-voltage/low-current from the inverter to the low-voltage/high-current required to heat

the workpiece efficiently.

We can think of the tank circuit incorporating the work coil (Lw) and its capacitor (Cw) as a

parallel resonant circuit. This has a resistance (R) due to the lossy workpiece coupled into the

Work coil due to the magnetic coupling between the two conductors.

FIGURE 5-1 Impedence matching


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In practice the resistance of the work coil, the resistance of the tank capacitor, and the

reflected resistance of the workpiece all introduce a loss into the tank circuit and damp the

resonance. Therefore it is useful to combine all of these losses into a single "loss resistance."

In the case of a parallel resonant circuit this loss resistance appears directly across the tank

circuit in our model. This resistance represents the only component that can consume real

power, and therefore we can think of this loss resistance as the load that we are trying to drive

power into in an efficient manner.

When driven at resonance the current drawn by the tank capacitor and the work coil

are equal in magnitude and opposite in phase and therefore cancel each other out as far as the

source of power is concerned. This means that the only loadseen by the power source at

the resonant frequency is the loss resistance across the tank circuit.

The job of the matching network is simply to transform this relatively large loss

resistance across the tank circuit down to a lower value that better suits the inverter

attempting to drive it. There are many different ways to achieve this impedance

transformation including tapping the work coil, using a ferrite transformer, a capacitive

divider in place of the tank capacitor, or a matching circuit such as an L-match network.

In the case of an L-match network it can transform the relatively high load resistance

of the tank circuit down to something around 10 ohms which better suits the inverter. This

figure is typical to allow the inverter to run from several hundred volts whilst keeping

currents down to a medium level so that standard switch-mode MOSFETs can be used to

perform the switching operation.

The L-match network consists of components Lm and Cm shown opposite. The L-

match network has several highly desirable properties in this application. The inductor at the

input to the L-match network presents a progressively rising inductive reactance to all

frequencies higher than the resonant frequency of the tank circuit. This is very important

when the work coil is to be fed from a voltage-source inverter that generates a squarewave

voltage output.


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The squarewave voltage generated by most half-bridge and full-bridge circuits is rich

in high frequency harmonics as well as the wanted fundamental frequency. Direct connection

of such a voltage source to a parallel resonant circuit would cause excessive currents to flow

at all harmonics of the drive frequency! This is because the tank capacitor in the parallel

resonant circuit would present a progressively lower capacitive reactance to increasing

frequencies. This is potentially very damaging to a voltage-source inverter. It results in large

current spikes at the switching transitions as the inverter tries to rapidly charge and discharge

the tank capacitor on rising and falling edges of the squarewave. The inclusion of the L-

match network between the inverter and the tank circuit negates this problem. Now the output

of the inverter sees the inductive reactance of Lm in the matching network first, and all

harmonics of the drive waveform see gradually rising inductive impedance. This means that

maximum current flows at the intended frequency only and little harmonic current flows,

making the inverter load current into a smooth waveform.

Finally, with correct tuning the L-match network is able to provide a slight inductive

load to the inverter. This slightly lagging inverter load current can facilitate Zero-Voltage-

Switching (ZVS) of the MOSFETs in the inverter bridge. This significantly reduces turn-on

switching losses due to device output capacitance in MOSFETs operated at high voltages.

The overall result is less heating in the semiconductors and increased lifetime.

In summary, the inclusion of an L-match network between the inverter and the parallel

resonant tank circuit achieves two things.

1. Impedance matching so that the required amount of power can be supplied from the

inverter to the workpiece.

2. Presentation of a rising inductive reactance to high frequency harmonics to keep the

inverter safe and happy.


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FIGURE 5-2 Impedence matching scheme

Looking at the previous schematic above we can see that the capacitor in the

matching network (Cm) and the tank capacitor (Cw) are both in parallel. In practice both of

these functions are usually accomplished by a single purpose built power capacitor. Most of

its capacitance can be thought of as being in parallel resonance with the work coil, with a

small amount providing the impedance matching action with the matching inductor (Lm.)

Combing these two capacitances into one leads us to arrive at the LCLR model for the work

coil arrangement, which is commonly used in industry for induction heating.


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CHAPTER 6 WORKCOIL:

6.1 INTRODUCTION

This arrangement incorporates the work coil into a parallel resonant circuit and uses

the L-match network between the tank circuit and the inverter. The matching network is used

to make the tank circuit appear as a more suitable load to the inverter, and its derivation is

discussed in the section above.

The work coil has a number of desirable properties:

1. A huge current flows in the work coil, but the inverter only has to supply a low current.

The large circulating current is confined to the work coil and its parallel capacitor, which are

usually located very close to each other.

2. Only comparatively low current flows along the transmission line from the inverter to the

tank circuit, so this can use

lighter duty cable.

3. Any stray inductance of the transmission line simply becomes part of the matching

network inductance (Lm.) Therefore the heat station can be located away from the inverter.

4. The inverter sees a sinusoidal load current so it can benefit from ZCS or ZVS to reduce its

switching losses and therefore run cooler.

5. The series matching inductor can be altered to cater for different loads placed inside the

work coil.

6. The tank circuit can be fed via several matching inductors from many inverters to reach

power levels above those achievable with a single inverter. The matching inductors provide


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inherent sharing of the load current between the inverters and also make the system tolerant

to some mismatching in the switching instants of the paralleled inverters.

Another advantage of the LCLR work coil arrangement is that it does not require a high-

frequency transformer to provide the impedance matching function. Ferrite transformers

capable of handling several kilowatts are large, heavy and quite expensive.

In addition to this, the transformer must be cooled to remove excess heat generated by the

high currents flowing in its conductors. The incorporation of the L-match network into the

LCLR work coil arrangement removes the necessity of a transformer to match the inverter to

the work coil, saving cost and simplifying the design.

However, the designer should appreciate that a 1:1 isolating transformer may still be

required between the inverter and the input to the work coil arrangement if electrical isolation

is necessary from the mains supply. This depends whether isolation is important, and whether

the main PSU in the induction heater already provides sufficient electrical isolation to meet

these safety requirements.

6.2 CALAULATION OF WORKCOIL

The number of turns of work coil is mainly based on the length of work piece and the pitch of

coil windings. Thus,

N =

Where,

N = number of turns of work coil

Lw = length of work piece to be hardened, m

And the inner diameter of work coil is

Din

= Dw

+ 2CP


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The outer diameter of work coil is

Dout = Din + 2dc

Where,

dw = diameter of work coil, m

dc = diameter of conductor, m

The total length of conductor for work coil is

Ic = 2Ilead +

Where,lc = length of conductor, m

Ilead = length of work coil lead, m

Rm = inner radius of work coil, m

The minimum thickness of conductor must be at least two times of depth of current

penetration in conductor itself.

Therefore, the minimum thickness of conductor is

Tc = 2c

Where,

tc = minimum thickness of conductor, m

c = depth of current penetration in conductor, m

The depth of current penetration in conductor is

c Where,

Oc = permeability of conductor, H/m

Oo = permeability of free space, H/m

c = electric conductivity of conductor, mho/m

f = applied frequency, Hz


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CHAPTER 7 ADVANTAGES AND APPLICATIONS

7-1 ADVANTAGES:

Optimized Consistency:Once the system is properly calibrated and set up, there is no guess work or variation; the

heating pattern is repeatable and consistent. With modern solid state systems, precise

temperature control provides uniform results; power can be instantly turned on or shut off.

With closed loop temperature control, advanced induction heating systems have the

capability to measure the temperature of each individual part. Specific ramp up, hold and

ramp down rates can be established & data can be recorded for each part that is run.

Maximized Productivity:

Production rates can be maximized because induction works so quickly; heat is developed

directly and instantly (>2000 F. in < 1 second) inside the part. Startup is virtually

instantaneous; no warm up or cool down cycle is required. The induction heating process can

be completed on the manufacturing floor, next to the cold or hot forming machine, instead of

sending batches of parts to a remote furnace area or subcontractor. For example, a brazing or

soldering process which previously required a time-consuming, off-line batch heating

approach can now be replaced with a continuous, one-piece flow manufacturing system.

Improved Product Quality:

With induction, the part to be heated never comes into direct contact with a flame or other

heating element; the heat is induced within the part itself by alternating electrical current. As

a result, product warpage, distortion and reject rates are minimized. For maximum product

quality, the part can be isolated in an enclosed chamber with a vacuum, inert or reducing

atmosphere to eliminate the effects of oxidation.

Extended Fixture Life:

Induction heating rapidly delivers site-specific heat to very small areas of your part,

without heating any surrounding parts. This extends the life of the fixturing and mechanical

setup.

Environmentally Sound:

Induction heating systems do not burn traditional fossil fuels; induction is a clean, non-

polluting process which will help protect the environment. An induction system improves

working conditions for your employees by eliminating smoke, waste heat, noxious emissions

and loud noise. Heating is safe and efficient with no open flame to endanger the operator or


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obscure the process. Non-conductive materials are not affected and can be located in close

proximity to the heating zone without damage.

Energy Saving:

Induction heating is much more efficient than traditional radiant or conduction heating

because there is no loss of heat. The magnetic field transfers energy directly into the pan, and

you can touch the induction burners with a bare hand even while they're turned on.

Conduction heating, by contrast, uses electric elements or gas burners, which lose heat to the

surrounding air and are consequently not as efficient.

Temperature Accuracy:

One of the biggest problems with traditional electric or gas stoves is the difficulty of

controlling the temperature of the burner. Certain cooking techniques and recipes require

precise temperatures, such as melting chocolate or making a custard. Induction burners allow

for accurate temperatures, since you can control the electromagnetic field precisely.

7-2 APPLICATION:

Metal melting

melt gold, silver, steel, aluminum, copper, brass and other metals.

melt your materials at rates to meet your production requirements.

Material testing

How do engineers choose materials with the right set of mechanical properties for a

given product or application? Actual considitions for materials in many industries


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involve high temperatures, and the properties of many materials change with

increasing temperature. Consider the aluminium alloy from which an airplane wing is

made, or the steel used to manufacture automobile axles.

Bonding

Bonding is the process of jointing a metal or a conductive material to a rubber or plastic

material without a third bonding agent. This includes applications where gaskets are bonded

to metal casings, thermoplastic composite bonding, and rubber washer assemblies.

Pipe bending

Induction heating is the preferred heating method for bending of large thicker walled pipes

due to the narrow band focused heating offered by the induction process with the resulting

higher quality bends with lower ovality and wall thinning than other bending methods.

Soldering

Soldering produces liquid and gas-tight joints quickly and at low cost. Most soldering

applications are carried out in air, with the flux acting as a barrier to surface oxidation and


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interation with the atmosphere. It is a convenient and economical way to produce joints when

more complex joining machines are not available or cost-effective.

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