basic induction heat

7/27/2019 basic induction heat

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Introduction

Induction heating is a non-contact heating process. It uses high frequency electricity

to heat materials that are electrically conductive. Since it is non-contact, the heating

process does not contaminate the material being heated. It is also very efficient since

the heat is actually generated inside the workpiece. This can be contrasted with other

heating methods where heat is generated in a flame or heating element, which is thenapplied to the workpiece. For these reasons Induction eating lends itself to some

unique applications in industry.

How does Induction Heating work ?

! source of high frequency electricity is used to drive a large alternating current

through a coil. This coil is known as the work coil. See the picture opposite.

The passage of current through this coil generates a very intense and rapidly changing

magnetic field in the space within the work coil. The workpiece to be heated is placed

within this intense alternating magnetic field.

"epending on the nature of the workpiece material, a number of things happen...

The alternating magnetic field induces a current flow in the conductive workpiece.

The arrangement of the work coil and the workpiece can be thought of as an electrical

transformer. The work coil is like the primary where electrical energy is fed in, and

the workpiece is like a single turn secondary that is short-circuited. This causes

tremendous currents to flow through the workpiece. These are known as eddycurrents.


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In addition to this, the high frequency used in induction heating applications gives rise

to a phenomenon called skin effect. This skin effect forces the alternating current to

flow in a thin layer towards the surface of the workpiece. The skin effect increases the

effective resistance of the metal to the passage of the large current. Therefore it

greatly increases the heating effect caused by the current induced in the workpiece.

(Although the heating due to eddy currents is desirable in this application, it is

interesting to note that transformer manufacturers go to great lengths to avoid this

phenomenon in their transformers. Laminated transformer cores, powdered iron

cores and ferrites are all used to prevent eddy currents from flowing inside

transformer cores. Inside a transformer the passage of eddy currents is highly

undesirable because it causes heating of the magnetic core and represents power that

is wasted.)

And for Ferrous metals ?

For ferrous metals like iron and some types of steel, there is an additional heating

mechanism that takes place at the same time as the eddy currents mentioned above.

The intense alternating magnetic field inside the work coil repeatedly magnetises and

de-magnetises the iron crystals. This rapid flipping of the magnetic domains causes

considerable friction and heating inside the material. eating due to this mechanism isknown as ysteresis loss, and is greatest for materials that have a large area inside

their #- curve. This can be a large contributing factor to the heat generated during

induction heating, but only takes place inside ferrous materials. For this reason ferrous

materials lend themselves more easily to heating by induction than non-ferrous

materials.

It is interesting to note that steel looses its magnetic properties when heated above

appro$imately %&&'(. This temperature is known as the (urie temperature. This

means that above %&&'( there can be no heating of the material due to hysteresis

losses. !ny further heating of the material must be due to induced eddy currents

alone. This makes heating steel above %&&'( more of a challenge for the inductionheating systems. The fact that copper and !luminium are both non-magnetic and very

good electrical conductors, can also make these materials a challenge to heat

efficiently. )*e will see that the best course of action for these materials is to up the

frequency to e$aggerate losses due to the skin effect.+

What is required for Induction Heating ?

In theory only things are essential to implement induction heating


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. ! source of igh Frequency electrical power,

/. ! work coil to generate the alternating magnetic field,

. !n electrically conductive workpiece to be heated,

aving said this, practical induction heating systems are usually a little more

comple$. For e$ample, an impedance matching network is often required between theigh Frequency source and the work coil in order to ensure good power transfer.

*ater cooling systems are also common in high power induction heaters to remove

waste heat from the work coil, its matching network and the power electronics.

Finally some control electronics is usually employed to control the intensity of the

heating action, and time the heating cycle to ensure consistent results. The control

electronics also protects the system from being damaged by a number of adverse

operating conditions. owever, the basic principle of operation of any induction

heater remains the same as described earlier.

Practical implementation

In practice the work coil is usually incorporated into a resonant tank circuit. This has a

number of advantages. Firstly, it makes either the current or the voltage waveform

become sinusoidal. This minimises losses in the inverter by allowing it to benefit from

either 0ero-voltage-switching or 0ero-current-switching depending on the e$act

arrangement chosen. The sinusoidal waveform at the work coil also represents a more

pure signal and causes less 1adio Frequency Interference to nearby equipment. This

later point becoming very important in high-powered systems. *e will see that there

are a number of resonant schemes that the designer of an induction heater can choosefor the work coil

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 be

sinusoidal. 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 thisreason 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 ne$t to the ob2ect 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 si0ed workpiece present inthe work coil to damp the circuit. This is not a problem in applications like rice


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cookers where the workpiece is always 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 e$perienced 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 powerapplications.

Parallel resonant tank circuit

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

capacitor placed in parallel with it. This causes the current through the work coil to be

sinusoidal. The parallel resonance also magnifies the current through the work coil,

far higher than the output current capability of the inverter alone. The inverter sees a

sinusoidal load current. owever, in this case it only has to carry the part of the loadcurrent 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. (onduction 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.

*ork 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 !mpere turns to do the induction heating.+

*ater cooling is common for all but the smallest of systems. This is needed to remove

e$cess heat generated by the passage of the large high frequency current through the

work coil and its associated tank capacitor.

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

load with a 3power factor correction3 capacitor connected across it. The 4F(

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 its capacitor, 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 34F(3

capacitor and the work coil. There is always some loss in this tank circuit due todielectric loss in the capacitor and skin effect causing resistive losses in the capacitor


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and work coil. Therefore a small current is always drawn from the inverter even with

no workpiece present. *hen 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 circuit increases when a

workpiece is entered into the coil.

Impedance matching

5r simply 36atching3. 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 T17678"59S current

to flow in the surface of the metal. owever 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 operates at 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 65SF7Ts )or fast I:#Ts+ to be used. The

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

inductance. It is the 2ob 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.

*e can think of the tank circuitincorporating the work coil )


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

power into in an efficient manner.

*hen 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 load seen by thepower source at the resonant frequency is the loss resistance across the tank

circuit.(Note that, when driven either side of the resonant frequency, there is an

additional out!of!phase component to the current caused by incomplete

cancellation of the wor" coil current and the tan" capacitor current. #his reactive

current increases the total magnitude of the current being drawn from the source but

does not contribute to any useful heating in the wor"piece.)

The 2ob 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, acapacitive divider in place of the tank capacitor, or a matching circuit such as an


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the inductive reactance of


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The LCL work coil

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

the


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Conceptual schematic

The system schematic belows shows the simplest inverter driving its


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!t still higher powers it is possible to use several seperate inverters effectively

connected in parallel to meet the high load-current demands. owever, the seperate

inverters are not directly tied in parallel at the output terminals of their -bridges.7ach of the distributed inverters is connected to the remote work coil via its own pair

of matching inductors which ensure that the total load is spread evenly among all of

the inverters.

These matching inductors also provide a number of additional benefits when inverters

are paralleled in this way. Firstly, the impedance #7T*778 any two inverter outputs

is equal to twice the value of the matching inductance. This inductive impedance

limits the 3shoot between3 current that flows between paralleled inverters if their

switching instants are not perfectly synchronised. Secondly, this same inductive

reactance between inverters limits the rate at which fault current rises if one of the

inverters e$hibits a device failure, potentially eliminating failure of further devices.

Finally, since all distributed inverters are already connected via inductors, any

additional inductance between the inverters merely adds to this impedance and onlyhas the effect of slightly degrading current sharing. Therefore the distributed inverters


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for induction heating need not necessarily be located physically close to each other. If

isolation transformers are included in the designs then they need not even run from

the same supply>

Fault tolerance

The


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Power control methods

It is often desirable to control the amount of power processed by an induction heater.

This determines the rate at which heat energy is transferred to the workpiece. The

power setting of this type of induction heater can be controlled in a number of

different ways

!. "aryin# the $C link volta#e.

The power processed by the inverter can be decreased by reducing the supply voltage

to the inverter. This can be done by running the inverter from a variable voltage "(

supply such as a controlled rectifier using thyristors to vary the "( supply voltage

derived from the mains supply. The impedance presented to the inverter is largelyconstant with varying power level, so the power throughput of the inverter is roughly

proportional to the square of the supply voltage. @arying the "( link voltage allows

full control of the power from &D to &&D.

It should be noted however, that the e$act power throughput in kilowatts depends not

only on the "( supply voltage to the inverter, but also on the load impedence that the

work coil presents to the inverter through the matching network. Therefore if precise

power control is required the actual induction heating power must be measured,

compared to the requested 3power setting3 from the operator and an error signal fed

back to continually ad2ust the "( link voltage in a closed-loop fashion to minimise

the error. This is necessary to maintain constant power because the resistance of theworkpiece changes considerably as it heats up. )This argument for closed-loop power

control also applies to all of the methods that follow below.+

%. "aryin# the duty ratio of the devices in the inverter.

The power processed by the inverter can be decreased by reducing the on-time of the

switches in the inverter. 4ower is only sourced to the work coil in the time that the

devices are switched on. The load current is then left to freewheel through the devices

body diodes during the deadtime when both devices are turned off. @arying the duty

ratio of the switches allows full control of the power from &D to &&D. owever, a

significant drawback of this method is the commutation of heavy currents between

active devices and their free-wheel diodes. Forced reverse recovery of the free-wheel

diodes that can occur when the duty ratio is considerably reduced. For this reason

duty ratio control is not usually used in high power induction heating inverters.

&. "aryin# the operatin# frequency of the inverter.


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The power supplied by the inverter to the work coil can be reduced by detuning the

inverter from the natural resonant frequency of the tank circuit incorporating the work

coil. !s the operating frequency of the inverter is moved away from the resonant

frquency of the tank circuit, there is less resonant rise in the tank circuit, and the

current in the work coil diminishes. Therefore less circulating current is induced into

the workpiece and the heating effect is reduced.

In order to reduce the power throughput the inverter is normally detuned on the high

side of the tank circuits natural resonant frequency. This causes the inductive

reactance at the input of the matching circuit to become increasingly dominant as the

frequency increases. Therefore the current drawn from the inverter by the matching

network starts to lag in phase and diminish in amplitude. #oth of these factors

contribute to a reduction in the real power throughput. In addition to this the lagging

power factor ensures that the devices in the inverter still turn on with 0ero voltage

across them, and there are no free-wheel diode recovery problems. )This can be

contrasted with the situation that would occur if the inverter were detuned on the low

side of the work coilEs resonant frequency. ?@S is lost, and the free-wheel diodes seeforced reverse-recovery whilst carrying significant load current.+

This method of controlling power level by detuning is very simple since most

induction heaters already have control over the operating frequency of the inverter in

order to cater for different workpieces and work coils. The downside is that it only

provides a limited range of control, as there is a limit to how fast power

semiconductors can be made to switch. This is particularly true in high power

applications where the devices may already be running close to ma$imum switching

speeds. igh power systems using this power control method require a detailed

thermal analysis of the results of switching losses at different power levels to ensure

device temperatures always stay within tolerable limits.

For more detailed information about power control by detunin# see the new

section below labelled LCLR network frequency response.

'. "aryin# the value of the inductor in the matchin# network.

The power supplied by the inverter to the work coil can be varied by altering the value

of the matching network components. The


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higher load impedance to be presented to the inverter. This lighter load results in a

lower power flow from the inverter to the work coil.

The degree of power control achieveable by altering the matching inductor is

moderate. There is a also a shift in the resonant frequency of the overall system - This

is the price to pay for combining the


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controlling power throughput by ad2usting the phase shift between the two bridge

legs.

*hen both bridge legs switch e$actly in phase, they both output the same voltage.

This means there is no voltage across the work coil arrangement and no current flows

through the work coil. (onversely, when both bridge legs switch in anti-phasema$imum current flows through the work coil and ma$imum heating is achieved.

4ower levels between &D and &&D can be achieved by varying the phase shift of the

drive to one half of the bridge between & degrees and & degrees when compared to

the drive of the other bridge leg.

This technique is highly effective as power control can be achieved at the lower

power control side. The power factor seen by the inverter always remains good

because the inverter is not detuned from the resonant frequency of the work coil,

therefore reactive current flow through free-wheeling diodes is minimised.

Induction Heating Capacitors

The requirements for capacitors used in high power induction heating are perhaps the

most demanding of any type of capacitor. The capacitor bank used in the tank circuit

of an induction heater must carry the full current that flows in the work coil for

e$tended periods of time. This current is typically many hundreds of amps at many

tens or hundreds of kilohert0. They are also e$posed to repeated &&D voltage

reversal at this same frequency and see the full voltage developed across the work

coil. The high operating frequency causes significant losses due to dielectric heatingand due to skin effect in the conductors. Finally stray inductance must be kept to an

absolute minimum so that the capacitor appears as a lumped circuit element compared

to the reasonably low inductance of the work coil it is connected to.

(orrect choice of dielectrics and e$tended foil construction techniques are used to

minimise the amount of heat generated and keep effective-series-inductance to a

minimum. owever, even with these techniques Induction heating capacitors still

e$hibit significant power dissipation due to the enormous 1F currents they must

carry. Therefore an important factor in their design is allowing the effective removal

of heat from within the capacitor to e$tend the life of the dielectric.

8ote the large surface area of the connection plates on the (elem conduction-cooled

components and the reactive power rating )G@!1+ printed on the rating label. igher

power units pictured above in aluminium cases have connections for water cooling

hoses to remove the heat generated internally.

LCL network frequenc! response

The


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voltages and currents within the network change as the drive frequency is altered. The

:1778 traces represent the current passing through the matching inductor, and

therefore the load current seen by the inverter. The 17" traces represent the voltage

across the tank capacitor, which is the same as the voltage across the induction

heating work coil. The top graph shows the !( magnitudes of these two quantities,

whilst the bottom graph shows the relative phase of the signals relative to the !(output voltage from the inverter.

From the amplitude part of the bode plot it can be seen that ma$imum voltage is

developed across the work coil )top red trace+ at one frequency only. !t this

frequency current through the work coil is also ma$imum and the largest heating

effect is developed at this frequency. It can be seen that this frequency corresponds to

the ma$imum load current drawn from the inverter )top green trace.+ It is worth

noting that the magnitude of the inverter load current has a null at a frequency only

slightly lower than that which gives ma$imum heating. This plot shows the

importance of accurate tuning in an induction heating application. For a high H


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system these two frequencies are very close together. The difference between

ma$imum power and minimum power can be only a few kilohert0.

From the bottom graph we can see that for frequencies below the ma$imum power

point, the work coil voltage )green+ is in-phase with the output voltage from the

inverter. !s the operating frequency increases the phase angle of the work coil voltagechanges abruptly through & degrees )phase inversion+ right at the point where

ma$imum power is being processed. The phase angle of the work coil voltage then

remains shifted by & degrees from the inverter output voltage for all frequencies

above the ma$imum power point.

From the bottom graph we can also see that the load current from the inverter e$hibits

not one but two abrupt phase changes as the operating frequency is progressively

increased. Inverter load current initially lags the inverterEs output voltage by &

degrees at low frequencies.


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The bode plot above shows the area of interest around the null frequency and the

ma$imum power point in more detail. It also shows a family of curves depicting the

behaviour of the induction heating tank circuit with a variety of different workpieces

present. This allows us to get a feel for how the network behaves with a large lossy

workpiece to having no workpiece present at all, and all loads in between.

*ith no workpiece installed, losses are low and H factor is high. This gives rise to the

sharply peaking currents and voltages in the top graph, and the abruptly changing

phase shifts in the bottom graph. !s a lossy workpiece is introduced the overall H

factor of the


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workpiece present.Therefore the current delivered from the inverter should be

decreased to prevent the work coil and tank capacitor currents sky-rocketing in the

absence of any significant loss in the system. Secondly, the inverter load current with

no load must be tracked very accurately if the inverter is not to see either a leading or

lagging load current because it slews so quickly through 0ero degrees.

(onversely we can say that with a lar#e lossy workpiece present- there will be less

resonant rise inherent in the LCLR arran#ement and the inverter will have to

supply more load current in order to achieve the required level of current in the

work coil.owever, the control electronics now do not need to track the resonant

frequency so closely since the diminished H gives a load current that shifts phase in a

more leisurely manner.

Finally a number of points are worthy of consideration from the plot above when

considering an automatic control stratergy to track the resonant frequency of an


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The white vertical line indicates the frequency at which the tank capacitor voltage

)and also the work coil voltage+ lag the inverter output voltage by & degrees. This is

also the point where ma$imum voltage is developed across the work coil and

ma$imum current flows through it. The white line is where you want to be to develop

the ma$imum possible heating effect in the workpiece. If we look at the inverter loadcurrent phase )bottom green plot+ we can see that this is always between & degrees

and -& degrees when it crosses the white line no matter how abruptly or slowly it

slews. This means that the inverter always sees a load current that is either in-phase or

at worst slightly lagging in power factor. Such a situation is ideal for supporting ?@S

soft-switching in the inverter and preventing free-wheel diode reverse-recovery

problems.


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shaded region the ideal place to operate in order to achieve control over induction

heating power. #y detuning the inverter drive frequency on the high-side of the

ma$imum power point, power throughput can be reduced and the inverter always sees

a lagging power factor.

(onversely, to the left of the white line we have a band of frequencies labelled3(apacitive


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Heating pictures


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Wa"eforms

This shows the inverter output current waveform when driving the


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This shows the inverter output current waveform when driving the


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This shows the output voltage from the inverter when it is mistuned to a frequency

that is below the natural resonant frequency of the work coil. 8otice the very fast rise

and fall times of the squarewave accompanied by e$cessive voltage overshoot and

ringing. These are all attributed to forced reverse-recovery of the 65SF7T body

diodes whilst enduring this undesirable operating mode. )5vershoot and ringing is due

to reverse recovery current spikes shock-e$citing stray inductance in the inverter

layout into parasitic oscillation.+

This shows the output voltage from the inverter when it is tuned very slightly above

the natural resonant frequency of the work coil. 8otice that the rise and fall times of

the squarewave are more controlled, and there is comparatively little overshoot or

ringing. This is due to the ?ero @oltage Switching )?@S+ which takes place when theinverter runs in this favourable operating mode.


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This shows the output voltage from the inverter when it is tuned precisely to the

resonant frequency of the work coil. !lthough this situation actually achieves

ma$imum power throughput, it does not quite achieve ?ero @oltage Switching of the

65SF7Ts. 8otice the little notches on the rising and falling edges of the voltage

waveform. These occur because the mid-point of the bridge leg has not been fully

commutated to the opposite supply rail during the dead-time before the ne$t 65SF7T

turns on. In practice a small amount of inductive reactance presented to the inverter

helps provide the required commutating current and achieve ?@S. For this reason the

situation described for the previous photograph is preferable to being precisely intune.

basic induction heat

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