老外自制的中频感应加热装置

DIY中频加热炉

Introduction

Induction heating is a non-contact heating process. It uses highfrequency electricity to heat materials that are electricallyconductive. Since it is non-contact, the heating process does notcontaminate the material being heated. It is also very efficient sincethe heat is actually generated inside the workpiece. This can becontrasted with other heating methods where heat is generated in aflame or heating element, which is then applied to the workpiece. Forthese reasons Induction Heating lends itself to some uniqueapplications in industry.

How does Induction Heating work ?

A source of high frequency electricity is used to drive a largealternating current through a coil. This coil is known as the workcoil. See the picture opposite.

The passage of current through this coil generates a very intenseand rapidly changing magnetic field in the space within the work coil.The workpiece to be heated is placed within this intense alternatingmagnetic field.

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

The alternating magnetic field induces a current flow in theconductive workpiece. The arrangement of the work coil and theworkpiece can be thought of as an electrical transformer. The work coilis like the primary where electrical energy is fed in, and theworkpiece is like a single turn secondary that is short-circuited. Thiscauses tremendous currents to flow through the workpiece. These areknown as eddy currents.

In addition to this, the high frequency used in induction heatingapplications gives rise to a phenomenon called skin effect. This skineffect forces the alternating current to flow in a thin layer towardsthe surface of the workpiece. The skin effect increases the effectiveresistance of the metal to the passage of the large current. Thereforeit greatly increases the heating effect caused by the current inducedin the workpiece.

(Although the heating due to eddy currents is desirable inthis application, it is interesting to note that transformermanufacturers go to great lengths to avoid this phenomenon in theirtransformers. Laminated transformer cores, powdered iron cores andferrites are all used to prevent eddy currents from flowing insidetransformer cores. Inside a transformer the passage of eddy currents ishighly undesirable because it causes heating of the magnetic core andrepresents power that is wasted.)

And for Ferrous metals ?

For ferrous metals like iron and some types of steel, there is anadditional heating mechanism that takes place at the same time as theeddy currents mentioned above. The intense alternating magnetic fieldinside the work coil repeatedly magnetises and de-magnetises the ironcrystals. This rapid flipping of the magnetic domains causesconsiderable friction and heating inside the material. Heating due tothis mechanism is known as Hysteresis loss, and is greatest formaterials that have a large area inside their B-H curve. This can be alarge contributing factor to the heat generated during inductionheating, but only takes place inside ferrous materials. For this reasonferrous materials lend themselves more easily to heating by inductionthan non-ferrous materials.

It is interesting to note that steel looses its magnetic materialswhen heated above approximately 700°C. This temperature is known as theCurie temperature. This means that above 700°C there can be no heatingof the material due to hysteresis losses. Any further heating of thematerial must be due to induced eddy currents alone. This makes heatingsteel above 700°C more of a challenge for the induction heatingsystems. The fact that copper and Aluminium are both non-magnetic andvery good electrical conductors, can also make these materials achallenge to heat efficiently. (We will see that the best course ofaction for these materials is to up the frequency to exaggerate lossesdue to the skin effect.)

What is Induction Heating used for ?

Induction heating can be used for any application where we want toheat an electrically conductive material in a clean, efficient andcontrolled manner.

One of the most common applications is for sealing the anti-tamperseals that are stuck to the top of medicine and drinks bottles. A foilseal coated with "hot-melt glue" is inserted into the plastic cap andscrewed onto the top of each bottle during manufacture. These foilseals are then rapidly heated as the bottles pass under an inductionheater on the production line. The heat generated melts the glue andseals the foil onto the top of the bottle. When the cap is removed, thefoil remains providing an airtight seal and preventing any tampering orcontamination of the bottle's contents until the customer pierces thefoil.

Another common application is "getter firing" to removecontamination from evacuated tubes such as TV picture tubes, vacuumtubes, and various gas discharge lamps. A ring of conductive materialcalled a "getter" is placed inside the evacuated glass vessel. Sinceinduction heating is a non-contact process it can be used to heat thegetter that is already sealed inside a vessel. An induction work coilis located close to the getter on the outside of the vacuum tube andthe AC source is turned on. Within seconds of starting the inductionheater, the getter is heated white hot, and chemicals in its coatingreact with any gasses in the vacuum. The result is that the getterabsorbs any last remaining traces of gas inside the vacuum tube andincreases the purity of the vacuum.

Yet another common application for induction heating is a processcalled Zone purification used in the semiconductor manufacturingindustry. This is a process in which silicon is purified by means of amoving zone of molten material. An Internet Search is sure to turn upmore 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 commonapplications because the induction process heats the surface of themetal very rapidly. Therefore it can be used for surface hardening, andhardening of localised areas of metallic parts by "outrunning" thethermal conduction of heat deeper into the part or to surroundingareas. The non contact nature of induction heating also means that itcan be used to sterilise metal instruments by heating them to hightemperatures whilst they are already sealed inside a known sterileenvironment in order to kill germs.

What is required for Induction Heating ?

In theory only 3 things are essential to implement induction heating:

1. A source of High Frequency electrical power,

2. A work coil to generate the alternating magnetic field,

3. An electrically conductive workpiece to be heated,

Having said this, practical induction heating systems are usually alittle more complex. For example, a matching network is often requiredbetween the High Frequency source and the work coil in order to getgood power transfer. Water cooling systems are also common in highpower induction heaters to remove waste heat from the work coil and itsmatching network. Finally some control electronics is usually employedto control the intensity of the heating action, ensure consistentresults, and to protect the system from adverse operating conditions.However, the basic principle of operation of any induction heaterremains the same as described earlier.

Practical implementation

In practice the work coil is usually incorporated into a resonanttank circuit. This has a number of advantages. Firstly, it makes eitherthe current or the voltage become sinusoidal. This minimises losses inthe inverter by allowing it to benefit from eitherzero-voltage-switching or zero-current-switching depending on the exactarrangement chosen. The sinusoidal waveform at the work coil alsorepresents a more pure signal and causes less Radio FrequencyInterference to nearby equipment. We will see that there are a numberof 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 operatingfrequency by means of a capacitor placed in series with it. This causesthe current through the work coil to be sinusoidal. The seriesresonance also magnifies the voltage across the work coil, far higherthan the output voltage of the inverter alone. The inverter sees asinusoidal load current but it must carry the full current that flowsin the work coil. For this reason the work coil often consists of manyturns of wire with only a few amps or tens of amps flowing.

This arrangement is commonly used in things like rice cookers wherethe power level is low, and the inverter is located next to the objectto be heated. The main drawbacks of the series resonant arrangement arethat the inverter must carry the same current that flows in the workcoil. In addition to this the series resonant action can become verypronounced if there is no workpiece present

to damp the circuit. Thisis not a problem in applications like rice cookers where the workpieceis always the same cooking vessel, and its properties are well known atthe time of design.

The tank capacitor is typically rated for a high voltage because ofthe resonant voltage rise experienced in the series tuned resonantcircuit.

Parallel resonant tank circuit

The work coil is made to resonate at the intended operatingfrequency by means of a capacitor placed in parallel with it. Thiscauses the current through the work coil to be sinusoidal. The parallelresonance also magnifies the current through the work coil, far higherthan the current capability of the inverter alone. The inverter sees asinusoidal load current. However, in this case it only has to carry thepart of the load current that actually does real work. The inverterdoes not have to carry the full circulating current in the work coil.This means that the work coil can be placed at a location remote fromthe inverter without incurring massive losses in the feed wires.

It also means that work coils using this technique often consist offew turns of a thick copper conductor but with large currents of manytens or hundreds of amps flowing. (This is necessary to get therequired Ampere turns to do the induction heating.) Water cooling iscommon for all but the smallest of systems. This is needed to removeexcess heat generated by the passage of the large high frequencycurrent through the work coil and its associated tank capacitor.

In the parallel resonant tank circuit the work coil can be thoughtof as an inductive load with a "power factor correction" capacitorinstalled across it. The PFC capacitor provides reactive current flowequal and opposite to the inductive current drawn by the work coil.Therefore the only current flow from the inverter is a small amountrequired to overcome losses in the "PFC" capacitor and the work coil.There is always some loss in this tank circuit due to dielectric lossin the capacitor and skin effect causing resistive losses in the workcoil. Therefore a small current is always drawn from the inverter. Whena lossy workpiece is inserted into the work coil, this damps theparallel resonant circuit by introducing a further loss into thesystem. Therefore the current drawn by the parallel resonant tankcircuit increases when a workpiece is entered into the coil.

Impedance matching

Or simply "Matching". This refers to the electronics that sitsbetween the source of high frequency power and the work coil we areusing for heating. In order to heat a solid piece of metal viainduction heating we need to cause a TREMENDOUS current to flowin the surface of the metal. However this can be contrasted with theinverter that generates the high frequency power. The invertergenerally works better (and the design is somewhat easier) if itoperates at fairly high voltage but a low current. This allows commonswitch mode MOSFETs to be used. The comparatively low currents alsomake the inverter less sensitive to layout and stray inductance. It isthe job of the matching network and the work coil to transform the highvoltage/low current from the inverter to the low voltage/high currentrequired to heat the workpiece efficiently.

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

allel resonant circuit.

This has a resistance (R) due to the lossy workpiece coupled intothe work coil due to the magnetic coupling between the two conductors.

See the schematic opposite.

In practice the resistance of the work coil, the resistance of thetank capacitor, and the resistance of the workpiece all introduce aloss into the tank circuit and damp the resonance. Therefore it isuseful to combine all of these losses into a single "loss resistance."In the case of a parallel resonant circuit this loss resistance appearsdirectly across the tank circuit. This resistance represents the onlycomponent that can consume power, and therefore we can think of thisloss resistance as the load that we are trying to drive power into asefficiently as possible.

When driven at resonance the current drawn by the tank capacitor andthe work coil are equal and opposite in phase and therefore cancel eachother out as far as the source of power is concerned. This meansthat the only load presented to the power source at the resonantfrequency is the loss resistance across the tank circuit. (Notethat, when driven either side of the resonant frequency, there is anadditional "out-of-phase" component to the current caused by incompletecancellation of the work coil current and the tank cap current. Thisreactive current increases the total magnitude of the current beingdrawn from the source but does not contribute to any useful heating inthe workpiece.)

The job of the matching network is simply to transform thisrelatively large loss resistance across the tank circuit down to alower value that better suits the inverter attempting to drive it.There are many different ways to achieve this impedance transformationincluding tapping the work coi

l, using a ferrite transformer, acapacitive divider in place of the tank capacitor, or a matchingcircuit such as an L-match network.

In the case of an L-match network it can transform the relativelyhigh load resistance of the tank circuit down to something around 10ohms which better suits the inverter. This figure allows the inverterto run from several hundred volts whilst keeping currents down to areasonable level so that standard switch-mode MOSFETs can be used toperform the switching operation.

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

The L-match network also has another highly desirable property. TheL-match network provides a progressively rising inductive reactance toall frequencies higher than the resonant frequency of the tank circuit.This is very important when the work coil is to be fed from an inverterthat generates a squarewave voltage output. Here is an explanation ofwhy this is so…

The squarewave voltage generated by most half-bridge and full-bridgecircuits is rich in high frequency harmonics as well as the wantedfundamental frequency. Direct connection of such a voltage source to aparallel resonant circuit would cause excessive currents to flow at theharmonics of the drive frequency! This is because the tank capacitor inthe parallel resonant circuit presents a progressively lower capacitivereactance to increasing frequencies. This is potentially very damagingto a voltage-source inverter. It results in large current spikes at theswitching transitions as the inverter tries to rapidly charge anddischarge the tank capacitor on rising and falling edges of thesquarewave. The inclusion of the L-match network between the inverterand the tank circuit negates this problem. Now the output of theinverter sees the inductive reactance of Lm in the matching networkfirst, and all harmonics see a gradually rising inductive impedance.

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 t

o the workpiece,

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

er safe and happy.

Looking at the previous schematic above we can see that thecapacitor in the matching network (Cm) and the tank capacitor (Cw) areboth in parallel. In practice both of these functions are usuallyaccomplished by a single capacitor. Most of its capacitance can bethought of as being in parallel resonance with the work coil, with asmall amount providing the impedance matching action with the matchinginductor (Lm.) Combing these two capacitances into one leads us toarrive at the LCLR model for the work coil arrangement, which iscommonly used in industry.

The LCLR work coil

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 LCLR 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 close

to each other.

2. Only low current flows along the transmission line from the inverter to the tank circuit, s

o this can use lighter duty cable.

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

ce (Lm.)

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 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 conductor. 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 LCLR work coil arrangement if electrical isolation is necessary

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

whether the PSU in the induction heater already provides sufficient

electrical isolation to meet these requirements.

Complete schematic

The complete schematic showing the inverter driving its LCLR work coil arrangement is shown belo

w.

Note that this schematic DOES NOT SHOW the MOSFET gate-drive and control electronics! Please dont ask for further information.

The inverter is a simple half-bridge consisting of two MTW14N50

MOSFETs made my On-semiconductor (formerly Motorola.) It is fed from a

smoothed DC supply with decoupling capacitor across the rails to

support the AC current demands of the inverter. However, it should be

realised that the quality and regulation of the power supply for

induction heating applications is not critical. Full-wave rectified

(un-smoothed) mains can work equally as well as smoothed and regulated

DC when it comes to heating metal. And there are many arguments for

keeping the size of the DC bus capacitor down to a minimum. In

particular it improves the power factor of current drawn from the main

supply, and it also minimises stored energy in case of fault conditions

within the inverter.

The DC blocking capacitor is merely to block the DC output from the

half-bridge inverter from causing current flow through the work coil.

It is sized sufficiently large that it does not take part in the

impedance matching, and does not adversely effect the operation of the

LCLR work coil arrangement.

Fault tolerance

The LCLR work coil arrangement is very well behaved under a variety of possible fault conditions.

1.

2.

3.

4. Open circuit work coil. Short circuit work coil, (or tank capacitor.) Shorted turn in work coil. Open circuit tank capacitor.

All of these failures result in an increase in the impedance being

presented to the inverter and therefore a corresponding drop in the

current drawn from the inverter. The author has personally used a

screwdriver to short-circuit between turns of a work coil carrying

several hundred amps. Despite sparks flying at the location of the

applied short-circuit, the load on the inverter is reduced and the

system survives this treatment with ease.

The worst thing that can happen is that the tank circuit becomes

detuned such that its natural resonant frequency is just above the

operating frequency of the inverter. Since the drive frequency is still

close to resonance there is still significant current flow out of the

inverter. But the power factor is reduced due to the detuning, and the

current begins to lead the voltage. This situation is undesirable

because the load current seen by the inverter changes direction before

the applied voltage changes sign. The outcome of this is that current

is force-commutated between free-wheel diodes and the opposing MOSFET

every time the MOSFET is turned on. This causes a forced reverse

recovery of the free-wheel diodes whilst they are carrying significant

forward current. This results in a large current surge through both the

diode and the opposing MOSFET that is turning on.

Whilst not a problem for special fast recovery rectifiers, this

forced recovery can cause problems if the MOSFETs intrinsic body diodes

are used to provide the free-wheel diode function. However, it should be realised that proper control of the inverter operating frequency should ensure that it tracks the resonant frequency of the tank

circuit. Therefore the leading power factor condition should ideally not arise, and should certainly not persist for any length of time. The resonant frequency should be tracked up to its limit, then the system shut-down if it has wandered outside of an acceptable range.

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:

1. Varying the DC link voltage.

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 DC supply such as a controlled

rectifier using thyristors to vary the DC supply voltage derived from the mains supply. The impedance presented to the inverter is largely constant with varying power level, so the power throughput of the

inverter is roughly proportional to the square of the supply voltage. Varying the DC link voltage allows full control of the power from 0% to 100%.

2. Varying 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. Power is only sourced to the work coil in the time that the devices are switched on. The load

current is left to freewheel through the devices body diodes during the deadtime when both devices are turned off. Varying the duty ratio of the switches allows full control of the power from 0% to 100%. The only significant drawback is forced reverse recovery of the free-wheel diodes that can occur when the duty ratio is reduced.

3. Varying the operating frequency of the inverter.

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. As 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 in the workpiece.

In order to control the power the inverter is normally detuned on the high frequency side of the tank circuits natural resonance. This causes the inductive reactance of the matching circuit to become

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. Both of these factors contribute to a reduction in real power throughput. In addition to this the lagging power factor ensures that the devices in the inverter still turn on with zero voltage across them.

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 be running close to maximum switching speeds anyway.

4. Varying the value of the inductor in the matching network.

The power supplied by the inverter to the work coil can be varied by altering the value of the matching network components. The L-match

network between the inverter and the tank circuit technically consists of an inductive and a capacitive part. But the capacitive part is in parallel with the work coil's own tank capacitor, and in practice these are usually one and the same part. Therefore the only part of the

matching network that is available to make adjustable is the inductor.

The matching network is responsible for transforming the load of the workcoil to a suitable load impedance to be driven by the inverter. Altering the inductance of the matching inductor adjusts the value to which the load impedance is translated. In general, decreasing the inductance of the matching inductor causes the work coil impedance to be transformed down to a lower impedance. This lower load impedance being presented to the inverter causes more power to be sourced from the inverter. Conversely, increasing the inductance of the matching inductor causes a higher load impedance to be presented to the inverter. This lighter load results in a lower power flow.

The degree of power control achieveable by altering the matching inductor is fairly small. There is a also a shift in the resonant

frequency of the overall system. This is the price to pay for combining the L-match capacitance and tank capacitance into one unit. The L-match

network essentially borrows some of the capacitance from the tank capacitor to perform the matching operation, thus leaving the tank

circuit to resonate at a higher frequency. For this reason the matching inductor is usually fixed or adjusted in coarse steps to suit the intended workpiece to be heated, rather than provide the user with a fully adjustable power setting.

Heating pictures

Waveforms