DIY中频加热炉
Introduction
Induction heating is a non-contact heating process. It uses highfrequency electricity to heat mat
erials that are electricallyconductive. Since it is non-contact, the heating process does notcont
aminate 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 He
ating 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 fi
eld in the space within the work coil.The workpiece to be heated is placed within this intense al
ternatingmagnetic 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 coili
s like the primary where electrical energy is fed in, and theworkpiece is like a single turn seco
ndary that is short-circuited. Thiscauses tremendous currents to flow through the workpiece. Thes
e areknown as eddy currents.
In addition to this, the high frequency used in induction heatingapplications gives rise to a phe
nomenon called skin effect. This skineffect forces the alternating current to flow in a thin laye
r towardsthe surface of the workpiece. The skin effect increases the effectiveresistance of the m
etal 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 theirtransform
ers. Laminated transformer cores, powdered iron cores andferrites are all used to prevent eddy cu
rrents from flowing insidetransformer cores. Inside a transformer the passage of eddy currents is
highly undesirable because it causes heating of the magnetic core andrepresents power that is was
ted.)
And for Ferrous metals ?
For ferrous metals like iron and some types of steel, there is anadditional heating mechanism tha
t takes place at the same time as theeddy currents mentioned above. The intense alternating magne
tic fieldinside the work coil repeatedly magnetises and de-magnetises the ironcrystals. This rapi
d flipping of the magnetic domains causesconsiderable friction and heating inside the material. H
eating 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 ma
terials 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 ca
n 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 f
or 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 wi
ll see that the best course ofaction for these materials is to up the frequency to exaggerate los
sesdue 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 plas
tic cap andscrewed onto the top of each bottle during manufacture. These foilseals are then rapid
ly 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 re
mains providing an airtight seal and preventing any tampering orcontamination of the bottle's con
tents 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 materialcal
led a "getter" is placed inside the evacuated glass vessel. Sinceinduction heating is a non-conta
ct 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 tur
ned on. Within seconds of starting the inductionheater, the getter is heated white hot, and chemi
cals 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 pr
ocess that I know little about.
Other applications include melting, welding and brazing or metals.Induction cooking hobs and rice
