Since Lidl have got 2kW induction hobs on offer for £30, What are the chances
that one could be used or modified do something useful? Possibly a new coil
could be reverse engineered and any interlocks defeated.
I'm feeling the need to go shopping...
Shout Kippers 3 times and the induction master will appear :)
Im interested to know too
... that's my cue ! Enter stage left.
There has been much talk over the past couple of years about using
them on the Yahoo group Electro_cast
But I'm not aware of anyone being sucessful in their use. The most
recent topic is by coincidence this subjuct under 'cook tops' -
presumably the colonials use this expression as their language drifts
away from ours! IIRC the files & links section of that group contains
the circuit diagram or maybe a link to the cct.
I have no experience on this particular application, but I am an ex
Induction Heating engineer.
The idea of getting as much energy into the peice you are working with
is to get it coupled up right. Maximum power transfer occurs when you
have it coupled up properly, such maximum current, and the volts are at
Generally on equipment I have worked are parelle tuned systems, but this
will be series tuned system, which means there is little/no reactive
current flowing, but there large reactive voltages appearing across the
inductance (otherwise known as the work coil), and the tank capacitor.
This means that you don't have to cool the inductor and the capacitor.
In parallell tunder circuits the reactive current is roughly input
current times to Q factor. Q is known as Xl/R. Similarly in a series
syste, reactive volts are roughly Vin times the Q factor. You normally
aim for a Q value of about 10 for various reasons.
So the point of what I am saying is that the coupling factor is crucial
to getting power into your work peice. As a guide, this works far
better with steel, and iron workpeices for obvious reasons, ie a magnet
will stick to it. Trying to get heat into an aluminium peice is much
more difficult. It's not impossible but you have to go for a higher Q
value in your work coil.
Another point is what is the frequency of operation? If you can hear it
then it's below about 10kHz. If it's higher, then it may be difficult
to get heat into large peices. If you are heating something large then
you will require a lower operating freuquency because of skin effect.
Has this helped? Ask me some questions!
No, the principle used there is the curie point of the metal - when it
ceases to be ferro magnetic the sensor coil no longer couples to the
drive coil by transformer action.
Yes that it true, when you get to Curie point, it is more difficult to
get any more energy into the piece. I have known systems whereby at
curie point you have a contactor that changes over so that the Q of the
coil is doubled, so you can put more ergs into the peice, but this is
difficult and technically can be unreliable because you are switching
I'm not sure your statement is correct about trnaformer action though.
When it's below curie, the heating is more to do with eddy currents and
molecular agitation in the peice and. When you're past curie, you *are*
using transformer action, as you are inducing ebergy into the peice and
basically driving a short circuit secondary.
I am now confused, the Wild Barfield induction heat treatment machines I
used to work on had a copper water cooled coil which the component, in this
case a diesel fuel injection nozzle, sat above but did not touch. In the
magic box sat a Mercury rectifryer which did its thing and the component
which was approx 3/4" in dia glowed red hot. What's the difference in plain
All is in the context! The Wild Barfield heat treatment furnace uses
resistive heating elements (not induction) controlled by a system
detecting the critical temperature by the curie point of the metal
My understanding maybe incorrect but in your last posting you appear to be
describing an actual furnace with heating elements embedded into brickwork.
The machine I am trying to describe had a single ring coil that was actually
cold unless a piece of metal was put in close proximity. I also remember a
sign not allowing people with pacemakers within a set distance because of
frequency generation playing hell with the pacemaker.
"> "Andrew Mawson" wrote in
No doubt W B made (make?) a range of kit - I'm describing their
classic heat treatment furnace
If it's a Water Cooled work coil, then that says that it was a parallel
resonant coil, with the Tank Capacitor in parallel with it. Because of
the reactive current circulating in the tuned circuit, the I^2.R losses
would be enourmous, hence the water cooling. Live water cooled Bus Bars
were common place, and still are.
Reistance furnaces would not have water cooled elements for blatantly
I actually did some work on a point nose heater, which was used for
hardening a point crossing casting.
Billet heaters were two a penny. I did do some interesting stuff with
Gas Turbine disks in that we used a single turn induction heating coil
to expand the shat cup, such that you would separate the disk from the
shaft. They used to use gas torches, and it took them hours to separate
disks from shafts. You would have to do this if the turbine disk had
shown up an anamoloy in the ultrasound test, but the shaft was still
good. Using induction heating the shaft would drop off the disk about a
couple of seconds of applying energy to it from the coil. We won a
technical innovation award for that one they were so pleased with it.
Below Curie point, alternating magnetic flux heats steel
workpieces by the combined effects of eddy currents and
Eddy current loss is the I squared R loss produced by circulating
currents induced into the workpiece resistivity.
Hysteresis loss is the result of the failure of the flux density
in the workpiece to pass through zero as the exciting flux passes
Permanent magnet materials have extremely high hysteresis loss
(they retain most of their magnetism when the magnetising field
is removed). Soft iron has a much lower but still significant
hysteresis loss. Hardened steel is intermediate.
When the Curie point is reached the iron is no longer
ferromagnetic which means that the permeabilty (the factor by
which it increases work coil inductance) drops to one and zero
flux is now coincident with zero exciting flux.
This means that the hysteresis loss disappears and there is a
reduction in work coil inductance. The disappearance of the
hysteresis loss increases the work coil Q and the reduction of
work coil inductance requires an increase in tank capacitance to
return it to its previous resonant condition.
You're making it far too complicated...
Above the Curie point the relative permeability is (to all intents and
purposes) 1. Below it, it is from 500 to 2000 for steels. The leakage
reactance of the work coil stays the same, the mutual reactance between work
coil and work goes to hell and the field through the work drops.
Therefore the induced currents become less because they are a function of the
rate of change of flux and less heat is produced.
Inductance isn't a function of hysteresis loss, it's a function of
permeability. Mumetal will heat better than Alnico!