At 04:52 PM 06/06/2013, you wrote:
>On 2013-06-06 12:08 PM, Dwayne Reid wrote:
> > 2) Consider using the copper tube as a shorted 1-turn secondary on a
> > transformer.  In other words, wrap however many turns of insulated
> > wire you need around the pipe and feed with enough high-frequency
> > current as you need to bring the pipe up to the desired temperature.
> >
> > Note that the primary wire does NOT get hot, so space it away from
> > the copper tube if desired.  Better yet, wrap the copper tube with
> > insulation, then wrap the primary coil over top of that insulation.
> >
> > Details left as an exercise for the student <grin>.
>
>I was considering this, and if done right, would be ideal. But it would
>complicate the electronics. With the NiCr wire, I could get away with a
>MOSFET and switch it to control temperature. With the induction heating
>method, I'd need at least an H bridge and potentially complicated PWM
>control to shape the output signal into a sine wave.
>
>Also, how would I go about calculating the inductance, coupling, number
>of turns, frequency, etc...? I'm assuming this is far from an ideal
>transformer and losses will be great.

Sure, since that is where the heating comes from.

If you're interested, have a look at the work of Tim
Williams- he has built rigs that work up into the kW range.

http://www.mindchallenger.com/inductionheater/induction10.html

If you want to buy a resistive heater, something like shown on page 509
here: http://www.watlow.com/products/heaters/cable-nozzle-heaters.cfm
would be good. For even better coupling, buy a wound MgO-insulated
coil and braze it to the outside of a relatively thin (to minimize
the heat capacity) copper tube. Stainless sheath is fine for your
low-temperature application.

Hmm... since you say you want fast heating it seems inevitable that a
failure of the controller would cause the flash point of the wax to be
exceeded. Suggest some kind of reliable safety agency approved thermal
fuse (cutoff) in the circuit if this is a concern.

--sp


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