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Induction heater results

Posted by fdavies 
Induction heater results
January 02, 2009 02:12PM
Inspired by the thread (http://forums.reprap.org/read.php?70,8467) in which Triffid_hunter suggested using induction heating in the extruder, as well as an incident involving the use of a steel washer in place of a ferrite core, I have built a prototype induction heater.

I briefly experimented with using a single MOSFET switch to keep the circuit as simple as possible. However, the inductive part of the heater needed a diode and load resistor to dump energy into every switch cycle, and the results were not very satisfactory.

I have since built a heater with totem pole MOSFETs (half bridge). This automatically takes care of inductive energy return and provides a lot more power.
I have used it to heat an M6 threaded rod as well as a chrome plated acorn nut, both shown in the attached picture.

The heating coil on the M6 rod consists of 25 turns of 24AWG magnet wire. 13 turns are would into the threads with the remaining 12 wound on top of the first layer. This heats a 6.0 gram M6 rod with a 3mm axial hole (cut from a steel bolt) from 24C to 126C in 30 seconds.

The heating coil for the acorn nut consists of about 15 turns of 24AWG wound into an unsupported circle with an inner diameter of about 15 mm and an outer diameter of about 20 mm. This heats a 9.1 gram steel acorn nut from 29C to 68C (delta T of 39C) in 30 seconds.

I chose 20 KHz to bring up the total power consumption to about 12W as determined by experiment.

I tried a coil wrapped around a brass tube, but heating non-ferrous (non-magnetic) materials requires higher currents. This means that the heating coil gets very hot which means that the situation is not much different from the Nichrome wire heater, but with a lot more complicated circuitry.

So, is this better than using a nichrome heater? Not obviously so. The circuit necessary to drive the coil is fairly complex, and may outweigh the simplicity of heater coil design. I can only speculate about reliability.

Frank Davies

Edited 1 time(s). Last edit at 01/02/2009 02:15PM by fdavies.
open | download - DSC02519.JPG (132.5 KB)
open | download - ind_01.pdf (12.6 KB)
Re: Induction heater results
January 02, 2009 03:04PM
Hmmm, nophead is experimenting with smaller heat zones.

If you used this with a ceramic feed tube and a metal nozzle you couldn
Re: Induction heater results
January 04, 2009 04:07AM
Interesting! This nicely removes the issue of electrically isolating the heating element from the barrel. We already depend upon the PWM driver to operate the heater, and in practice, this circuit might not be that much worse (although less adaptable to other uses). After all, you only have to build it once. If the frequency determines the energy output (and thus the rate of temperature increase), how easily can it be adjusted?

How high a temperature can you get things up to, given plenty of time?

Edited 1 time(s). Last edit at 01/04/2009 04:10AM by Ralith.
Re: Induction heater results
January 04, 2009 07:06AM
The first limit on how hot it can get is the insulation on the magnet wire. Mine begins to smoke after a while. Given wire with high temperature insulation, eventually either the copper might melt or the steel would change its magnetic properties when it heats to above its Curie temperature. Of course, in practice the hotter it gets, the faster heat flows away from it by conduction, convection and radiation.

Frank Davies
Re: Induction heater results
January 04, 2009 07:42AM
I imagine one could use uninsulated magnet wire, wrapped (carefully) around a furnace cement (or similar) coated barrel to circumvent the first issue, while drastically reducing losses thanks to the thermal insulation offered by the cement. I'm not sure what steel's Curie temperature is, but judging by your comparison of it to the melting point of copper I imagine it's plenty high. Specifically, I'm still wondering if your setup can currently (or with few changes, such as the removal of insulation) compare to the ceiling temperatures enabled by nichrome; if so, it seems like a great alternative. If your board's not too expensive, and further tests produce good results, perhaps you could even propose it for distribution through the RRRF.
Re: Induction heater results
January 04, 2009 11:07AM
You mentioned it doesn't work too well on brass, how about a steel nut on top of brass.

Isn't induction heating a skin effect?

Maybe stack various width washers for more skin surface?
Re: Induction heater results
January 04, 2009 12:40PM
I don't think it works like that, you would still have a low impedance turn of brass effectively short circuiting the steel unless you had steel on the inside as well.

To get non ferrous metals to work you can increase the frequency. That reduces the skin depth where the current flows, so the impedance is higher. Of course that would affect the primary as well so the wire would need to be thick. I think litz wire is often used. [en.wikipedia.org]

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