Saturday, May 09, 2009

 

Induction Heating

For my Final Year Project (I am studying Master of Engineering) I have designed an extruder with induction heating. This technique uses a varying magnetic field generated by the induction coil to heat the surrounded (ferromagnetic) metal. Below a photo of the extruder, the induction coil can be seen left at the end of the extruder pipe.


The thick blue and brown wires are connected to the induction coil, whereas the yellow ones are leading to a thermistor for temperature measurement. A cutaway view of the extruder pipe is shown in the next picture.

The induction coil heats the sleeve made from mild steel (ferromagnetic) whereas the stainless steel is not heated directly because it is not ferromagnetic. The main reason why I have used stainless steel for the pipe is that this metal has low thermal conductivity compared with e.g. mild steel (about 16W/mK versus 59W/mK). This, along with a wall thickness of only 1mm in the middle part of the pipe, increases the temperature gradient from the heated zone to the right part of the pipe. This helps to reduce the 'filament sticking' problem described in the HydraRaptor Blog (thanks to "nophead" for his work). I have measured the temperature inside the pipe at different distances from the nozzle, the result can be seen in the following diagram.

The higher the temperature the steeper the temperature gradient at distances > 2cm (end of heated zone). This can be explained by the higher difference to the ambient temperature at high nozzle temperatures.

At the next photo the used electronics for induction heating is shown. The top PCB is the power stage circuit, connected below (the green PCB) is a "44-Pin Demo Board" (with PIC16F887) from Microchip which was handy. It is used for driving the FET (see picture centre) of the power stage and to control the heating. It also controls the DC-Motor used for filament feeding. The two potentiometers on the lowest board are for setting the extruder temperature and filament speed.


Comments:
Hi Stefan, nice diagram, just to clarify, the stainless steel had 1mm walls and a 3mm diameter hole?

Also can you put up the schematic?

Cheers
From Ian McCrum (Stefan's supervisor - University of Ulster)
 
I tried something similar and had problems with the copper wire heating (the magnet wire insulation began to smoke). What frequency are you operating at?

Frank Davies
 
I second that request for a schematic.

Also, does this circuit put out appreciable amounts of interference?
 
Wow cool toys. Well done Stefan.

Hmm the former might benefit from being made (turned) out of PEEK.

Although to get your figures Stefan you must have used something heat resistant and non conductive.

What did you use ??

It would be quite something to have an extruder heater that was almost completely insulated but could be still heated.

Cheers

aka47
 
How do you measure the temperature? using a thermistor? or is it set by the pot?
 
And, how well did it operate?
 
Wouldnt it be better to put the mild steel hot bit inside the tube? It would then directly heat the filament and eliminate losses through the stainless.
 
Take a look at the no-lathe heater & nozzle on reprap.blogspot.com

This used a thin brass barrel and an iron nozzle. Might be a good recipient for the inductive heating concept?

Vik :v)
 
To Ian:
The wall is 1mm and the hole has (to be exact) 3.5mm. I will put up the schematic.

To Frank Davies:
I am operating at about 30kHz, currently I am using three twisted copper wires parallel (each 0.56mm diameter) for the induction coil. So it's enough for the high peak current flowing (up to 11A).

To Forrest Higgs:
Up to now I haven't done tests regarding possible interferences of the circuit. This has to be examined further. At least it doesn't interfere with a radio (FM) that is close to the circuit.

To be continued ...
Stefan
 
Interesting work.
I have a question: Doesn't inductive heating work on any (electrically) conductive material (and thus on the stainless steel also), whether it's ferro-magnetic or not? It may deposit more heat in ferromagnetic material than in a non-ferrous one, but shouldn't it still heat any conductor, via eddy current losses? Please straighten my out if I'm misunderstanding.

Thanks,

Larry
 
To aka47:
Thanks for the compliment. The former is made from laminate (phenol resin with cotton cloth, I don't know the English name for it). The material is only specified for temperatures up to 110°C but for my first tests I have heated it to 200°C and for a short time to 250°C. But than it begins to smell and to smoke. So it is not intended to be used for the final version of the extruder. A possibility would be to use fibreglass cloth as insulating material between the former an the heated extruder pipe. Indeed this is one of the advantages of induction heating: The extruder can be heated from "outside", so a complete insulation is no problem.

To Renoir:
I am using a glass sealed thermistor that goes up to 300°C. It is very small (about 2.3mm diameter), so I was able to place it under the former in a hollow of the extrusion pipe. The desired temperature is set by the potentiometer, a closed loop controller is implemented in the PIC to control the temperature.

To Dave:
The induction heating is working very well. As mentioned above I have already reached 250°C. Even higher temperatures would be no problem when using a insulation between the former and the heated pipe.

To Lampbus:
That's a good idea. Theoretically the stainless steel doesn't influence the magnetic field, so it probably works if I put the mild steel inside the pipe. Perhaps the smaller diameter is a problem, because then the magnetic flux through the mild steel is also smaller and therefore the heating is less. But if I have time I will have a try!

To Vic Olliver:
Interesting way to make an extruder when no lathe is available. Because brass is not ferromagnetic (as well as stainless steel) but the nozzle is, induction heating would work. A disadvantage is the high thermal conduction of brass.
 
I once did some uncontrolled induction heating of small items, just for fun. The system I used was to simply wind a coil of enough turns that no ferrite material was needed, and attach it to a 12V 50W halogen electronic transformer. They have an AC output of around 20KHz at 50W. No control, but lots of power, very cheap and everything is available over the counter. Also If you want to have some ferrite to increase the magnetic field, what about using a large toroid around the outside of the coil? that way you can heat the stainless directly.
 
What material did you use for the coil body?
 
To Larry_P:
You are right, in principle induction heating is also working on non-ferromagnetic metals. But induction heating is working far better with ferromagnetic materials. This is mainly caused by the skin effect, i.e. at high frequencies the eddy currents are flowing only at the surface of the heated material. The skin depth (the thickness of the layer were the currents are mainly flowing) decreases with the permeability of the material. Ferromagnetic materials have a high permeability, this means the skin depth is low compared with non-ferromagnetic materials. This leads to a higher resistance "seen" by the eddy currents. Therefore more power (heat) is generated inside the material (P=I²R). Moreover the resistivity of mild steel is innately higher compared with other commonly used metals like copper or brass. Another effect which contributes to the heat generated (amount < 10%) is the magnetic hysteresis of ferromagnetic material. Heat is generated here because the remagnetization needs energy.

To Murray:
Good idea, using an electronic transformer. But there are, as you already said, disadvantages:
- Amount of heat generated not easy to control
- You need a lot of turns to increase the impedance of the coil
- I don't know what is happening to the transformer when high voltages are generated through self-induction, maybe it will be destroyed

To xsainnz:
I am using a laminate material (further details see the answer I gave 'aka47').
 
Ah yes, sorry missed that.
 
Very interesting work!

Is the schematic of the power supply available somewhere?

Thanks,

Frédéric
 
For those of you that are interested a good source of information unrelated to this project but has very similar overlap is a project to build a driver for inductive soldering iron tips. It has alot of cool additions. Temp sensing (using RF reflective feedback). Temp ramping and idling. Here is the link: http://www.eevblog.com/forum/projects/diy-metcal-13-56-mhz-rf-supply/

Hope this info helps!

Daniel Roseman
 
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