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Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Hi guys,
I need to heat up small aluminum disks (D = 6mm - 9mm, T = 1.5mm)to about 400°C within 4sec or less. I have only access to the disks from below, not from the side. It comes to mind that induction heating with a pancake coil might work.
However, i have serious trouble getting started in this subject. I understand the basics but lack the experience. As i am trying to heat up a non-ferrous material, eddy currents are all i can work with. I need to deliver 92J to the aluminums heat capacity and compensate for convective cooling during the heating process. I guess i need 40W-80W of power at the spot.
Due to size constraints, i understand that i need to go into RF-territory (>10MHz). The frequency sets the skin depth and therefore the sheet resistance of the aluminum and therefore the power transfer. For such high frequencies, the simplest topology to go with is some self-oscillating class-E circuit. However class-E is a bit bitchy about load resistance. This is my main road block right now - i have no idea which resistance to assume.
Other topologies, such as half-bridges seem very lossy at high frequency. I simply find no suitable circuit to even start with that promises a predictable result.
Happy to hear any suggestions how to tackle the problem
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Is there any reason why you can't use a ferrite core, e.g. wrap a drive coil around a 0.5" diameter ferrite rod and then put the end of the rod underneath, as close to the bottom of the disk as you can get it? I didn't do any calculations, but this approach might get you down into a lower frequency range and make it easier to couple power into the disk.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Nice idea with the ferrite. Need to mind the power lost in ferrite, not from eddy currents but from hysteresis. I don't know if ordinary induction heating gets to field strengths where ferrite would saturate.
Have you got an estimate of the sheet resistance and ampere turns to get 40 watts of heating in aluminum workpieces? It might help that common engineering alloys are more resistive than pure Al by factors of 1.5 to 2.0, though the benefit gets square-rooted because skin depth is greater. Don't forget the increase in rho as temperature approaches 400 °C.
Depending on reflectivity of the Al surface, you might be able to use radiant heat. Not necessarily a laser, I am thinking of projector lamp with compact incandescent filament. Then traditional condenser optics with a small f/ ratio to form image of filament (1:1 scale or smaller) on workpiece to be heated.
How about a pointy little torch flame? Or intense glow discharge/ arc between refractory electrodes and the work?
Stream of hot air, as from a heat gun? Bath of molten metal or high temperature oil?
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Instead of a pancake coil, why not consider a strip wound coil? (i.e. copper strip and insulator rolled together with your 'load' placed on one of the ends with an insulating disk underneath it) This will help shape the magnetic field in your favour, and should allow you to get the cirulating currents high enough so that the load is negligible. With a coil diameter of say 10mm and a length of 20mm you might get around 100nH for 2.5nF/10MHz.
Just thinking out loud but that's how I'd approach it.
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RF-Induction Heater - having trouble getting started
