Coil wire - maximum current? 1

[RayJohnson2]

Hi all,

I am designing an electromagnet, a coil, with roughly the following specifications:
- diameter about 30 mm, length about 40 to 50 mm
- maximum field strength: 50 mT (500 Gauss)
- AC, 0 to 330 Hz
- environment temperature: 150°C
- core: mu-metal
- winding: copper enamelled wire, diameter around 0.25 mm (SWG 33, AWG 30)
The biggest question I have is: what value could I use as ampacity? What is the maximum current that I can use on this wire, at this temperature?
I have found tables, listing max. currents at high temperatures, but that was for power cables. Not for small diameter wires like the one I mentioned.
Can anyone point me to data or tables?

Another question:
I usually use a coil winding fill percentage around 65% (Meaning 65% of the coils cross section is copper, 35% is not).
Does that seem realistic to you?

Kind regards

 

[MagBen]

That was a good question! with big gaps in the magnetic circuit, you are not taking advantage of high permeability of mu metal.

 

[EdStainless]

And you can both eliminate the 'back' air gap and perhaps add coil winding there as well, that would reduce the winding depth on all coils and help with heat dissipation. I have seen coils wound on thin Ti spools (with large, thin end flanges) to help with cooling.

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P.E. Metallurgy, Plymouth Tube

 

[RayJohnson2]

The gap below is due to the fact that there will be 3 coilpairs at an angle of 120°. The picture before the last one is a top view and this shows the principle.
So, at the bottom in the center, 6 cores will come together. I could probably reduce the size of the gap. If I use laminated mu metal, I could make the cores more or less V-shaped where they come together to reduce the gap.

Good comments, all. Thanks!