Primary coil heating

[flyingperson23]

I've been having some problems with my primary coil heating up very quickly. It uses EVR's supports, and I'm running it at 400A OCD and 300us/12% duty cycle max. After a few minutes, the primary is hot enough to soften the polycarbonate and melt the plastic end caps. The aluminum frame also gets fairly warm, though there are no large loops. Is this reasonable/inevitable with those on times or does it sound like something else is going on?
Thanks!

[Mads Barnkob]

400A OCD and 300us/12% duty cycle max. After a few minutes, the primary is hot enough to soften the polycarbonate and melt the plastic end caps. The aluminum frame also gets fairly warm.

400A is fairly low and should cause no problems, 300us/12% duty cycle is pretty high, but again at 400A, I would not consider it a problem.

Aluminium frame heating is not a good sign, you need to fix that, you should not have losses in your frame. Its forming a loop, break it.

Primary coil losses is a product of at least 3 things
1: Losses from the inductance/resistance of the coil itself.
2: proximity heating, the outer turns heating the inner turns, its a self-heating induction heater.
3: bad tuning, will result in higher circulating current that is not transferred to the secondary coil / spark load.

I wrote much more about all this in: https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/busbar-and-primary-circuit/

[Dominic@TWHV]

Seems like to me the primary coil is inducing some eddy currents into the casing and causing that to heat up. That is extra losses you want to avoid. Again, a sidenote, cooling options are always there when things gets out of hand (like Mads said, 400A is usually low for any active cooling to be needed).

Two main ways of primary cooling:

Air (blowing air through the center of the tube)

Distilled water (pump *DISTILLED* water through the tube)

These are my personal viewpoints and *may* contain mistakes. Take it with a grain of salt. Best of luck fixing your issue!

[Manz]

Primary heating is expected when using a small diameter primary and long ontimes/duty cycle.
My pulseskipping coil's primary heats up quite a bit when running at 10% duty cycle 300A but the primary is only 4mm in diameter.
If you think the heating is excessive you should make sure the tube you used is 100% copper and not just the outside layer.
You can calculate the DC resistance and if you measure it to be much higher it could tell you something.