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Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Dr. KV -
I forgot to ask, what's the operating frequency of your coil? More than 400 KHz?
Standard PVC pipe is very commonly used as a secondary coilform for systems operating up to 250-300 KHz. I don't recall very many reports of "excessive" heating originating from dielectric loss within the coilform material itself.
Do you have access to a non-contact optical IR thermometer? This would make it very easy to take "spot" temperature measurements of the primary conductor, any exposed secondary coilform, and the secondary winding itself, both the windings under the primary coil as well as windings further up.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Herr Zapp, By checking the temperature with a somewhat limited sensor (my hand), I found the secondary heats a lot just where the primary is around, on the top it is ambient temperature. Maybe there is more Volt/turn induced right under the primary, which results in higher dielectric loss there.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys,
Well, all CW coils with lots of power throughput will tend to heat up a lot. The base of the secondary will tend to heat the most. PVC or any other plastic with low melting point is a poor choice of form material for a secondary that is going to see a lot of heating.
I wouldn't use polypropylene tube, as far as I know it melts at even lower temperature than PVC and certainly the majority of heating comes from ohmic losses in the wire. I'm not sure how big dielectric losses really are here. The plastic of the secondary form doesn't really see a very strong electric field here, and your primary form would melt first if this was a problem.
Even cardboard tube is a big improvement, especially when dried well and coated with varnish or epoxy/polyester resin. Even better would be a fiberglass, ceramic, teflon or glass tube, depending on what you can get or make.
For my large SSTC I made my own fiberglass tube on the inside of a PVC tube which was a pain to remove later, but the secondary performed great, was extremely tough and didn't even heat closely as much as I expected (perhaps due to large thermal mass of the form.)
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Secondary coil too hot
