Re: Hot spot

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

 >
 > Commercial xmfr mfgrs overcome this by using vacuum epoxification
 > techniques.  The windings are shorted out and heated with a primary
current
 > while the epoxy cures which drives out the air bubbles and the vacuum
sucks
 > them away.

Or.. don't even bother with shorting... they just run DC through them...
dissipated power is dissipated power... all you need is a bit of heat, but
not too much (you don't want to kick the resin too fast).. It's the vacuum
that's important.. that, and viscosity, and mixing, and pot life, and
care...   potting for HV is a non-trivial operation.  Even the best gets
hashed up occasionally (as some co-workers of mine have recent found for a
18 kV power supply destined for space...) with everything done just right.
I've heard that mixing and potting under a helium atmosphere helps.. high
diffusibility, low gas viscosity, high thermal conductivity...All bogus
probably.. most of the time, it works, and sometimes, you can do everything
perfect and it still gets voids and bubbles, and you just have to throw it
away and start over.  If you make 100 a day (or year), it's a 1% oopsie, and
no big deal.  If you make one a year (or every 5 years), then the failure is
a BIG problem.