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Re: [T3] Coils

On 20 Nov 2001, at 21:57, Y and C Everhart wrote:

> What I've noticed about coils are that coils filled with oil (shake them and you
> can hear the oil slosh around) are usually the cheaper ones.  The O.E. Bosch
> coils and other good quality coils are epoxy filled (solid when shaken).  I
> won't use anything but Bosch, especially here in Arizona where the heat isn't
> very good for the oil filled ones.

I think the Bosch coils are also oil-filled, it's just that they are full, with no air 
space so they don't slosh. I'm not sure about this, though, they might just be 
filled with tar, but you will notice that they develop an oily region around all 
the seals as the years pass.

There's nothing wrong with an oil filled transformer, in fact it's the best 
insulation money can buy. At my former job I designed, built, and tested a 
1:1 230V power transformer isolated for 175kV. It was oil filled and worked 
like a charm. It's now a standard product for that company.

Epoxy has all kinds of problems as an insulator: It expands and contracts at 
different rates from the parts inside and cracks as a result. The cracks 
become leakage and spark paths, and once sparked it leaves a track which 
never heals. The big advantage of fluid insulators is that they can't "track" so 
even if they spark they will heal.

Jim Adney
Madison, WI 53711-3054

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