[Hallicrafters] Repotting Transformers.

[Roger K8RI]

> We do this all the time at my work place only on transformers up to 1000
> kva.  Your friend is doing it in the wrong order.  First the vacuum, then
> the varnish, and then the pressure.

With limited resources and home made equipment it's far easier to do it his 
way.
The hot varnish has a very low viscosity so the air bubles still get out 
which is the main reason for heating it.

>
> Its called VPI or Vacuum Pressure Impregnation.  After the vacuum stage, 
> the
> varnish is pumped into the tank and the work is allowed to soak and 
> absorb.

I agree, this is the proper order if you have the equipment.
With a good roughing pump you can maintain 10^-1 Torr and a half hour at 
that will get rid of almost any air that would be a problem as long as it's 
not trapped in old varnish.  We had the equipment to go to 10^-6, but not 
for transformer potting. That was float zone refining as well as some 
electron beam heaters.  It takes quite an electron beam to melt a 7 or 8" 
diameter chunk of of Silocon.  The mass spectrometer would make 10^-7, but 
it used liquid Nitrogen cold traps  in addition to Ion pumps after the 
difussion pumps made 10^-5 or 6.

We used to use a two part silicone to pot some very high voltage 
transformers.    They were dipped under vacuum and we just let the 
atmospheric pressure force it into the windings. It was a relatively slow 
cure at room temp, or about an hour at 250F "as I recall".

> Then the air is pumped in under pressure forcing the varnish into the
> windings and crevices.  The work piece is then taken to a large oven and
> baked for a few days.  It makes the windings, and the core a one piece 
> unit

He used to do that in his first wire's oven.

> with no movement of the windings at all and very quiet laminations.
>

As to those who asked, the vapor pressure of the varnish is high enough that 
none of it gets into the vacuum cleaner<:-))  However, vacuum cleaners 
depend on the air flow through them to cool the motor.  You don't want to 
let them run very long that way.

73

Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com

> K2WH
> Electrical Engineer in Power Generation,
> Transformer Design Engineer and all around
> Ham Radio Operator.
>
>
> -----Original Message-----
> From: hallicrafters-bounces at mailman.qth.net
> [mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of Roger K8RI
> Sent: Friday, July 29, 2005 2:51 PM
> To: Hallicrafters at mailman.qth.net
> Subject: [Hallicrafters] Repotting Transformers.
>
> I may have mentioned this before, but in case I didn't:
> A friend of mine fashioned a metal canister with a fitting on the top that
> would allow him to connect a vacuum cleaner.  He'd fill the container with
> enough "hot" transformer varnish so he could completely imerse the
> transformer without the thing running over.  The transformer, sans covers,
> was then lowered into the varnish, the cover placed on the canister, the
> vacuum cleaner hooked up and turned on.
>
> I don't think he'd run the vacuum cleaner for more than a minute, then 
> while
>
> the varnish was still hot he'd let the air back into the canister. That
> would force the varnish into small internal places that would never be
> reached by dipping alone.  It doesn't take a big vacuum system to do this.
>
> Roger Halstead (K8RI and ARRL 40 year Life Member)
> N833R - World's oldest Debonair CD-2
> www.rogerhalstead.com
>
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