[AMPS] Re: HT XFMR

[Steve Thompson]

-----Original Message-----
From: Jim Thomson <jim_thomson@bctel.com>
To: amps@txrx.demon.co.uk <amps@txrx.demon.co.uk>
Date: 22 June 1999 20:48
Subject: HT XFMR


>Steve. Along the same lines,I have 4 oil filled caps in my
HV PS. rated
>at 47uf @ 4.5kv. I run them series parallel to give 47uf
@9kv. (PS puts
>out 6800vdc). I grounded all 4 cases. After 2 yrs,top
insulator blew
>out,and have corona ever since.
>
>-My electrican friend said to remove the case grnding,and
mount all 4
>caps on a sheet of micarta.We also placed sheets of
micarta(HV
>fiberglass) between oil caps.
>
>-Two oil caps in series to increase V ratings works,but
6800vdc to
>chassis on a cap rated at 4500vdc is a bit much.

>
>-I have seen Plate xfmr's mounted on plywood to do the same
thing. Float
>the case of plate xfmr,on any kind of insulator,and you
have removed the
>stress from it. I have seen this done on commercial
broadcast HV
>supplies. This gets around any leakage current from hi-pot
testing.


I've convinced myself it's ok to do this, although I worry
about the effects on other equipment should the primary to
core insulation fail. G3RZP suggested MOVs across the
winding and from each end to ground which I reckon should
protect things.

>
>-Richs method of drying the plate xfmr's out works well !
Simply apply a
>low voltage via variac/auto xfmr to the secondary winding.
Put a dead
>short across primary 1st. With a low v applied to
secondary,adjust v
>till rated AC duurent flows in secondary.     EG:
36vac/18ohms=2amps
>Let it sit like that for several hrs with xfmr wrapped in a
beach towel.
>This keeps heat in,and moisture gets absorbed by
towel/blanket etc. It's
>a perfect way to cook the moisture out without having
several kv
>appearing across secondary.


Great idea. I can cheat a bit - I have an oven which will
take a good sized transformer, but it's unusual in that I
can pump it to a modest vacuum at the same time. Should
speed the drying process a bit  - I guess it could help with
potting too.

Steve


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