I could be wrong about this but my recollection is that not only should the
through bolts be insulated, but also the hardware at both ends as well. The
idea is to prevent an accidental shorted turn that has high current and hot
spots as a result.
So. in addition to the insulating sleeve, you may need some insulated
shoulder washers as well.
I can't see that a significant voltage would be involved so no need for
exotic insulating material; most anything should be ok that can handle a bit of
heat from the core. For convenience, a piece of heat shrink tubing might work
well for the bolts.
73/k5gw
In a message dated 5/3/2008 7:31:59 P.M. Central Daylight Time,
n3db@radix.net writes:
I assumed not since it was built as it is, but that said a dead SB220 HV
transformer I pulled apart did indeed have insulated bolts. Should I try
doing so, maybe with a teflon tube as opposed to paper?
73 Dave
----- Original Message -----
From: <TexasRF@aol.com>
To: <n3db@radix.net>; <amps@contesting.com>; <km1h@jeremy.mv.com>
Sent: Saturday, May 03, 2008 8:24 PM
Subject: Re: [Amps] HV transformer issue
>
> Are these through bolts supposed to be insulated?
>
> 73/gw
>
>
> In a message dated 5/3/2008 7:20:02 P.M. Central Daylight Time,
> n3db@radix.net writes:
>
> Hello all,
>
> I am seeking input on a potential problem with my "almost new" B+
> transformer. It is a 240V input, output 4 taps from 2800-3000V rated 2
> amp full load.
> Transformer was tested in situ today, first stand-alone. Has a slight
> hum
> but not significant, and as mounted no detectable vibration on the
> laminations. AC output tested fine on all taps. Second test was with
> full wave bridge
> feeding a 12 x 800MF (series) homebrew cap bank attached- expected DC
> Voltage
> measured fine on the cap bank. 3rd test was with 100k ohm 250w R across
> the
> cap bank from B+ to B-, ~100 volt drop.
> Now the "problem": The transformer is a typical E I Core (roughly
> 9"x7"x7")
> with primary taps on one side fed to binding strip & secondary on the
> other
> side to an EU style strip opposite the mains. Core is held together with 4
> pieces of allthread, nuts on each side, around 1/4" in diameter.
> Transformer
> is not potted. With mains fed to the transformer & even with cap bank
> removed from the secondary side, the 4 allthread "bolts" get hot. Not a
> "little
> hot" but "blister your finger hot" with just a minute or 2 of mains
> voltage
> applied. I understand this was new when I bought it- that said, I can
> smell some
> heating varnish, apparently from the bolts. The laminations themselves
> stay
> completely cool, but granted it is so massive (84lbs) that it would take
> a
> good long while for it to heat up. I am not sure but the allthread
> appears to
> be standard steel. I have never run into this strange effect before-
> this
> can't be normal, can it? I am guessing this is hysteresis, magnet
> ostriction and heavy eddy currents in the bolts- does this sound
> plausible?
> What is going on here?
> Because they are getting so hot after just a short time under power I am
> concerned about using the transformer in the amp (GS35b 50 MHz in
> mid-build).
> It has been suggested that I pull one of the allthreads and check for
> hotspots
> on it, which I plan to try. If they are steel would it pay to replace
> them
> with non-magnetic stainless bolts & get rid of the allthread completely?
> Also, in its current configuration I cannot tell whether the bolts are
> actually
> a proper fit for the holes in the E I plates- it appears that it may be
> slightly undersized, and of course they are threaded the whole way, ans
> therefore
> may not be effectively transferring heat to the laminations.
> Any input would be most appreciated.
>
> 73 Dave N3DB
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>
>
>
>
>
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