[Amps] HV transformer issue
David H Craig
n3db at radix.net
Sat May 3 22:58:07 EDT 2008
Thanks much for the ideas gents. So far I tried the following:
1.) insulated the core from the chassis with a scrap 1/4" lexan sheet
(no bolts just let it sit there) & powered it up with no cap bank in
circuit. Bolts still heated, and a potential appeared on the amp chassis.
I was unable to accurately measure it, but it did get my attention. (using 3
wire 220, green to chassis of course) Shut it down & bolted it back.
2.) Ran it bolted down to chassis for about 5-10 minutes, again with no
cap bank in circuit. Potential mentioned above was gone. Just for the heck
of it I measured the temp of each bolt- top 2 ran around 160-170 deg F & 2
bottom ones were higher- one was 185 deg F & the other bottom bolt a
whopping 275 deg. A fair amount of dissipating heat on the sides of the
core after power-down, just a tad on the top. Could touch sides but was
pretty warm for a short, no-load run.
I guess that leaves insulating the bolts to try (and the shoulder
washers of course).
Thanks again/73 Dave N3DB
----- Original Message -----
From: <TexasRF at aol.com>
To: <n3db at radix.net>; <amps at contesting.com>; <km1h at jeremy.mv.com>
Sent: Saturday, May 03, 2008 9:26 PM
Subject: Re: [Amps] HV transformer issue
>
> 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 at 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 at aol.com>
> To: <n3db at radix.net>; <amps at contesting.com>; <km1h at 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 at 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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>
>
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