Hi guys:
This is definitely an eddy currents issue. Don't forget this is a
potentially a 6kVA transformer: it doesn't take much of losses to generate
enough current for that kind of heating. Many manufacturers relied on the
lacquer impregnation of the transformer to serve as insulation for the
bolts, especially at the ends to eliminate shorts, so that this can explain
why the bolt close to the bent bracket heated up most. The paint or whatever
was damaged and allowed contact.
Use whatever insulating material that's thin enough to go in easily on the
bolts' bodies and some fiber washers under the heads, at least on one side.
The point is to insulate one of the ends to prevent forming a turn, like
somebody pointed out.
As a side line, just to get some impression: with one single turn on the
secondary side of a soldering gun, you have about half a volt of AC but it
generates a few tens of amperes for generating the heat. All this at about
100W level only; think what it can generate at 6000VA.
73 and all the best:
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of David H Craig
Sent: Sunday, May 04, 2008 5:58 AM
To: TexasRF@aol.com; amps@contesting.com; Dick Hanson
Subject: Re: [Amps] HV transformer issue
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@aol.com>
To: <n3db@radix.net>; <amps@contesting.com>; <km1h@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@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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>
>
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