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Re: [Amps] HV transformer issue

To: <TexasRF@aol.com>, <amps@contesting.com>, "Dick Hanson" <dick@dkhanson.com>
Subject: Re: [Amps] HV transformer issue
From: "David H Craig" <n3db@radix.net>
Date: Sat, 3 May 2008 22:58:07 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>
    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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