Hello Tom,
I have been following this thread with interest, and I would just make this
comment on your recommendation to dispense with the electrostatic
shield.......Over the past year or so, I have had a major revamp, to reduce
the electrical noise etc. infiltrating my shack, the source probably being
my neighbours electronic junk 24/7.....I viewed the incoming AC on a scope,
and the neutral, which in the UK is tied to ground, was covered in spikes
etc. which indicates that the so called ground isnt!( 10 ohms is the IEE
specified GR)....I decided to isolate myself from the public supply, and in
so doing, I had a local transformer manufacturer, produce a 20amp
continuously rated 1:1 240v isolation transformer. At the same time, I asked
that two electrostatic screens were fitted, one on the primary and one on
the secondary. This beast is heavy, and you need a fork lift to move it. The
reason I specified two electrostatic screens is that in an American paper I
read, on the internet, on feeding AC supplies to EMC/Faraday screened rooms,
one or two isolation transformers were recommended, with two electrostatic
screens per transformer, the reasoning being complex, so I just accepted
it.......I also had a proper ground system laid, by a firm of lightning
protection specialists...certified ground resistance now just 2.1ohms...the
incoming public supply is taken to the primary winding of the isolation
transformer, the output, via a screened lead, to the shack. Both
electrostatic sceens are taken back to the newly installed ground system,
and there are now no connections between the public supply and my
shack....the result, all the electrical noise has now gone, and apart from
the radiated occasional radiated emissions, my radio background noise has
dropped back to that of 40 years ago....In text books of that time, the
electrostatic screen was described as a method of preventing the feed back
of interference into the public supply, by the consumer, however, it was
explained to me, by my local transformer manufacturer, that it was only
looked upon as a safety measure, by the transformer manufacturers, and with
the improved high termperature varnishes for the wires used in winding, they
could save a small amount of money, by not including the copper
electrostatic screens....which may be, why our received noise levels have
been climbing in the past decades.....Sorry if this varies the
thread...sincerely John G3JVC.
----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: "Borislav Trifonov" <bdt@shaw.ca>; <amps@contesting.com>
Sent: Saturday, July 01, 2006 11:45 AM
Subject: Re: [Amps] Serious transformer problem
> Boris,
>
> I can understand your frustration. It sounds like everything
> is identical and nothing obvious is wrong.
>
> It is not likely to be residual magnetisim since the soft
> iron (magnetically soft) used in transformer laminations is
> not able to be magentized. If it was able to be noticably
> magnetized, it would have to be a bad or defective
> lamination and would need replaced. Degaussing it would not
> help a thing, because every time it carried flux it would
> remagenteize to the same level. If the core is having a
> major problem with residual magnetisim, it will make a poor
> transformer.
>
> My guess would be you either are on the right track, that
> something is shorted. Another possibility is you are
> measuring at much too low of power levels and things that
> do not matter in operation, like a small hysteresis curve
> difference, are showing up and the buzzing is just a
> coincidence.
>
> Here is what I would look at:
>
> 1.) There should not be a conductive path between
> laminations. That means the laminations must be insulated
> from each other, and the fasteners securing the laminations
> must be insulated from the laminations. Make sure you don't
> have the fasteners shorting the core from side to side. I've
> seen people actually drill into laminations to mount
> components!
>
> 2.) Run the transformers full voltage ( be SURE they are in
> a grounded enclosure when powered up) with no load. See if
> one gets significantly warmer or starts to smell. NEVER
> touch the core while it is powered on!! A single shorted
> turn will behave much like you descibe, and evetually make
> the transformer fail. Generally you can smell that turn
> getting hot if you run the transfomer for a while without
> load.
>
> Get rid of the electrostatic shields if you can the next
> time. They really don't do anything in our applications,
> they take up room, they reduce voltage breakdown, and they
> are a potential souce of problems if they become shorted.
>
>
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