The xfmr I rewound has two tapped primaries on a split bobbin
(I didn't need to touch the primary windings). Magnetizing
current is about 2A. It runs dead cold.
Manfred's info online (below) helped yuuugely with the rewind. As tight as I
rewound the secondary, I had a little magnetostrictive
noise. A local transformer shop was nice enough to vacuum-
pressure impregnate it free gratis with polyester resin.
Photos + text: http://www.tinyurl.com/wa7prc-transformer
vy 73,
Bryan WA7PRC
----------------------------------------------------------------------
Date: Tue, 10 Oct 2017
From: Jim Thomson
Subject: [Amps] capacitor across mains input to transformer
Date: Tue, 10 Oct 2017
From: Steve Wright
Subject: [Amps] capacitor across mains input to transformer
<Can someone direct me to reading material on this?
<I've wound a HT transformer and it's got a decent hum to it and draws
<about 1A @230V input with no load.
<Thanks,
<Steve
### That 1 A of current you are seeing on the primary...with no load on the
sec, is
called...magnetizing current. If you dont use enough primary turns.....keeping
the
turns ratio constant, it will result in higher magnetizing current. Microwave
oven
transformers typ have high magnetizing currents.
[snip]
Jim VE7RF
------------------------------
Date: Wed, 11 Oct 2017
From: Manfred Mornhinweg
Subject: Re: [Amps] capacitor across mains input to transformer
Steve,
> Can someone direct me to reading material on this?
Plug alert: My web site, of course... The articles on "transformers and
coils", and "practical transformer winding":
http://ludens.cl/Electron/Magnet.html
http://ludens.cl/Electron/trafos/trafos.html
> I've wound a HT transformer and it's got a decent hum to it
That hum typically comes from vibrating parts, specially the
laminations, and particularly the section of the laminations that's
inside the bobbin and thus somewhat hard to compress properly. Try
compressing the pack a bit harder, and use wedges to compress the part
inside the bobbin. If that doesn't kill the hum, you have to impregnate
the transformer. Get some transformer varnish, loosen up the core, soak
both the core and the winding assembly with as much varnish as you can
get in, then tighten the bolts of the core, insert wedges if possible to
compress the center leg, and let dry. Ideally heat it up to speed
drying. You can use DC through the high voltage winding to heat it up.
Roughly the nominal secondary current, or even a little more.
> and draws about 1A @230V input with no load.
That doesn't sound bad at all. That current is strongly phase-shifted
relative to the voltage. So the actual power consumed while idling (core
loss) is likely to be far lower than 230W. If it's a transformer for a
legal-limit amp, then this amount of idling current looks plenty low enough.
Do you have a true power meter? If so, you can measure how much of the
230VA is real power, and there you have the core loss - because
essentially all the real power taken up by a transformer when idling is
core loss.
Manfred
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