<div dir="ltr">Hey Bill,<div><br></div><div>Thanks for the reply. I suspect if this were isolated on its own breaker or an industrial breaker there wouldn't be a problem, but this is on a residential 15A breaker and the breaker services more than just the transformer. Also my understanding is that residential breakers tend to have different (less forgiving) time/current curves than industrial breakers designed for big inductive loads. This is why I suspect the problem to be caused by inrush current and not a fault in the device or in the breaker and why I hope limiting the inrush will resolve the problem.</div><div><br></div><div>I think I'm going to try the series 75w bulb approach that Karl suggested and if that mitigates the problem then I'll probably go the NTC thermistor route for a long-term fix, but if you or Bill Fenn can suggest some test or another that I can perform that would identify a problem with the windings in the transformer or some other problem then I'd love to hear about it -- don't want to turn an annoyance into a house fire, right?</div><div><br></div><div>Thanks all for your help thus far.</div><div><br></div><div>-Mark</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, May 23, 2018 at 10:23 AM, Bill Danielson <span dir="ltr"><<a href="mailto:anviljenkins@gmail.com" target="_blank">anviljenkins@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<p>Back to the OP, I am a Maintenance Technician in a large mfg
plant, we have a LOT of transformers in the plant and when power
is turned off an back on there is never any issue at all. Sounds
kind of strange that this is going on. I understand some basics of
how an inductor works, that there will be an initial surge until
the magnetic field builds up, but Industrial stuff must be made to
just work on power up and not blow a properly sized fuse ??<span class="HOEnZb"><font color="#888888"><br>
</font></span></p><span class="HOEnZb"><font color="#888888">
<p>Bill<br>
</p></font></span><div><div class="h5">
<br>
<div class="m_1653725220471390560moz-cite-prefix">On 05/23/2018 10:04 AM, Karl W4KRL
wrote:<br>
</div>
<blockquote type="cite">
<div class="m_1653725220471390560WordSection1">
<p class="MsoNormal">Jerry,<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">My speculation is that the minuscule motion
of the laminations against each other transfer mechanical
energy to the coating on the laminations thereby warming them
slightly. As they become stickier or softer they do a better
job of dampening the emitted sound. <u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Karl W4KRL<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<div style="border:none;border-top:solid #e1e1e1 1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal"><b>From:</b> jerome whelan
<a class="m_1653725220471390560moz-txt-link-rfc2396E" href="mailto:whelanjh@gmail.com" target="_blank"><whelanjh@gmail.com></a> <b>On Behalf Of </b>Jerome
Whelan<br>
<b>Sent:</b> Tuesday, May 22, 2018 10:17 PM<br>
<b>To:</b> <a class="m_1653725220471390560moz-txt-link-abbreviated" href="mailto:W4KRL@arrl.net" target="_blank">W4KRL@arrl.net</a><br>
<b>Cc:</b> Mark Whittington <a class="m_1653725220471390560moz-txt-link-rfc2396E" href="mailto:markwhi@gmail.com" target="_blank"><markwhi@gmail.com></a>;
Tacos <a class="m_1653725220471390560moz-txt-link-rfc2396E" href="mailto:tacos@amrad.org" target="_blank"><tacos@amrad.org></a><br>
<b>Subject:</b> Re: Advice sought - isolation transformer
inrush current limiting<u></u><u></u></p>
</div>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Please pardon my jumping in on this issue,
but I think this might be an opportunity to increase my
knowledge on magnetostrsiction… I have observed that a large
transformer, when initially energized at no-load, generates a
loud power-frequency sound which fades out over ten to fifteen
seconds… <u></u><u></u></p>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">What causes the time-decaying character
of the hum sound ?<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">Jerry AB3SX<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<div>
<p class="MsoNormal"><br>
<br>
<u></u><u></u></p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal">On May 22, 2018, at 14:29 PM,
Karl W4KRL <<a href="mailto:W4KRL@arrl.net" target="_blank">W4KRL@arrl.net</a>>
wrote:<u></u><u></u></p>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<div>
<p class="MsoNormal">Mark,<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal">This is due to remanent flux in
the transformer core. If the first half cycle of
the line tries to magnetize the core in the same
polarity, it will saturate the core and draw high
current. One possible approach is to put a
resistor in series with the line and cut out the
resistor after the transformer core resets. An
incandescent bulb, say 75W if you can find one,
would be a good resistor.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal">That said, try an SL22 10008
NTC.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><a href="https://www.ametherm.com/inrush-current/transformer-inrush-current.html?gclid=CjwKCAjw_47YBRBxEiwAYuKdw6_IzT-R5wYlOv0eMMWHP5owpuxpxYzNUo4YC3SoXzx-6Xj7CdVNiBoCnDsQAvD_BwE" target="_blank"><span style="color:#954f72">https://www.ametherm.com/<wbr>inrush-current/transformer-<wbr>inrush-current.html?gclid=<wbr>CjwKCAjw_47YBRBxEiwAYuKdw6_<wbr>IzT-<wbr>R5wYlOv0eMMWHP5owpuxpxYzNUo4YC<wbr>3SoXzx-6Xj7CdVNiBoCnDsQAvD_BwE</span></a><u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal">Karl W4KRL<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><b>From:</b><span class="m_1653725220471390560apple-converted-space"> </span>Tacos <<a href="mailto:tacos-bounces+karl.w4krl=gmail.com@amrad.org" target="_blank">tacos-bounces+karl.w4krl=<wbr>gmail.com@amrad.org</a>><span class="m_1653725220471390560apple-converted-space"> </span><b>On
Behalf Of<span class="m_1653725220471390560apple-converted-space"> </span></b>Mark
Whittington<br>
<b>Sent:</b><span class="m_1653725220471390560apple-converted-space"> </span>Tuesday,
May 22, 2018 6:57 AM<br>
<b>To:</b><span class="m_1653725220471390560apple-converted-space"> </span>Tacos
<<a href="mailto:tacos@amrad.org" target="_blank">tacos@amrad.org</a>><br>
<b>Subject:</b><span class="m_1653725220471390560apple-converted-space"> </span>Advice
sought - isolation transformer inrush current
limiting<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
<div>
<div>
<p class="MsoNormal">I've got a 1KW isolation
transformer that usually trips the 15A breaker
at the breaker box when I turn it on, load or
no. I'm considering installing a pair of NTC
thermistors on the input side of the
transformer, but I've no idea how to size them
or if there might be a better option.<u></u><u></u></p>
</div>
<div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
</div>
<div>
<div>
<p class="MsoNormal">Any advice would be
appreciated.<u></u><u></u></p>
</div>
</div>
<div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
</div>
<div>
<div>
<p class="MsoNormal">Cheers.<u></u><u></u></p>
</div>
</div>
<div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
</div>
<div>
<div>
<p class="MsoNormal">-Mark<u></u><u></u></p>
</div>
<div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
</div>
<div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
</div>
</div>
</div>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"LucidaGrande",serif">______________________________<wbr>_________________<br>
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</div>
</blockquote>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
</div>
</div>
<br>
<fieldset class="m_1653725220471390560mimeAttachmentHeader"></fieldset>
<br>
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</pre>
</blockquote>
<br>
</div></div></div>
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