Advice sought - isolation transformer inrush current limiting

[Mark Whittington]

Hey Bill,

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.

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?

Thanks all for your help thus far.

-Mark

On Wed, May 23, 2018 at 10:23 AM, Bill Danielson <anviljenkins at gmail.com>
wrote:

> 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 ??
>
> Bill
>
> On 05/23/2018 10:04 AM, Karl W4KRL wrote:
>
> Jerry,
>
>
>
> 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.
>
>
>
> Karl W4KRL
>
>
>
> *From:* jerome whelan <whelanjh at gmail.com> <whelanjh at gmail.com> *On
> Behalf Of *Jerome Whelan
> *Sent:* Tuesday, May 22, 2018 10:17 PM
> *To:* W4KRL at arrl.net
> *Cc:* Mark Whittington <markwhi at gmail.com> <markwhi at gmail.com>; Tacos
> <tacos at amrad.org> <tacos at amrad.org>
> *Subject:* Re: Advice sought - isolation transformer inrush current
> limiting
>
>
>
> 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…
>
>
>
> What causes the time-decaying character of the hum sound ?
>
>
>
> Jerry AB3SX
>
>
>
>
>
> On May 22, 2018, at 14:29 PM, Karl W4KRL <W4KRL at arrl.net> wrote:
>
>
>
> Mark,
>
>
>
> 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.
>
>
>
> That said, try an SL22 10008 NTC.
>
> https://www.ametherm.com/inrush-current/transformer-
> inrush-current.html?gclid=CjwKCAjw_47YBRBxEiwAYuKdw6_IzT-
> R5wYlOv0eMMWHP5owpuxpxYzNUo4YC3SoXzx-6Xj7CdVNiBoCnDsQAvD_BwE
>
>
>
> Karl W4KRL
>
>
>
> *From:* Tacos <tacos-bounces+karl.w4krl=gmail.com at amrad.org> *On Behalf
> Of *Mark Whittington
> *Sent:* Tuesday, May 22, 2018 6:57 AM
> *To:* Tacos <tacos at amrad.org>
> *Subject:* Advice sought - isolation transformer inrush current limiting
>
>
>
> 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.
>
>
>
> Any advice would be appreciated.
>
>
>
> Cheers.
>
>
>
> -Mark
>
>
>
>
>
> _______________________________________________
> Tacos mailing list
> Tacos at amrad.org
> https://lists.amrad.org/mailman/listinfo/tacos
>
>
>
>
> _______________________________________________
> Tacos mailing listTacos at amrad.orghttps://lists.amrad.org/mailman/listinfo/tacos
>
>
>
> _______________________________________________
> Tacos mailing list
> Tacos at amrad.org
> https://lists.amrad.org/mailman/listinfo/tacos
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.amrad.org/pipermail/tacos/attachments/20180523/212bc896/attachment-0001.html>