Advice sought - isolation transformer inrush current limiting

Wed May 23 13:40:27 EDT 2018

I'm really hoping to avoid running a separate circuit for this.  It's not
something that gets a lot of use, just need a way to isolate a device under
test in the rare case where I need to put scope probes on a mains-powered
circuit.  I definitely would not be comfortable over-sizing the existing
breaker and compensating with fuses in this case, because that branch
circuit powers something like a third of the outlets on the first floor of
my house.  This is also a key reason why I suspect that it's just inrush
current and not a fault -- it only intermittently trips when I turn the
transformer on, and whether it trips or not depends on what else is turned
on around the house.

As an aside, this house has the most haphazard electrical layout of any
modern residence I've ever seen.  For example, there's one 15A breaker that
services one wall in my office, one wall in the living room, a ceiling fan
and one wall of outlets in the master bedroom upstairs on the other side of
the house, and the lighting fixtures in two bathrooms.  I'm pretty sure the
electrician was drunk when he wired this place.

If I do have to run wiring to fix the problem I'll install a type D breaker
and never have to worry about it again, but that is definitely the last
resort.

Also thanks for the article link, very interesting reading.

Best,

-Mark

On Wed, May 23, 2018 at 11:35 AM, Bill Danielson <anviljenkins at gmail.com>
wrote:

> really we just wire them up and use them, and put in a new one when they
> fail :-).I am hoping we all learn something in the replies :-).
>
> http://www.cooperindustries.com/content/dam/public/
> bussmann/Electrical/Resources/solution-center/technical_
> library/BUS_Ele_Tech_Lib_Transformer_Protection.pdf
>
>
> That document there would suggest that what you have going on is not
> typical to Industrial stuff. I know our overloads for a lot of things (not
> fuses) do have two modes of overload, one is inductive and one is
> thermal...this helps cope with the high inductive current when an AC motor
> starts ? Industrial fuses also many of them apparently have a dual mode
> that allows a higher inrush current, and then there are "slow blow"
> versions as well ??
>
> I am wondering if you would be better to hook your transformer to a
> circuit a larger breaker, but use a separate fuse(s) to protect it ?? Just
> to fiddle around if you have an electric dryer that existing circuit and
> plug would give you safe access to what a 50A 230VAC circuit ?? Then size
> an industrial fuse like the document tells us to ??
>
>
> Bill
>
>
>
> On 05/23/2018 10:55 AM, Mark Whittington wrote:
>
> 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-R5wYlOv0
>> eMMWHP5owpuxpxYzNUo4YC3SoXzx-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
>>
>>
>>
>>
>>
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>>
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>
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