Advice sought - isolation transformer inrush current limiting

Wed May 23 14:28:07 EDT 2018

Just a comment: HV differential probes useful for work on mains-powered
circuits became cheap enough to have one (or more) in a toolbox "just in
case". You can find Micsig probes on ebay for less than $150, and they look
like pretty decent product:
http://www.eevblog.com/forum/testgear/new-low-cost-($170)-100mhz-differential-scope-probe-from-micsig/

Jacek
kw4ep

On Wed, May 23, 2018 at 1:40 PM, Mark Whittington <markwhi at gmail.com> wrote:

> 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-c
>>> urrent.html?gclid=CjwKCAjw_47YBRBxEiwAYuKdw6_IzT-R5wYlOv0eMM
>>> WHP5owpuxpxYzNUo4YC3SoXzx-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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>>> Tacos at amrad.org
>>> https://lists.amrad.org/mailman/listinfo/tacos
>>>
>>>
>>>
>>>
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>>>
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>>
>>
>
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-- 
Given a choice between two theories, take the one which is funnier
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