Advice sought - isolation transformer inrush current limiting

Bill Danielson anviljenkins at gmail.com
Wed May 23 11:35:47 EDT 2018 

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 <mailto: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>
>>     <mailto:whelanjh at gmail.com> *On Behalf Of *Jerome Whelan
>>     *Sent:* Tuesday, May 22, 2018 10:17 PM
>>     *To:* W4KRL at arrl.net <mailto:W4KRL at arrl.net>
>>     *Cc:* Mark Whittington <markwhi at gmail.com>
>>     <mailto:markwhi at gmail.com>; Tacos <tacos at amrad.org>
>>     <mailto: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
>>         <mailto: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
>>         <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
>>         <mailto: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 <mailto: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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