Advice sought - isolation transformer inrush current limiting

[Jerome Whelan]

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 <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
>  
>  
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> Tacos at amrad.org
> https://lists.amrad.org/mailman/listinfo/tacos

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