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Joined: Jun 2007
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1st post. How do you calc transformer inrush current?
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Joined: Jun 2007
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Thanks Brian, looks like what I need.
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Joined: Jul 2002
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OK Chris so you are looking to know what the inrush current on a MVA transformer, should be?. Is this for the purposes of fusing or cable sizing?. I personally think you are out of your depth, if you have to ask the question in the first place. Just my opinion. Cheers, Mike. 
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Joined: Jan 2003
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I personally think you are out of your depth, if you have to ask the question in the first place. What the heck does that mean?  He should not try to learn something new?  I 'know' Chris from another forum, he is a competent professional who appears he wants to expand his knowledge of the trade. It happens that here under the NEC we can be capable of overcurrent device and conductor sizing for transformers without actually knowing how to calculate the actual inrush currents. Mike, do you know how to calculate the actual inrush currents for a particular transformer? I don't, but I am not out of my depth when I have to choose overcurrent protection for a transformer installation.
Last edited by iwire; 06/17/07 06:17 AM.
Bob Badger
Construction & Maintenance Electrician
Massachusetts
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Is this for the purposes of fusing or cable sizing?.
In part,yes. The two things I love most about my work are when the lights on a big project come on for the first time and the sound of a large transformer being energized. The sound of the inrush. I want to know whats happening at that moment. Thanks all for your help and opinions.
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Joined: Jul 2002
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Chris,
I would like to un-reservedly apologise for the nasty reply I provided you with last night.
This sort of behaviour, I'm not proud of at all.
Sorry to anyone else that read my reply also,
this will not be happening again.
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Joined: Jul 2002
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Having read the PDF file from Brian, that top formula seems wildly familiar. With that said, is an inrush current sinusoidal? (as in a pure sine wave?). We wind our own transformers at work (PoCo) and one part of my Linemans apprenticeship was 6 months in the Transformer shop, which I only go into these days if it's too cold in the Faultsman's office!.  I must ask our head Transformer guy at work how they calculate the inrush current, because we get a Post-It note tacked to the side of the tranny stating what size fusing to use, for both run and in-rush current. Bear in mind Chris, we use totally different voltages here in New Zealand, but the formula should still hold true.
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Joined: Jul 2004
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Pardon my ignorance but why wouldn't you have inrush on every cycle? I guess I don't understand all I know about transformers.
Greg Fretwell
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Joined: Jun 2006
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Once the transformer is started there is always a residual magnetic field to control the incomming current. IE it opposes the incomming rush of current. When a transformer is energised there is no magnetic opposition to the flow of current hence the Inrush. Once the transformer has an established magnetic field the 0 crossing of the AC does not cause a complete collapse of the opposing EMF and magnetic field.
Last edited by mikesh; 06/18/07 12:50 PM. Reason: spelling
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