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Transformer inrush #165024 06/16/07 04:01 PM
Joined: Jun 2007
Posts: 4
C
ChrisKennedy Offline OP
New Member
1st post. How do you calc transformer inrush current?

Work Gear for Electricians and the Trades
Re: Transformer inrush [Re: ChrisKennedy] #165028 06/16/07 05:48 PM
Joined: Oct 2005
Posts: 40
G
gibbonsseabee80 Offline
Member
Hello Chris,
This is a big formula that has some symbols that can not be typed in this block. I found a link for you. Maybe this will will help you out. I breaks it all down.
http://www.transformerscommittee.org/info/F01/IEEE_Inrush_Tutorial.pdf




Brian Gibbons
Re: Transformer inrush [Re: gibbonsseabee80] #165036 06/16/07 11:01 PM
Joined: Jun 2007
Posts: 4
C
ChrisKennedy Offline OP
New Member
Thanks Brian, looks like what I need.

Re: Transformer inrush [Re: ChrisKennedy] #165045 06/17/07 02:09 AM
Joined: Jul 2002
Posts: 8,379
Trumpy Offline
Member
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. cool

Re: Transformer inrush [Re: Trumpy] #165046 06/17/07 05:16 AM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
Originally Posted by Trumpy
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? confused

He should not try to learn something new? mad

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 05:17 AM.

Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: Transformer inrush [Re: Trumpy] #165051 06/17/07 07:49 AM
Joined: Jun 2007
Posts: 4
C
ChrisKennedy Offline OP
New Member
Originally Posted by Trumpy

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.

Re: Transformer inrush [Re: ChrisKennedy] #165083 06/17/07 11:12 PM
Joined: Jul 2002
Posts: 8,379
Trumpy Offline
Member
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.

Re: Transformer inrush [Re: Trumpy] #165088 06/18/07 01:11 AM
Joined: Jul 2002
Posts: 8,379
Trumpy Offline
Member
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!. laugh
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.

Re: Transformer inrush [Re: Trumpy] #165101 06/18/07 11:41 AM
Joined: Jul 2004
Posts: 9,571
G
gfretwell Offline
Member
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
Re: Transformer inrush [Re: gfretwell] #165103 06/18/07 11:48 AM
Joined: Jun 2006
Posts: 613
M
mikesh Offline
Member
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 11:50 AM. Reason: spelling
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