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ekstra   ara
 Post subject: Autotransformers and Arc Flash
PostPosted: Tue Feb 15, 2022 6:43 am 

Joined: Mon Mar 09, 2020 9:27 am
Posts: 30
I am going through prints to build my preliminary SKM model and came across an autotransformer that is being used to adjust voltage possibly? The notation on the prints is "Auto transformer +10% voltage range". i have not worked with autotransformers, but that is my understanding of them. Its feeding and old theatrical dimmer rack system.

My questions is, how do they effect short circuit/Arc flash? Would this be modeled as an impedance device? What special considerations needed to be taken with them? SKM does not have a component for an autotransformer as far as i can see. So i am curious.


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 Post subject: Re: Autotransformers and Arc Flash
PostPosted: Mon Mar 28, 2022 7:56 am 
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Joined: Thu Jan 10, 2008 8:49 pm
Posts: 514
Location: New England
The secondary winding is in series between the source and load. The primary winding is in parallel between line and neutral.
So its like having a reactor(or a large inductor) in the circuit. The problem is that its difficult to get specifications on just the secondary winding.


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 Post subject: Re: Autotransformers and Arc Flash
PostPosted: Mon Mar 28, 2022 2:06 pm 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 584
An auto transformer has a single winding. An auto is like any other transformer, it trades voltage for current. Neglecting the impedance of the transformer, a 10% boost in voltage will result in a 10% drop in fault current. Fault current will drop further due to transformer impedance. A two winding transformer model might be adaptable if you could enter a sufficiently low impedance. But is this a fixed 10% booster, or is it a step voltage regulator? If it's a regulator, it will have multiple steps between 0 and 10%. When on neutral, a voltage regulator will have no effect on fault current.


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 Post subject: Re: Autotransformers and Arc Flash
PostPosted: Mon Apr 11, 2022 8:16 am 
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Joined: Thu Jan 10, 2008 8:49 pm
Posts: 514
Location: New England
I had this issue a few years ago. I went to another site for help. This is the response.

https://www.eng-tips.com/viewthread.cfm?qid=386462

the one suggestion to measure impedance was a good one if you have the test capability. This is the way manufacturers measure impedance so there is no reason you can't do it yourself. Use a variac to limit input voltage into a shorted secondary and measure current.


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