# Transformer secondary voltage too high



## postalizer (Feb 9, 2010)

Was wondering what are the possible causes for your secondary voltage on a transformer to be higher than expected. In the situation I am looking at I have a 5kva transformer 480/240 primary 240/120 secondary. It is wired for 480 primary 120 secondary but the secondary voltage reads over 200 volts. The input voltage is right around 480 give or take a few. There are several of these in use all wired the same, and all showing too high of a voltage on the secondary ranging from 160-240. The transformers are all around 30-35 years old if this makes any difference.

Thanks
Ed


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## Chris Kennedy (Nov 19, 2007)

Do these trannies have an ungrounded secondary?


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## MDShunk (Jan 7, 2007)

Got any make and model information? 

Some junky transformers do read high until you put a load on them, then the calm down. I've seen up to 20% high, but not quite like you're describing.


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## brian john (Mar 11, 2007)

Are these single phase or delta/wye?

Is (as noted) the secondary properly grounded?

I doubt a 5 kva, may have several different voltage connections, are you sure all connections are right?


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## drsparky (Nov 13, 2008)

Are your voltage readings taken with the transformers under load?


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## brian john (Mar 11, 2007)

MDShunk said:


> Got any make and model information?
> .



That would be a major help in this.


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## 480sparky (Sep 20, 2007)

postalizer said:


> Was wondering what are the possible causes for your secondary voltage on a transformer to be higher than expected. In the situation I am looking at I have a 5kva transformer *480/240 primary 240/120 secondary*. It is wired for 480 primary 120 secondary but the secondary voltage reads over 200 volts. The input voltage is right around 480 give or take a few. There are several of these in use all wired the same, and all showing too high of a voltage on the secondary ranging from 160-240. The transformers are all around 30-35 years old if this makes any difference.
> 
> Thanks
> Ed


 
I don't get it... it's basically a 2:1 ratio. You're putting 480 into it, you should be getting 240 out of it. If you want 120 out, you need only 240 in. 

Or am I having one of my regular Monday-after-the-Stupor-Bowl


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## MDShunk (Jan 7, 2007)

480sparky said:


> I don't get it... it's basically a 2:1 ratio. You're putting 480 into it, you should be getting 240 out of it. If you want 120 out, you need only 240 in.
> 
> Or am I having one of my regular Monday-after-the-Stupor-Bowl


Could be, that's why I asked for make and model information. It could have a number of primary and secondary connections, such that you can put 480 in and get 120 out. 

I've got my money on a miswire, like maybe primary and secondary interconnection.


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## brian john (Mar 11, 2007)

MDShunk said:


> Could be, that's why I asked for make and model information. It could have a number of primary and secondary connections, such that you can put 480 in and get 120 out.
> 
> I've got my money on a miswire, like maybe primary and secondary interconnection.


Most likely.


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## SparkYZ (Jan 20, 2010)

Did you use the proper taps inside the transformer?


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## postalizer (Feb 9, 2010)

The readings are taken while under load. I will grab the make and model today, but the transformer definitely allows for the configurations as stated in my first post. My thought process today is to unhook the load from the secondary and take readings, hopefully steering me in the direction of the issue.


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## JohnJ0906 (Jan 22, 2007)

postalizer said:


> Was wondering what are the possible causes for your secondary voltage on a transformer to be higher than expected. In the situation I am looking at I have a 5kva transformer 480/240 primary 240/120 secondary. It is wired for 480 primary 120 secondary but the secondary voltage reads over 200 volts. The input voltage is right around 480 give or take a few. There are several of these in use all wired the same, and all showing too high of a voltage on the secondary ranging from 160-240. *The transformers are all around 30-35 years old if this makes any difference.*
> 
> Thanks
> Ed


Were these existing and started putting out over-voltage, or were they just installed?

Is the secondary grounded?


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## brian john (Mar 11, 2007)

Check connections (as noted numerous times) the factory connections may be mis-labled.

Megger Primary to secondary
Primary to ground
Secondary to ground with neutral ground bond lifted.

When we get the make and model number, we can add more.


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## Zog (Apr 15, 2009)

postalizer said:


> The readings are taken while under load. I will grab the make and model today, but the transformer definitely allows for the configurations as stated in my first post. My thought process today is to unhook the load from the secondary and take readings, hopefully steering me in the direction of the issue.


Have you done a TTR test?


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## JayH (Nov 13, 2009)

Zog said:


> Have you done a TTR test?


 
Most contractors don't have a TTR tester (I've only worked for a few that did their own testing and certification), that's what we hire you guys for.


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## MDShunk (Jan 7, 2007)

JayH said:


> Most contractors don't have a TTR tester (I've only worked for a few that did their own testing and certification), that's what we hire you guys for.


You can do the TR and TRL test with little more than a 9volt battery, a multimeter, and a calculator.


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## brian john (Mar 11, 2007)

MDShunk said:


> You can do the TR and TRL test with little more than a 9volt battery, a multimeter, and a calculator.


Or a variac can be helpful.


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## JayH (Nov 13, 2009)

MDShunk said:


> You can do the TR and TRL test with little more than a 9volt battery, a multimeter, and a calculator.


That test equipment won't meet the specification requirements of the projects in which I'm involved.


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## brian john (Mar 11, 2007)

JayH said:


> That test equipment won't meet the specification requirements of the projects in which I'm involved.


There is a difference between trouble shooting to solve an issue and testing to meet spec.


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## JayH (Nov 13, 2009)

brian john said:


> There is a difference between trouble shooting to solve an issue and testing to meet spec.


You're overstating the obvious.

I just had to look up what a Variac was. Found out it is a trademarked name for a variable transformer.

Variac has many models, but I doubt many guys are carrying these guys around either.


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