EC&M Article on Transformer

[Dennis Alwon]

I just read an article in EC&M about a factory that had been operating for 40 years with voltages at 194v phase to phase and 112v phase to ground. Apparently there is a 480/208-120v labeled transformer that was tapped incorrectly. What I don't get is the writer stated the trany was tapped for 480v so he changed it to 460v tap and the voltages on the secondary went 208/120.

If the trany is marked 480 why would you have to wire it to 460v to get the proper voltage

Here is the article but I think I got the gist of it

StackPath

 

[hillbilly1]

The service must have been 460 instead of 480.

 

[infinity]

The issue is that the primary voltage is too low so it doesn't match the 480 volt tap setting.

 

[augie47]

That's the reason for the taps, is it not ?

 

[hillbilly1]

Probably an old 460 ungrounded, or corner grounded delta service.

 

[infinity]

That's the reason for the taps, is it not ?

Yes. I recently changed a 75 kva that had about 197 VAC instead of 208 at the panel. They couldn't understand why their new 230 volt pumps didn't work correctly. New building, new transformer.

 

[Dennis Alwon]

Yes. I recently changed a 75 kva that had about 197 VAC instead of 208 at the panel. They couldn't understand why there new 230 volt pumps didn't work correctly. New building, new transformer.

Apparently the issue is the tap at the trany.... So you think the building wasn't getting 480V? The article was not clear about that.

 

[augie47]

Apparently the issue is the tap at the trany.... So you think the building wasn't getting 480V? The article was not clear about that.

Either the building in general or the location of the transformer due to feeder voltage drop.

 

[Dennis Alwon]

Either the building in general or the location of the transformer due to feeder voltage drop.

It would have been good if they mentioned that...

 

[Dennis Alwon]

I was assuming 480v and trying to figure why hooking it to 460v would work....LOL

 

[LarryFine]

Changing primary taps alters the transformer's turns/voltage ratio.

 

[hillbilly1]

I was assuming 480v and trying to figure why hooking it to 460v would work....LOL

You know what happens when you assume! LOL!

 

[infinity]

Apparently the issue is the tap at the trany.... So you think the building wasn't getting 480V? The article was not clear about that.

The building was supplied by 208Y/120 from the utility and all the 480 was provided by several step-up substations. That transformer had a lower output and never had the taps changed.

 

[David Castor]

The voltage ratings of the transformer indicate the turns ratio. If connected for 480 V to 208 V, the turns ratio is 2.31. It doesn't matter what voltage is actually applied - the turns ratio is still 2.31. If the transformer is connected for 460 to 208 V, the turns ratio is 2.21, so it is stepping down the voltage LESS - or in other words for the same input voltage, the output voltage will be higher. To raise the secondary voltage on a step-down transformer, the turns ratio must be decreased. So for the same input voltage, LOWERING the primary tap will INCREASE the secondary voltage because you are reducing the turns ratio and stepping down the secondary voltage less.