transformer

[omega83]

hey guys

can you guys give me a tip on this? your primary voltage is tap at 480v . you have 4 taps at 2-1/2% FCBN intervals and 2taps at 2-1/2% FCAN. The building 480v voltage is not true and is at 460v. I need the secondary voltage at 208v. With my 460v building tap at 480, Im not getting 208v off the secondary side. So I would have to change the primary taps to compensate for the building voltage. This is a test question I had on journeyman test.

1)2-1/2 FCBN
2)2-1/2 FCAN
3)5% FCAN
4)5% FCBN
5)7% FCBN

I did the problem and came out with these voltages
2.5 %fcbn 0f 480 is 468v
5%fcbn of 480 is 456V

the ratio of 460 to 208 v is 2.211/1

so what I did was
at 2.5% of 480 is 468v/2.211(ratio)= 211v secondary
at 5% of 480 is 456v/2.211(ratio)=206v secodary

What voltage should I use for the secondary that is suppose to be at 208v? Both calculation came higher or lower voltages. Should I go with higher or the lower voltage from the calulation?

 

[templdl]

So your source voltage is 460. How much below normal is that? 480/480=.958? Isn't that aclose that a 5% difference? Select the tap that is as close to that below normal voltage as you have. The tap is based upon a below normal voltage i.e. tap.
My figure would be 5% which would end up being a tad more than the target of 480v

 

[omega83]

So your source voltage is 460. How much below normal is that? 480/480=.958? Isn't that aclose that a 5% difference? Select the tap that is as close to that below normal voltage as you have. The tap is based upon a below normal voltage i.e. tap.
My figure would be 5% which would end up being a tad more than the target of 480v

So I should go with the 480v tap at 5%fcbn at 460v primary(building voltage) and 206v secondary. I need it at 208v and 206v is closer than if I tap it at 2.5% which is 211v. I just was not sure if I should be lower or higher for the compensation. I should go with the closest one right?

 

[templdl]

You are correct with the 5%FCBN tap.

The 2-1/2% FCBN would give you a lower secondary voltage on the secondary not higher in your situation. That tap would be used for about 468v supply to arrive at 208v on the secondary. Should you use that tap for 460v your secondary voltage would end up to be lower than 208v desired.

If you apply 460 on the 480v tap you will end up with a little less than 200v when you need 208v close to 5% more. As such using the -5%FCBN tap will give you the voltage that you need on the secondary.

You aren't going to have a perfect 460v supply and you only are provided a given set of taps which is dependent upon the way transformer is wound, as such you know what voltage you want to end up with and pick a tap based upon the anticipated supply voltage to get to close to the 208 secondary voltage as you can.

(2) 2-1/2%FCAB and (4) 2-1/2%FCBN taps are quite common which gives provides a range of up to 5% above and 10% below