VOLTAGE DROP AT XFMR 4

[anbm]

How to calculate voltage drop at transformer secondary if we only know primary voltage and transformer size?

Ex: I have 112.5kVA xfmr, 3-phase (480/208), actual primary voltage is 479.47V.

Thanks!

 

[jghrist]

Thanks prc. So to conclude, the % impedance of the trafo has no direct influence on regulation, am I right ?

How on earth did you conclude that? Wasn't the % impedance a key part of prc's calculation?

 

[7anoter4]

Sorry, it was an error in my above formulae.
My good old professor told us: "don't take anything for granted, you have to demonstrate it before"
So, I took the load constant a=Sload/Srated from a known hand book. But actually a=Iload/Irated only.
Then I have to introduce KU=VS/VSrated and finally the voltage regulation formula is:
uft=1+a*Ufi1%/100-SQRT(1-(a*Ufi2%/100)^2)
and VL=VSrated*KU*(1-uft)
Now in order to fix the O.P. example:
a=2
uft=1+a*Ufi1%/100-SQRT(1-(a*Ufi2%/100)^2)=1+2*3.25/100-sqrt(1-(2*1.443/100)^2)= 0.0654
VL=VS*KU?*(1-UFT) =480*0.95*(1-0.0654) = 426.18 = 88.8%
For the rated VS and rated I, KU=1 and a=1 so the last result is still ok.
Mea culpa!

 

[waross]

In the simplest terms, a transformer and a load consist of the transformer resistance, and the transformer inductive reactance and the load and the load reactance.
The voltage drop depends on all of these factors.
Additional comments;
Load power factor, depends on the load resistance and load reactance.
Transformer impedance, depends on transformer resistance and transformer reactance.
You have a series circuit with transformer resistance and reactance and load resistance and possibly load reactance.
Assuming an infinite source, these values along with the source voltage determmine the other values.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

In principle you are right, waross. But as these parameters are not constant –as for instance –the induction motor start case , it is very difficult to appreciate the total resistance and reactance in order to calculate the voltage drop up to motor terminals. It is more convenient to state the load current and the resultant pf and from the catalog, data of the transformer [as uk% and pk [kw] –and to calculate, enough accurately, the voltage drop starting down-stream from the transformer. It is from my experience.
Best Regards