Spin Up Exam Week 1 QFTW Question Solution

For the 3 phasy wye transformer what is Ia? Given on diagram VAB = VBC = VCA= 480 V, Load KVA = 25 KVA and pf = 0.75 lagging.

I solved this this way:

S/3 = 8.33 KVA angle -41.4

VAB/sqrt (3) = 480/sqrt (3) = 277 angle -30 

IA = 8333/277 = 30.08 angle -11 and even if I do this using line to line and 3 phase apparent I get the same. What am I doing wrong?

If you see the drawing you will notice the following: 

1) The 25KVA load is connected line-to-neutral (not line-to-line). Therefore, you don't have to divide 25KVA by 3.

2) Van is the zero angle reference. Therefore, Van = 277 angle (0). (You used angle -30 in your calculation)

Ok I was confused as to why it said VAB then and then as you said showed it from line to neutral. I put -30 because phase voltage lags line to line by 30. However what you say makes sense.

Is Van always the zero angle reference, ir can I designate what I want? I guess this will be given.

Dodgeviper1017 said:

Is Van always the zero angle reference, ir can I designate what I want? I guess this will be given.

Click to expand...

If the problem asks to find the voltage or current in PHASOR form (magnitude and angle), then you need a reference. Phasors are defined relative to a reference phasor (zero phase phasor).

For example, if the voltage at bus 1 leads the voltage at bus 2 by 15 deg, and the voltage at bus 2 leads the voltage at bus 3 by 20 deg, we can say the following:

V1 = V ANG(0)
V2 = V ANG (-15deg)
V3 = V ANG(-35deg)

OR

V1 = V ANG(15deg)
V2 = V ANG (0deg)
V3 = V ANG(-20deg)

OR

V1 = V ANG(35deg)
V2 = V ANG (20deg)
V3 = V ANG(0deg)

All of the above are correct, the only difference is the voltage chosen as our reference.

If you are just finding magnitudes (not phasors), then you can choose any voltage as your reference.

Ok so the 30 degree phase shift is not always necessary. If they show a voltage already referenced as phase to neutral? If this had been line to line I would have shifted it correct?

Dodgeviper1017 said:

Ok so the 30 degree phase shift is not always necessary. If they show a voltage already referenced as phase to neutral? If this had been line to line I would have shifted it correct?

Click to expand...

If you are working with magnitudes, you don't have to worry about the 30 deg phase shift when you convert between L-L and L-N. Just multiply or divide by sqrt(3). But, if you are working with phasors (magnitude and angle), you must take into account the 30 deg phase shift. There is a natural 30 phase shift between L-N quantities and L-L quantities.