# Find primary current in 3 phase transformer I am really confused with the above problem.

I get the logic of the solution

Since,the line voltage ratio is 2. So, current will be half. Thus 50.

But I am confused should we take +30 or -30.

Shouldn't it depend on the connection whether it is Yd1 or Yd11.

Please help me in understanding this concept.

## -- EDIT --

I was trying to draw some phasors which I have attached below Assuming IA winding and Iab winding are in phase

In this case taking Ib = 100 <0 gives me IA = 50 <30

Please help me with current phasors, I am really confused.

• R loads give same I phasors as V phasors. Wye the confusion? mysite.du.edu/~jcalvert/tech/threeph.htm – Tony Stewart EE75 Apr 8 '18 at 5:08
• mysite.du.edu/~jcalvert/tech/wyedelta.gif – Tony Stewart EE75 Apr 8 '18 at 5:13
• @TonyStewartEEsince1975, just tell me will IA be in phase with Iab or 180 out of phase with Iab ?? – Nikhil Kashyap Apr 8 '18 at 13:34
• Look at the picture do you see 180 or 0 deg? No becuase WYE primary to Delta secondary is rotated -30 degrees – Tony Stewart EE75 Apr 8 '18 at 13:44
• but unlike analog clock and CW Notation for + phase , + angle is CCW rising above the zero axis at 3 o'clock – Tony Stewart EE75 Apr 8 '18 at 14:01

## 2 Answers

WYE to DELTA or DELTA to WYE results in positive or + 30 deg shift.
where a leads b leading c

Yes it matters how it is wired. There is never any phase displacement or shift between two common windings, but there MAY be a phase displacement or shift between the line voltages of two transformers depending on how the windings are connected.

In this example B+ goes to A' or A'-B or C'-A
Yd11 with 30 deg lead. (secondary leads primary by 30 deg.) and primary WYE lags secondary DELTA

##  Why does math use CCW for positive angles and clocks rotate the opposite direction?

... because in math everyone uses Archimedes and Cartesian standards with right axis as + and vertical axis at top is +ve ( same in schematics for V+ to top)  Rotating a circle past any reference point in the orientation as you are told (CCW is positive) then Delta to WYE or WYE to DELTA if in same sequence given results in 30 rotation to left or CCW or POSITIVE 30 deg.

Positive angles in Trigonometry oppose direction of clock. It does not mean we go backward in time. The cicle rotation std is also show here as CCW for positive degrees when rotating the circle past any reference point.

See again for b to lag a the triangle must be rotated CCW for POSITIVE ANGLES with positive time. $\phi=\omega t$

A balanced (positive sequence) three phase AC voltage source is connected to a balanced, Y-connected load through a $$\Y-\Delta\$$ transformer as shown in the signal. The line to line voltage testing is 230V on the Y side and 115V on the $$\\Delta\$$-side. If the magnetizing current is neglected and $$\I_s=100A, \Phi=0\$$, then whar is the value of $$\I_p\$$ in ameres? After understanding the connection, now we will try to draw voltage and current phasors.

Since, the line voltage ratio is 2. So, we can conclude that the magnitude of primary line current will be 50. Now, we will try to get its phase using current phasor. 