# Three phase

Discussion in 'Homework Help' started by IState21, Sep 20, 2010.

1. ### IState21 Thread Starter New Member

Sep 20, 2010
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0
The problem I am working on is:
A 480 V, star connected, three phase ac generator supplies power to a delta connected load through a transmission line. The per phase impedeances of the load and of the transmission line are (16+j20) and (1+j20) respectively. Determine:
(a) The line current
(b) The magnitude of the line to line voltage across the load
(c) The efficiency of the transmission

For a I have I=480/(1+j20)=1.197-j23.94
but should I instead add both impedances together and divide 480 by them?

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Yes, to find the line current you have to consider the total load plus line impedance - not just the line impedance. You may find it simpler to refer everything back to a 'per phase' representation. In that case calculate the generator line-to-neutral voltage and apply this to the line impedance plus the delta load impedance converted to its star equivalent. Remember the total impedance is a complex number addition. The subsequent complex division of the line-to-neutral voltage by the total line + load impedance will give you the line current.

The rest is complex number manipulation.

%Efficiency=(Real Load Power)/(Real Generator Power) expressed as a percentage.

3. ### IState21 Thread Starter New Member

Sep 20, 2010
4
0

What I did was:
Vln=480/sqrt(3) <-30 = 277.13 <-30°
Zy=ZΔ/3= (16+j20)/3 = 5.3+ j6.67 <== Not sure why it needs to be converted to ZΔ

Ztotal= (5.3+j6.67)+(1+j20)= 6.3+j26.67

Iline= (277.13 <-30°)/(6.3+j26.67) = (240-j138.57)/(6.3+j26.67) = -2.9 - j9.69

This doesn't seem right to me though

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783

So how would you have otherwise determined the total load seen by the generator?

5. ### IState21 Thread Starter New Member

Sep 20, 2010
4
0
Ahh makes sense, does the work that I showed look like I'm on the right track?

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Yes - it looks fine by me.