# Homework Help: 3 phase system

1. Oct 10, 2014

### influx

Why is Vab = Van -Vbn? I'm doing this after a while so maybe I'm getting something basic wrong but surely the potential difference between lines A and B is the voltage drop across phase a (Van) plus (rather than minus) the voltage drop across phase b (Vbn)?? So Vab = Van + Vbn?

Thanks

2. Oct 10, 2014

### zoki85

Notice orientations of '+' poles of source voltages Van and Vbn as you go through the loop a-n-b-a

3. Oct 11, 2014

### influx

I thought it had something to do with the signs but I've kind of forgotten. Could you elaborate please?

Cheers

4. Oct 11, 2014

### zoki85

Ok let me illustrate this with DC circuits:

Can you see the difference between case I and II ?

5. Oct 11, 2014

### Staff: Mentor

In a simple DC circuit, if one point is at +6 volts, and another point is at +6 volts, what is the potential difference between the two? Is it 12 volts, or is it zero?

If one point is at +6 volts and another is at -6 volts, what is the potential difference?

6. Oct 13, 2014

### influx

Case I: Going from a to b is Van + Vbn ?
Case II: Going from a to b is Van - Vbn?

7. Oct 13, 2014

### influx

The first is 0V
The second is 12V?

How does that apply to this problem?

8. Oct 13, 2014

### zoki85

:)

9. Oct 13, 2014

### Staff: Mentor

There is no difference. In either case the P.D. is given by the same equation.

Van is voltage at a referenced to n. Vbn is voltage at b referenced to n, that convention is clear.
So the P.D. is always Vab = Van - Vbn, and consistent with your output arrow direction.

Did you intend something different?

10. Oct 13, 2014

### zoki85

Arrow denotes voltage Vab (missing in the drawings). Vbn changes polarity so the outputs Vab aren't same in both circuits

Last edited: Oct 13, 2014