Is Vab really equal to Van minus Vbn?

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vbn.png


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
 
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Notice orientations of '+' poles of source voltages Van and Vbn as you go through the loop a-n-b-a
 
zoki85 said:
Notice orientations of '+' poles of source voltages Van and Vbn as you go through the loop a-n-b-a
I thought it had something to do with the signs but I've kind of forgotten. Could you elaborate please?

Cheers
 
Ok let me illustrate this with DC circuits:
Untitledcbe56.png

Can you see the difference between case I and II ?
 
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?

Do you add the readings, or do you subtract them?
 
zoki85 said:
Ok let me illustrate this with DC circuits:
Untitledcbe56.png

Can you see the difference between case I and II ?

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

Do you add the readings, or do you subtract them?
The first is 0V
The second is 12V?

How does that apply to this problem?
 
influx said:
Case I: Going from a to b is Van + Vbn ?
Case II: Going from a to b is Van - Vbn?
:)
 
zoki85 said:
Ok let me illustrate this with DC circuits:
Untitledcbe56.png

Can you see the difference between case I and II ?
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?
 
Arrow denotes voltage Vab (missing in the drawings). Vbn changes polarity so the outputs Vab aren't same in both circuits
 
Last edited:

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