3-ph circuits

[sonaiko]

hi all..
what i know is that in (+) sequence balanced 3-ph circuits...
Vab (ab is a subscript) leads Van with 30 degrees and it is larger by the square root of 3... (voltages are Y-connected

what is the relationship between these 2 voltages in (-) sequence system?

 

[Dean Huster]

I'm not sure I follow where you're going with this. In a 3-phase system, each phase leads or lags the other two phases by 120° -- I don't know where your 30° fits in here.

Dean

 

[John Sorensen]

Suppose you have a three phase grounded neutral Y, and Van phase is V<0 (V at an angle of 0), Vbn is V<-120 (Vbn lags Van by 120 deg) and Vcn is V<-240 (Vcn lags Vbn by 120 deg).

Then if you put your meter between A phase and B phase, you'll measure Vab=Van-Vbn= V<0 - V<-120. If you do the vector math, you'll find that the result is V*sqrt(3)<30, or Vab leads Van by 30 deg.

If you're asking about positive, negative and zero sequences, that's a different problem. Basically, if you have three vectors, say Vab, Vbc, and Vca, rotating counterclockwise in a sequence Vab, Vbc, Vca, if they're not symmetrical (equal in magnitude and angle) then they can be represented by three sets of three symmetrical vectors, the sums of which add up to the original vectors. It's very complicated. I have a good link about it, but it's at work.

j.

 

[John Sorensen]

Here's the link I was thinking of:
**broken link removed**

The math gets kind of intense. Actually the math isn't particularly hard, there's just a lot of it.

There are also a few small programs that demonstrate the positive negative zero sequence concept:

https://www.ece.utexas.edu/~grady/ABC012.html

j.