poopypants
Member
saw this in 404.8:
Obviously, there is a phase shift between 277 and 120 causing 277+120 to not equal 397.
1. Can someone please explain how this comes into play, and if my graphs are wrong, how would you change them?
2. If these are from the same source, how does the XFMR change phase rotation?
On another post in 2006, someone else solved it and got these results, which I just can't figure out how. Can someone please explain this to me?
FROM CLOSED THREAD:
I just did a quick model in AutoCAD. The voltage from a Phase A 277 volt line to a Phase A 120 volt line derived from that same 480/277 source measured out at 183 volts and change. If you go from a Phase B or C on the 277 volt side to a Phase A on the 120 volt side, you get either 303 volts or 386 volts.
Charles E. Beck, P.E., Seattle
You can also solve this question using the law of cosines to add the voltage vectors. It matches up with what Charlie posted except I get 302 where he got 303. It is possible to get 397 volts between two conductors, but for that to happen you would have to have both primaries of both the 480/277 and 208/120 transformers fed from the same source, but have the phase rotation different on the primary side. This give on secondary a +30 degree shift and the other a -30 degree shift making it possible to have one phase of one system 180 degrees out of phase with the other system.
Don
Last edited by don_resqcapt19; 11-02-07 at 11:39 PM.
Obviously, there is a phase shift between 277 and 120 causing 277+120 to not equal 397.
1. Can someone please explain how this comes into play, and if my graphs are wrong, how would you change them?
2. If these are from the same source, how does the XFMR change phase rotation?
On another post in 2006, someone else solved it and got these results, which I just can't figure out how. Can someone please explain this to me?
FROM CLOSED THREAD:
I just did a quick model in AutoCAD. The voltage from a Phase A 277 volt line to a Phase A 120 volt line derived from that same 480/277 source measured out at 183 volts and change. If you go from a Phase B or C on the 277 volt side to a Phase A on the 120 volt side, you get either 303 volts or 386 volts.
Charles E. Beck, P.E., Seattle
You can also solve this question using the law of cosines to add the voltage vectors. It matches up with what Charlie posted except I get 302 where he got 303. It is possible to get 397 volts between two conductors, but for that to happen you would have to have both primaries of both the 480/277 and 208/120 transformers fed from the same source, but have the phase rotation different on the primary side. This give on secondary a +30 degree shift and the other a -30 degree shift making it possible to have one phase of one system 180 degrees out of phase with the other system.
Don
Last edited by don_resqcapt19; 11-02-07 at 11:39 PM.
