AC Vectors ? 3

[etronics]

Why do the voltage vectors rotate 180 deg. when the ground is moved from the neutral to say phase 2. This is for wye connected phases??? Can't seem to wrap my head around this one. Thanks.

 

[davidbeach]

Wye wouldn't they?

 

[jghrist]

Draw a vector diagram sketch of what you mean and attach it. This would help us understand what you're problem is.

Chances are, though, if you do this, you will answer your own question.

 

[TestBeforeTouch]

If you have ABC rotation
then Vab = V at 0 , Vbc = V at -120, Vca = V at 120

if the meter measures voltages to neutral then
Van = Va - Vn
Vbn = Vb - Vn
Vcn = Vc - Vn

now if you ground B phase as is done on open delta PT
Vb = Vn

then
Van = Va - Vb = Vab
Vbn = Vb - Vb = 0
Vcn = Vc - Vb = Vcb = -Vbc so your voltage now appears negative or with a 60 degree phase angle instead of 120 as compared to the phase rotation above

See

http://pm.geindustrial.com/FAQ/Documents/469/GET-8414A.pdf

 

[waross]

Try measuring the voltage of a battery. You may get +12 volts.
Now reverse your meter leads. The voltage will now be -12 volts. The voltage has not changed, nothing has rotated. You have just changed your reference point. Kind of the same for vectors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[etronics]

See attached HTML dwg.

If I measure from A to A', B to B' and C to C' I read 0 vac and 0 deg. difference. AB = 115 vac, BC = 115 vac, AC = 115 vac 120 deg. apart.

After moving the ground to B phase (B') I measure from A to A', B to B' ; C to C' and read 67 vac and 180 deg. difference. The 2nd vector on the right is drawn to show this relationship. Obviously the 2 vectors were equiv. before changing the ground ref.
The ph-ph readings after moving the ground to B'.
AB'= 67 vac, AC'= 136 vac, BC'=67 vac, BA'=180 vac, CA'=136 vac CB'=67 vac. Didn't record angles for these. Some of these make sense but most don't.

I'm getting a headache !

Thanks again.

 

[etronics]

Sorry for the pic. not working, here it is in GIF format.

 

[Skogsgurra]

I think your headache would be a lot less if you would let the vectors stay in their original position when you move the ground connection from N to B. Nothing there to wonder about. Your sketch is OK if you leave the vectors and do not rotate them.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jghrist]

What do A-B-C and A'-B'-C' represent? Are they two separate sources. Your discussion about moving the ground from neutral to B' makes it sound like they are the same source with different locations of the ground. But then what does the voltage A-A' mean? I'm totally confused. I thought the vector diagram would clarify, but it obfuscates.

 

[waross]

I believe that the battery analogy is still valid. Consider "B" the battery. You have changed your point of reference relative to "B". That is, your reference point is now the free end of "B" instead of the common end of "B".
That reverses "B" (rather than rotating it!! Some of us make a distinction). The other issues follow from that.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[etronics]

To clarify, both sources are identical balanced 3 phase sources. Phases are designated by the letters A,B,C. The second source's (A'B'C') ground ref. is moved from N. to B'.

The measurements refer to voltage and phase prior to the ground change, and after the ground change. The drawing shows the relationship between the two sources AFTER the ground change since first and second source phases become 180 deg. apart from each other.

Hence, measuring A to A' = 67 vac and 180 deg apart.
Whew ! hope this helps.

Thanks for the responses so far.

 

[slavag]

Hi Etronics.
Only one additional Q:
Are you have some some secondary load connections to those sources?
Regards.
Slava

 

[slavag]

Hi Etronics.
Please pay attention. you changed your reference voltage B' and now it's same reference point, as neutral of first source, taht means A-B', C-B' and B-B' now are 67V.
I don't understand 180V, for my pinion is unpossible, max voltages with 180deg is 134V, ( 136V in you post)

 

[slavag]

Sorry, of course possible.
Please see attached file, you have noe something like to this.
Regards.
Slava

 

[jghrist]

After moving the ground to B phase (B') I measure from A to A', B to B' ; C to C' and read 67 vac and 180 deg. difference. The 2nd vector on the right is drawn to show this relationship. Obviously the 2 vectors were equiv. before changing the ground ref.
The ph-ph readings after moving the ground to B'.
AB'= 67 vac, AC'= 136 vac, BC'=67 vac, BA'=180 vac, CA'=136 vac CB'=67 vac. Didn't record angles for these. Some of these make sense but most don't.

Slavag's sketch is what your voltage would be if you made the change exactly as you said. Note that V_AA', V_BB', and V_CC' would all be 67V at 120°. V_AB'=133V at -150°, V_BC'=176V at 169°, V_BA'=175V at 131°, V_CA'=133V at 90°, V_CB'=67V at 30°.

 

[7anoter4]

Great answers Slava and jghrist!

 

[rockman7892]

Slavag can you explain your sketch a little more I am still a bit confused. Are the origonal AB&C vectors 120deg apart. For the second set of vectors I understand that since B is grounded its vector is in the middle of the wye vectors but why are the A' and C' vectors in the direction that they are?

So if I'm understanding all of this correctly, these are two different identical sources with one source gounded at B. We are then measuring between the two sources?

 

[jghrist]

Correction to my 21 Jul post:

V_AC'=133V at -150°
V_AB'=67V at -90°

 

[7anoter4]

I am not so familiarized with vector diagram but I get other angles [see attachment].
Am I wrong?
Best Regards

 

[slavag]

Hi 7another4.
Two p0ints I don't understand.
From where angle 101deg and 139.1deg
it's 90deg and 150deg.
It's math, not electricity .
Best Regards.
I must say you, I don't know who are you, but with you Im ready make any project.