On Fri, 12 Oct 2001, Donald H Locker wrote:
>
> Hmmmm. I'll try some ASCII art.
>
> C
> / \
> / \
> / \
> / \
> / \
> / \
> / \
> / \
> A--------n--------B
>
> Phases A and B are the 120 legs, n is neutral, C is the third phase.
With you so far.
> The angle between A and B is 180 degrees,
Not sure I'm believing this one yet... I'll explain later
> while the angles CAB, ABC, and BCA are each 120 degrees.
Actually they are 60 degrees in you diagram. Now if you want to
create another neural in the center of the triangle (called in N), then the
angles ANB ANC and BNC are 120 degrees. This probably has something to do
with my misunderstanding, but I'll elaborate further below. Unfortunately
my power electronics textbook is at home.
> A-n is 120 volts; B-n is 120; C-n is 208 (and at 90 degrees to A-n and B-n,
> FWIW); A-B is 240; B-C is 240; > C-A is 240.
Okay, time for my ASCII Art. Since I haven't thought about Power in a
while, the phasor abstractions aren't working for me and I'm going to need
to do this in the time domain.
According to my understanding, for a typical house wiring system, you
need two phases A & B - 180 degrees out of phase (120Vrms to ground). If you
read accross them you would get 240Vrms because they are completely out of
phase.
** ** ** **
* * * * * * * *
* * * * * * * *
A ----------------------------------------------------
* * * * * * *
* * * * * * *
** ** ** *
** ** **
* * * * * *
* * * * * *
B ----------------------------------------------------
* * * * * * *
* * * * * * *
** ** ** *
Now from what I remember about three phase power, the lagging waveforms
would need to each follow the previous by 120 degrees as in the following
** ** ** **
* * * * * * * *
* * * * * * * *
A ----------------------------------------------------
* * * * * * *
* * * * * * *
** ** ** *
** ** ** **
* * * * * * * *
* * * * * * * *
B ----------------------------------------------------
* * * * * * * *
* * * * * * * *
** ** ** ** *
** ** ** **
* * * * * * * * *
* * * * * * * * *
C ----------------------------------------------------
* * * * * * * * *
* * * * * * * * *
** ** ** ** *
Now if you were to measure the voltage between A and B you would get
only 180 volts RMS (254V peak) because of the phase offsets.
So I guess my question is how can the phases be 120 degrees apart for
three phase and simultaneously 180 degree apart for house wiring? Or am I
remembering something wrong
Now I could see it working if there was a transformer in between the
Delta and the house, but then it wouldn't be a simple case of adding a third
conductor to get three phase power.
<snip>
Tim
<>---------------------------------------------------------------------<>
Tim Hoerning / hoerni@cooper.edu
'91 Honda VFR 750, '84 Honda VT500 Ascot, '73 Honda CB350G Super Sport
'87 Chevy El Camino, '90 Plymouth Laser
<>---------------------------------------------------------------------<>
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