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240VAC single phase from three-phase? - Page 5

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Posted by amdx on April 3, 2010, 2:59 am

 I'll give it my best shot.
I'll define the system 240v, 2 wires, no neutral. let me know if that is a
We'll start with your scenario as below.
 >> If it were single phase connection of two 120 volt legs, the voltage

We'll start with a sinewave at 0 degrees and 0 Volts. Both wires, since they
are in phase.
As the sinewave starts to rise and gets to 30 degrees the voltage is 147
at 60 degrees the voltage has reached 260 volts and then at 90 degrees, 294
Now lets get out the oscilloscope, one lead on each wire, at 30 degrees the
voltage on
Line 1 has reached 85 volts and the voltage on Line 2 has reached 85 volts
also, because they're in phase
So the oscilloscope measures the potential difference between L1 and L2 and
gets 0 volts.
This continues through the whole cycle, since the are in phase there is no
potential difference.
If they are in phase, how can there be any potential difference to do work.
*If they are out of phase the real question is, at 30 degrees, does L1 go
negative 42.5 volts and
L2 go positive 42.5 volts. :-)
                                           Mike (amdx)
*Depends on the reference point.

Posted by amdx on April 3, 2010, 3:37 am

See third graph and description.

See either drawing.

See first graph and description

                Mike  (amdx)

Posted by amdx on April 6, 2010, 9:26 pm


Where did ya go?
 Was my argument persausive or did you just give up trying to
persaude me.
          Mike (amdx)

Posted by Josepi on April 7, 2010, 4:08 am



Where did ya go?
 Was my argument persausive or did you just give up trying to
persaude me.
          Mike (amdx)

Posted by m II on April 7, 2010, 7:19 am

amdx wrote:

This could go on forever, but it won't. This is really my last posting
on this topic.

Take a transformer secondary.

It makes 240 volts.

It has only ONE current flow in it.

Put a tap in the winding at exactly the mid point.

Measure the voltage in each half, moving the meter leads up in unison.

Both readings are identical and at the same phase angle.

Putting a centre tap on that winding has NOT reversed the current in
half the winding. The current is still the same as it was, at the SAME
angle all the way through the winding.

You are making things harder than they are. (-120 volts at 180 degrees)
***IS*** (120 volts at ZERO degrees). They are one and the same, now and

This is KEY:

******Any *two* wires in a single OR three phase circuit constitute a
SINGLE phase circuit. They have to. It's the LAW. Call your local
University. If you don't like their answer, Call God.******

Next, 208 volts phase to phase, in a three phase circuit, is obtained by
  the result of 120 volts at zero degrees and 120 volts at 120 degrees.

There is 120 degrees between the rms peaks of the sine waves.

Try it. Break down the voltage to it's rectangular components.

120 volts at zero degrees equals 120 v + j 0
120 volts at 120 degrees equals 120 * (cos 120) +j 120 * sin 120   =
(-60 volts + j 103.92)

So, we have:

  (120 + j 0)
-(-60 + j 103.92)

180+ j 103.92   Which is the resultant of the two vectors at 120 degrees

Now, re-convert back to polar notation. Pythagoras comes in handy.

180^2 + 103.92^2 =  43009

square root of 14399.36  = 208 volts.

Note that when the sine waves are the same peak value, 120 degrees
apart, multiplying the 120 line voltage by the square root of 3 also
gives you the 208 phase voltage

The newer 347 line voltage gives you 600 phase voltage. That voltage is
getting more popular for high rise building distributions. They now put
various small transformers in to feed office area 120 V needs.

In a Delta connection, the line voltage IS phase voltage and the root 3
is used to find current instead of voltage.

In a Delta connection, line current is root 3 times phase current.

Back to single phase. The centre tapped wire (neutral) in a three wire
system is NOT grounded because it's at zero potential. It is grounded
there in order to limit the maximum fault voltage to ground at the LINE

That screws up a lot of people. They mistakenly believe in erroneous and
  quite fictitious, non existent, phase angles.

The only phase angles that exist in single phase system are caused by
lagging or leading power factors and show the relationship between
voltage and current.

Wake up, people...it's called single phase for a reason. Two phase and
three phase systems are birds of a completely different colour. Do your


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