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240VAC single phase from three-phase? - Page 9

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Posted by Josepi on April 3, 2010, 2:34 am

Sorry, You are using the wrong common wire for your vestors.

Using the neutral to each hot leg the phases are out of phase by 180

Your battery analogy is errored unless you use the tie point between the two
batteries as equivalent to the common or neutral conductor. When you measure
from that common point to the end of each battery you get positive 12 volts
and negative 12 volts at each end. Out of phase by 180 degrees and they add
up to the total voltage of 24 or 240vac.

Bob F wrote:

If you are thinking of the neutral current only, you are exactly right.
Pressure in one direction cancels out the pressure in the other. I'm
talking about source and LINE currents. The source currents/voltages
*have* to be in phase.

Think of the neutral as playing two roles. One, it's a return for one
half the circuit. Two, it's a feed for the other half of the circuit.
The currents, if identical will cancel out. Keep in mind that this
shared, common conductor is NOT the same as the feeds coming out of the
sources and if the loads are equal, it's not even needed.

The picture below shows what I mean. Note the polarities.


Instead of a single 240 volt power supply, we use two 120 volt supplies
(in phase with each other!) in series to produce 240 volts, then run a
third wire to the connection point between the loads to handle the
eventuality of one load opening. This is called a split-phase power
system. Three smaller wires are still cheaper than the two wires needed
with the simple parallel design, so we're still ahead on efficiency. The
astute observer will note that the neutral wire only has to carry the
difference of current between the two loads back to the source. In the
above case, with perfectly “balanced” loads consuming equal amounts of
power, the neutral wire carries zero current.



Posted by m II on April 2, 2010, 2:00 am

Bob F wrote:

This page will clear up everything once and for all, for everyone involved.



Posted by m II on April 2, 2010, 2:06 am

Bob F wrote:

This is the CORRECT web page. The author has easy to read and non
confusing markings. This will clear things up for everyone involved.


The last posting with the wrong URL went out by mistake and I can't
cancel it.


Posted by Ulysses on April 3, 2010, 5:33 pm

I probably shouldn't say this but it muddies it up for me.  Without seeing
the sine waves and seeing the phases I can't tell if they are in-phase or
out-of-phase.  And aside from all of that I thought you got 240 from the two
120 volt coils if they were OUT of phase with each other, not IN phase.
Perhaps I'm just not understanding what IN and OUT of phase means.  I
thought OUT of phase meant that when one sine wave is peaking at the top the
other is at zero.  Is this wrong?  I have rewired generator heads many times
in order to either get 240 volts OR 120 volts in parallel (to balance the
load on the coils) and, in my mind (which can be a frightening place) the
sine waves were going up and down at the same time so 120 was all I was
going to be able to get.  Kinda like connecting two batteries in parallel.

Posted by hubops on April 3, 2010, 9:00 pm

<    snips   >

  H a  !  
I love this line.
  John T.

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