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240VAC single phase from three-phase? - Page 8

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Posted by Josepi on April 1, 2010, 11:28 pm

If two voltages are 180 degrees apart and the currents are in phase with
their voltages then the two return current in the neutral will add to zero.

Adding current 180 degrees out of phase is the same as subtracting currents
that in zero degress out of phase (clones) The result is zero or the

Metrology people do not consider voltages as vectors. Vectors are missing
time elements. They call them phasors. This became the standard about twenty
years ago. WE still draw vectors...LOL

BTW: Two cars going in the same direction at the same speed are vectorially
in phase with each other = 0 degrees. The difference between the tow is
zero. Note: Difference means subtract means 180 degrees out of phase.

I have and I did. Many years ago.

Take two identical cars at the same speed having a head on collision.

The total vector force (mass X velocity) of both cars becomes ZERO.

That is what happens when you have a 180 degree difference between two.

Now, for electrical stuff. The proof of what I say may be found in the
common, or neutral conductor. If both hot sides are feeding identical
loads, the neutral of the circuit is carrying zero amperes.

The neutral carries only the *difference* in load currents.

If the voltage polarities were 180 apart, the neutral would carry the
SUM of the load currents. It doesn't.

Check out the Edison three wire circuit. It's applicable to batteries as
well as 240/120 volt residential distribution.

What you two are confusing is the sum of the current flows in the
neutral and the flow in the sources. The sources are IN PHASE and at 0

Bruce in alaska wrote:

Posted by Bob F on April 1, 2010, 11:49 pm

m II wrote:


Posted by m II on April 2, 2010, 1:44 am

Bob F wrote:

Yes. This is why in many cases, the neutral sizing in circuits and
service conductors may be reduced in size compared to the 'hot'
conductors. The neutral carries the DIFFERENCE in line currents.

If you have two seriesed (sp) 12 volt batteries in series with two 12
volt bulbs, there can be a neutral wire from between the bulbs and
between the batteries. There will be NO current in the neutral wire and
the circuit works fine without it. The sum of the voltage drops around
the circuit exactly matches the total supply voltage.

If the batteries were 180 degrees out of phase, the bulbs would NOT work
without the centre wire, as the batteries would be canceling each other
out. A wire from between the bulbs and the mid connection of the
batteries would make this work, but at a cost of having to carry TWICE
the current of each light bulb.

Look at the secondary of a transformer. Suppose it's 240 volts. The
current through that winding is the same from one end to the other. It
has to be.

Now, placing a centre tap on that winding is NOT going to change the
phase relationship of half the winding compared to another. All we have
done is added another reference point.

Label the transformer winding as A = start, B = centre point, C = end.

When the voltage is measured, you have to be careful not to use the
centre tap as the common lead. That causes an error of 180 degrees to be
  shown in the reading because you are reading the wrong instantaneous
polarity on one end of the winding.

The proper way to measure is to put the meter common on Terminal A and
then move the other lead to first the centre tap then up to C.

You now have a point of origin, A, which will show that the voltages at
B and C are indeed in phase and additive.

If, by magic, the current in half the transformer secondary switched to
180 degrees out of phase, you would get a reading of ZERO volts at
terminals A and C.

An oscilloscope with two sets of leads will show what I mean. Try it.



Posted by Bob F on April 2, 2010, 2:18 am

m II wrote:

As I see it, if the voltage polarities were the same - 0 degrees apart, the
neutral would carry the sum of the load currents. If they are 180 degrees apart,
the neutral would carry 0 current.

Posted by m II on April 2, 2010, 2:32 am

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.



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