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240VAC single phase from three-phase? - Page 3

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Posted by m II on March 31, 2010, 10:43 pm
 


amdx wrote:


If indeed they were 180 degrees apart, a meter would read zero volts
instead of 240. Think of two batteries in series, with the meter
connected to the '+' of one and the '-' on the other.

If they are 12 volt batteries, then you will read 24 volts on the meter.
The current is at the same angle in both batteries. It has to be,
they're in series.

If we reverse one of the batteries, so the meter is attached to the '-'
posts on both, the meter would read zero volts.

The current in this case is the sum of one battery at zero degrees and
the other at 180 degrees. They effectively cancel each other out.

The instantaneous voltage in an alternating system has polarity and can
be looked at like a battery source.

Transformer windings can also be looked at as batteries and the
instantaneous polarity of the terminals is shown by a dot. That just
means that at any given moment in time, if one dot marked terminal is in
the positive cycle of the sine wave, the other dot marked terminal will
be the same.





mike

Posted by Josepi on March 31, 2010, 11:06 pm
 


The dots on transformers only relate phasing between the primary and
secondary of related windings.

Between phases the dots mean nothing exept they should be in phase relation
to each other, provided the primary is in phase relation to each other.

Many electricians believe the dot means polarity of a winding, and must be
connected to the live line. It originally meant the grounded polarity for
most. This was important to put the least potential strain or diference from
that end of the winding to the metal core. (winding start). In later years
of usage it is only a relational marker from primary to secondary.


If indeed they were 180 degrees apart, a meter would read zero volts
instead of 240. Think of two batteries in series, with the meter
connected to the '+' of one and the '-' on the other.

If they are 12 volt batteries, then you will read 24 volts on the meter.
The current is at the same angle in both batteries. It has to be,
they're in series.

If we reverse one of the batteries, so the meter is attached to the '-'
posts on both, the meter would read zero volts.

The current in this case is the sum of one battery at zero degrees and
the other at 180 degrees. They effectively cancel each other out.

The instantaneous voltage in an alternating system has polarity and can
be looked at like a battery source.

Transformer windings can also be looked at as batteries and the
instantaneous polarity of the terminals is shown by a dot. That just
means that at any given moment in time, if one dot marked terminal is in
the positive cycle of the sine wave, the other dot marked terminal will
be the same.





mike



Posted by amdx on April 1, 2010, 3:30 pm
 



electrical

electrical

Snip 12 volt battery data.

If you want to talk DC polarity and (phase) note one end of your two
batteries is
 '+' of one and the '-' or opposite polarity (phase)

I tried to post graphics from the page, they didn't post so please go
to the URL and page down 3/4 page.
Here is a website that describes the phase relationship of a typical
120/240V system showing the 180 degree phase.
http://www.allaboutcircuits.com/vol_2/chpt_10/1.html

Also so see the second graph about half way down on this page, and
description.
http://www.tpub.com/content/construction/14027/css/14027_75.htm

                               Mike



Posted by m II on April 1, 2010, 3:28 pm
 

amdx wrote:

The source from the page:
=============================================================
If we mark the two sources' common connection point (the neutral wire)
with the same polarity mark (-), we must express their relative phase
shifts as being 180o apart. Otherwise, we'd be denoting two voltage
sources in direct opposition with each other, which would give 0 volts
between the two “hot” conductors. Why am I taking the time to elaborate
on polarity marks and phase angles? It will make more sense in the next
section!
=============================================================



Note where he says 'IF'. He then goes on to say that in THAT case the
voltage sign has to be changed for the math to work. He uses the
standard, normal sign notation for all the other diagrams. Minus 120
volts at 180 is identical to 120 at 0 degrees (which it IS).



More source:
=============================================================
Single phase power systems are defined by having an AC source with only
one voltage waveform.
=============================================================

The current in BOTH voltage sources is identical and in the same
direction. He was showing that if polarities on one source are
arbitrarily changed, then the sign of the voltage has also got to be
changed.



Even in a three phase system, any TWO of the wires constitute a single
phase source.




mike



Posted by Bruce in alaska on April 1, 2010, 5:04 pm
 



Nope, you got it backwards... If they are 0 Degrees apart the meter
would read Zero Volts, HOWEVER if they are 180 degrees apart the Meter
would read 240 Vac. Draw it out, and you can see the Phase relationship.

--
Bruce in alaska
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