240VAC single phase from three-phase? - Page 2

 

amdx wrote:


I don't know where you guys are getting these numbers from. 120 volts
per leg, at a coincidental 120 electrical degrees (360 / 3) to another
leg gives you 208 volts. This is a 'wye' or 'star' connection. There is
a common 'neutral' point.

If it were single phase connection of two 120 volt legs, the voltage
would be 240. In a single phase setup, both currents are at the same
electrical angle, so they add directly, 120 + 120 = 240.

In a three phase addition, you add 120 volts at zero degree plus 120
volts at 120 electrical degrees. That 120 degrees makes the difference.
The multiplier to get 120 leg to ground voltage is the square root of
three (1.732).

208v line to line / (root 3) = 120v.
120 phase voltage to ground X (root 3) = 208 volts line to line.

Root three, coincidentally saves you having to use that awful trig stuff
like sines and cosines.

The root 2 (1.414) you mention above is used to find the peak of the
sine wave, or the effective voltage if the peak is known.

120 volts X root 2 =  +/- 169.7 volts peak. That is the very top and
bottom of each and every sine wave coming into your house, as it would
be in mine if I paid my utility bills.

Calculating peak voltage per phase should not be confused with adding
two phase voltages.

The single sine wave math:
http://www.bcae1.com/voltages.htm

The three phase math follows. There are three of the above mentioned
sine waves happening together, but shifted in time by 1/180 second from
each other. That 1/180 second is what you get given a 120 electrical
degree separation at 60 cycles per second.

http://av.rds.yahoo.com/_yltgeunp54LJL5KgAzCPzHaMX;_ylu=X3oDMTBvdmM3bGlxBHBndANhdl93ZWJfcmVzdWx0BHNlYwNzcg--/SIG h8h25sk/EXP70100473/**http%3a//www.voltech.com/support/articles/51/Three%2520Phase%2520Measurements%2520(104-022).pdf

If Microsoft's wonderful system chops that URL into pieces, please use
the following:


http://tinyurl.com/yfno3kh




mike

 



 I was responding to Gordon where he said,
"So on a 3 phase supply, one leg will rise up to 120volts, but
 the other leg will not be at -120 volts. It will be at about -88volts"
This is true but only at one instant in time, and that would NOT be at
a peak.
 A little study of  rms vs peak vs peak to peak will show you where
my numbers came from.
 I thought a graph showing the voltage over time would be a good way
to show the voltages. I was disappointed that I didn't find an accurate
graph. None have the proper voltage, when line A peaks positive, line B
should be at -124V on the graph. Any graph I have found is either to high
or to low. This just a poor drawing of a sinewave.

 >120 volts per leg, at a coincidental 120 electrical degrees (360 / 3) to
another

 Yes it is 208 (Vrms)


 It would seem to me they would be 180 degrees out of phase.
                                  Mike

 


I am from the UK, but it runs in my mind that unlike in the UK, the
secondary of local transformers is "zig-zag" connected in the USA

http://en.wikipedia.org/wiki/Zigzag_transformer
This not a very clear explanation.

The zig zag system has the advantage having six phases at 60deg
intervals for very little extra complication.  There are three
secondary windings with centre taps which are connected together to
from the "neutral " or grounded side.  The extrematies of these
windings form the six phases.  There are numerous possible voltages
available. And more possibilties of a wrong connection if all possible
phases are available to you!

 



   I am not from the UK - and you have completely flambustered
what very little understanding that I might have had about
zig-zag  ....   but that's ok.  
Maybe we need a new thread for our zig-zag .. ?
  John T.
  

 

Zig-zag is a secondary winding configuration, usually in large transformers,
over 10-20 MVA that attempts to eliminate third harmonics from all the HID
and CFL lighting load the green-washed crowd install.

The secondaries still only have three phases of output, Inside they are
composed of 6 different vectors. The effect is the negatively add third
harmonics from two fiffent phases and they become zero, reflected back into
the generating station.

This all assumes balanced three phase third harmonics.

Think this: With three phases drawn on a sinewave graph, one pahse starts
every 120 degree. Put a third harmonic on each phase (yup.. negative peak in
line with the fundamental peak) and you will find all the third harmonics on
all th4 phases are the same phase! . Windings can make them add to zero.

This is what the zig-zag secondary transformers do at the cost of extra
complexity.







   I am not from the UK - and you have completely flambustered
what very little understanding that I might have had about
zig-zag  ....   but that's ok.
Maybe we need a new thread for our zig-zag .. ?
  John T.