Posted by *m II* on March 31, 2010, 7:02 am

amdx wrote:

*> Your math is ahh, hmm, depends how you think about it :-)*

*> You almost need to think in peak and peak to peak terms when you*

*> are discussing 3 phase and measuring phase to phase.*

*> When one phase peaks it is 169.68 volts above ground, (1.414 x 120V)*

*> The other leg must be -124.43V, because 124.43 + 169.68 = 294.11 Vpp.*

*> And 294.11V x .707 = 208 V*

*> And yes you're correct 88V x 1.414 = 124.43Vp*

*> What I'm a bit puzzled by is I can't find a proper graph.*

*> The negative leg is never at the correct point to show what I have said.*

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

Posted by *amdx* on March 31, 2010, 5:36 pm

*> amdx wrote:*

*>> Your math is ahh, hmm, depends how you think about it :-)*

*>> You almost need to think in peak and peak to peak terms when you*

*>> are discussing 3 phase and measuring phase to phase.*

*>> When one phase peaks it is 169.68 volts above ground, (1.414 x 120V)*

*>> The other leg must be -124.43V, because 124.43 + 169.68 = 294.11 Vpp.*

*>> And 294.11V x .707 = 208 V*

*>> And yes you're correct 88V x 1.414 = 124.43Vp*

*>> What I'm a bit puzzled by is I can't find a proper graph.*

*>> The negative leg is never at the correct point to show what I have said.*

*> I don't know where you guys are getting these numbers from.*

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

*> leg gives you 208 volts.*

Yes it is 208 (Vrms)

*> 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.*

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

Mike

Posted by *harry* on March 31, 2010, 6:17 pm

*> > amdx wrote:*

*> >> Your math is ahh, hmm, depends how you think about it :-)*

*> >> You almost need to think in peak and peak to peak terms when you*

*> >> are discussing 3 phase and measuring phase to phase.*

*> >> When one phase peaks it is 169.68 volts above ground, (1.414 x 120V)*

*> >> The other leg must be -124.43V, because 124.43 + 169.68 = 294.11 Vpp.*

*> >> And 294.11V x .707 = 208 V*

*> >> And yes you're correct 88V x 1.414 = 124.43Vp*

*> >> What I'm a bit puzzled by is I can't find a proper graph.*

*> >> The negative leg is never at the correct point to show what I have said.*

*> > I don't know where you guys are getting these numbers from.*

*> 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*

*> > leg gives you 208 volts.*

*> Yes it is 208 (Vrms)*

*> > 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.*

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

*> Mike- Hide quoted text -*

*> - Show quoted text -*

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!

Posted by *hubops* on March 31, 2010, 8:26 pm

*>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.

Posted by *Josepi* on March 31, 2010, 9:50 pm

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 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.

> Your math is ahh, hmm, depends how you think about it :-)> You almost need to think in peak and peak to peak terms when you> are discussing 3 phase and measuring phase to phase.> When one phase peaks it is 169.68 volts above ground, (1.414 x 120V)> The other leg must be -124.43V, because 124.43 + 169.68 = 294.11 Vpp.> And 294.11V x .707 = 208 V> And yes you're correct 88V x 1.414 = 124.43Vp> What I'm a bit puzzled by is I can't find a proper graph.> The negative leg is never at the correct point to show what I have said.