Three phase analysis

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ronaldrc

Senior Member
Location
Tennessee
Hello

Rattus if this analysis of the 208 voltage is correct how many times would the voltage of 208 appear in one complete cycle positive or negative in this graph?


Edited to show another graph which one would you say is more correct?

Graph #1

Graph #2

Ronald :)

[ March 23, 2005, 07:26 PM: Message edited by: ronaldrc ]
 

rattus

Senior Member
Re: Three phase analysis

ronald, first off, you should plot the peak voltages (170V) of the three phases. Then when you plot the difference of any two phase voltages, the frequency of the 208V (294Vpeak) waves will be the same as that of the 120V waves. The plot will get rather busy if you put all six voltages on it.

Every time two phases are equal and opposite, you get a 294V peak. Every time two phases are equal (when they cross), you get a zero crossing.

Got it this time.

[ March 21, 2005, 10:55 PM: Message edited by: rattus ]
 

ronaldrc

Senior Member
Location
Tennessee
Re: Three phase analysis

Edited to save room. Rattus I take it you see six locations also? Is the graph correct as far as showing the one created with A and B phase?

Ronald :)

[ March 21, 2005, 10:45 PM: Message edited by: ronaldrc ]
 

ronaldrc

Senior Member
Location
Tennessee
Re: Three phase analysis

Rattus I would rather view the 170 Vac as 120 Vac since it is the effective value of the 170 Vac and this happens at the same instant.

Ronald :)
 

rattus

Senior Member
Re: Three phase analysis

ronald, looks OK to me, but I am getting bleary eyed. You show one full cycle of 3-phase 120V waves. You will show one full cycle of 3-phase 208V waves when you complete the sketch.
 

ronaldrc

Senior Member
Location
Tennessee
Re: Three phase analysis

Rattus would it be better if I reduced the size?
I wasn't going to run it on out to the end of the C phase or blue wave. :)
 

rattus

Senior Member
Re: Three phase analysis

ronald, a swmaller diagram would help, and I would sketch in just one of the 208V waves. You have the right idea, and you understand a bit more about 3 phase don't you?
 

Ed MacLaren

Senior Member
Re: Three phase analysis

Guys, sine waves diagrams, by their very nature, represent only instantaneous values.

120 volts and 208 volts are effective values, which are a sort of average over time.

Using effective values in reference to a sine wave makes no sense.

Ed
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Re: Three phase analysis

Ron, I'd just make it a
url.gif
instead of an
image.gif


We can open the image in a seperate window, and the margins would come back to normal. Saves time over resizing, probably.

[ March 22, 2005, 08:06 AM: Message edited by: georgestolz ]
 

rattus

Senior Member
Re: Three phase analysis

Originally posted by Ed MacLaren:
Guys, sine waves diagrams, by their very nature, represent only instantaneous values.

120 volts and 208 volts are effective values, which are a sort of average over time.

Using effective values in reference to a sine wave makes no sense.

Ed
I agree with Ed. You should plot peak values from which effective (RMS) values are calculated. You could just label the peaks as "Vp" which would fit all cases. Then, instead of 104V as you show, you would show 0.866Vp, then the line to line peak is 1.732Vp. Voila! Whatever that means.
 

ronaldrc

Senior Member
Location
Tennessee
Re: Three phase analysis

I agree with you I am showing a little more than is needed. Rattus is the 104 not 104 at that time makes good sense to me to show it as it is.

Most like myself just think its as simple as 3 peak 208 peaks and thats enough for my line of work.But if we don't understand math like most on here doesn't hurt to try to make it simple.Showing the graph the way it is and having a little understanding of the way the voltages aid and oppose each other when on the same conductor then the numbers make good sense.

George I think its fine my monitor is not distorted was before I make it a little smaller.
 

rattus

Senior Member
Re: Three phase analysis

Originally posted by ronaldrc:
I agree with you I am showing a little more than is needed. Rattus is the 104 not 104 at that time makes good sense to me to show it as it is.

Reply: Ronald, should be 147V which would make a peak of 294V.

Most like myself just think its as simple as 3 peak 208 peaks and thats enough for my line of work.But if we don't understand math like most on here doesn't hurt to try to make it simple.Showing the graph the way it is and having a little understanding of the way the voltages aid and oppose each other when on the same conductor then the numbers make good sense.

Reply: Ronald, it is too simple to plot it this way. It will leave the wrong impression. The math is quite simple, Vrms = Vpeak x 0.707. You should plot the actual peak values.
 

ronaldrc

Senior Member
Location
Tennessee
Re: Three phase analysis

Ok Rattus and Ed I will change them later but are the voltages shown right? Other that using effective voltage than peak?

I still think I should show all three waves so you can understand how the resultant voltages of 147 v. upper ands lower are added to created 294 peak to peak. In others words we are adding the A phase and B phase voltages at this point right?
 

rattus

Senior Member
Re: Three phase analysis

Originally posted by ronaldrc:
Ok Rattus and Ed I will change them later but are the voltages shown right? Other that using effective voltage than peak?

I still think I should show all three waves so you can understand how the resultant voltages of 147 v. upper ands lower are added to created 294 peak to peak. In others words we are adding the A phase and B phase voltages at this point right?
Ronald, I believe the graph to be correct except for the peak values.

Now be careful. Peak to peak voltage is the difference in positive and negative peaks of the same wave. That is, 340V for 120V and 588V for 208. You can't measure this with a voltmeter; you just know that Vpp = 2 x Vp.

Remember too, that you are subtracting algebraically the two waves, so 147 -(-147) = 294.
 

ronaldrc

Senior Member
Location
Tennessee
Re: Three phase analysis

Rattus it is my belief that the resultant voltage made from adding the A and b phase at that point would be 294 peak to peak.When I say adding don't forget about us math deprived people.

Ronald :)
 

rattus

Senior Member
Re: Three phase analysis

Ronald, you can say "adding" if you wish, but peak to peak has a well defined meaning. The peak value (measured relative to zero) of the 208Vrms wave is 294V. The peak to peak value is 588V. You must use those terms properly in order for people to follow your discussion.
 

ronaldrc

Senior Member
Location
Tennessee
Re: Three phase analysis

Rattus

You are right peak would refer to the top and we are looking at a graph of 170 peak to peak instead of a graph showing the resultant wave.
 
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