................

 

Welcome to the forum.

 

http://www.electricalknowledge.com/forum/forum.asp?FORUM_ID=11

 

Hi everyone
Thanks in advance for your help

If I understand things correctly 120/208v implies the Line voltage (voltage measured between any two line conductors) is 208 volt and the phase voltage (voltage measured between a line an the neutral) is 120 volt.

Where does this 208v come from? How does 120v lines add up to make 208v?

Are they any books out there that you would recommend?

Well its time rotation of sine wave and the wye connection kinda if A phase is going thur its peak cycle B & C are at different levels in voltage .

Meaning there not at peak voltage in there cycle moments in time .

the difference is the measured 208 volts between these phases A B / B C / C A .

The wye common point connection makes this a neutral point three phase cycle has 120 moments in any second in time when a sine wave is crossing zero volts in the cycle of the sine wave each phase is at a point the voltage will not be 240 volts but 208 volts on a wye transformer . basically there not at the same peaks or points in time so its less voltage between phases . Take care

 

Well said Nick.

 

Very well said, explainations like that is why I joined this forum!

 

Could this graph I made explain it?

Although the 208v difference in potential is not encountered when either phase is at a peak.

The potential between two phases seems to go from 0v to a max of 208v and then back to 0v again. Could this explain where the 208v comes from?

Measuring potential between blue and yellow phase:

∆ = 0v at point A and D
∆ = 208v (104v + 104v) at point C

My next question is: Do I have 240v total potential in position B (if all 3 phases are used)?

 

Bingo

 

phase to phase voltage is found by taking the single phase voltage and multiplying it by 1.73.

120V phase A-120v Phase B= 120 X 1.73= 207.6

If you really want to know where 1.73 came from I can find it, I really just remembered the "magic" number.

EDIT: this would obviously be for a wye connection

 

1.73 is the square root of 3 think 3 phases

 

this is also how you get the 480V with 277V phases.

277V (277phase A- 277V phase B) X 1.73= 479.21

Like I said before, I kind of lost the explanation of this number over the years but 1.73 as a multiplier always stuck with me.

 

The 1.73 comes from the square root of 3. That must some how relate to the graph.

The next thing about 3 phase current I would like to figure out is if 208v is available for a 3 phase motor or 240v (position B of the graph: blue line 120v, yellow line 60v, red line 60v)

 

Well look at this http://www.windstuffnow.com/main/3_phase_basics.htm

 

Thanks great site

I think I also need to improve my basic alternator and motor knowledge.

Any recommended reading?

 

I understand the fact that in a single phase unit the power falls to zero three times during each cycle. E, F and G in the following graph. Power oscillates constantly between 0v and 120v

But the web site also claims that in a 3 phase unit the power delivered to the load is the same at any instant. Is it really? Doesn’t the power oscillate constantly between 208v and 240v as illustrated in the previous graph?

 

The peak voltage is not 120 volts. 120 volts is the effective (RMS) voltage. The peak voltage is 120 volts multiplied by 1.414. So from 0 to the peak, the voltage would be 170 volts.

Chris

 

I'm surprised at the number of licensed guys that don't understand 3 phase systems..
However even more surprised at those who can't get 3way and 4ways switches to work.

 

I'm not an electrician; I'm a HVAC technician who just graduated. Our training was pretty good but I want to learn more about electricity.

Where does the 1.414 come from?

Just trying to understand the science behind it all.

 

Hi just briefly ac voltages are vector quantities , their calculations include both magnitude and direction. 1.732 is the sq. root of 3; 1.414 is the sq. root of 2.

 

I came across those values a few times but where do they come from?

 

Well If a lamp was connected first to a dc voltage and then to a ac voltage the lamp will light up more brilliantly when connected to dc .This is do to dc voltage its constant it remains at lets say 100 voltages continuously whereas the ac voltage reaches 100 volts peak only at two points during the cycle .
In order for the lamp to light with equal brilliance on ac as well as dc we must raise the ac voltage to 141 peak volts .
Effectively then 141 peak volts of ac will light up a lamp as does 100 volts of dc . Thats were they came up with the formula E peak = 1.41 x Eeff


EFFECTIVE VALUE of an ac wave can be calculated from the peak value by the following formula Eeff= 0.707 x E peak


For all sine waves whether voltage or current the value given to an ac wave will always be the effective value unless stated otherwise .
AC voltmeters and ammeters will always read the effective value of the ac wave unless it is indicated otherwise .





Take care

 

To expound on what nick is explaining so brilliantly...
Effective voltage is also known as RMS which is an acronym for RootMeanSquare (which nick also mentions)....which is a laborious calculation of voltage readings at every angle of rotation in a sine wive...all 360 instantaneous values....The Root squared of the averages will give you effective voltage.... or is it the average root squared??? or the squared average root??? i dont know...its a complicated calculation but it works out, that 1.414 times rms will give you peak voltage...and peak voltage times .707 will give you rms... that is WHERE those values come from. RMS

 

Well yes and yes RMS is what is read and its the effective value .

The meter takes many readings it averages these many readings to calculate the true R M S value . Take care

 

According to wikipedia, the efficient value (also known as RMS) of an alternate current equals the value of a direct (constant) current dissipating the same power (heat) by an equivalent resistance. Your example explains it better.

But when do I need to know this value, when is important to know for instance that 141v AC is equivalent to 100v DC? And what is the relation of this value with the nominal current provided by my service company. I know for instance they try to keep the residential current between 106v and 127v (for 120v nominal).

 

According to wikipedia, the efficient value (also known as RMS) of an alternate current equals the value of a direct (constant) current dissipatingthe same power (heat) by an equivalent resistance. Your example explains it better.

But when do I need to know this value, when is important to know for instance that 141v AC is equivalent to 100v DC? And what is the relation of this value with the nominal current provided by my service company. I know for instance they try to keep the residential current between 106v and 127v (for 120v nominal).

Well you dont need to know unless you just want to know your meter does that for you .
The same value is current or voltage they are both treated as the same in the same circuit .
power companys voltage changes hundreds of different values during the normal day up or down .

Heres how i know some times we install a voltage current monitor kit day/week logger on a breaker or switchboard to monitor its resent problem or history like lots of tripping.

Our service dept will monitor a service and look at the results of the week to check out stuff the voltage changes on a paper graph of record voltages can vary from 480 volts down to say 450 then back up to 495 this is the power company loading the lines or switching the lines or just plain old generator generating .

Its lots of different voltage changes up or down my personal home service is coming in at 126 volts and 245 volts single phase its my poco supply .

Take care

 

Try this.....:thumbsup:

Electricity is actually just simple geometry.

Here is how to explain a 277/480 VAC, 3 phase electrical system visually using geometry.

Using the scale of .1” equals 1 volt

On a floor draw a circle with a radius of 27.7” (27 ¾”) that is equal to 277 volts and mark the center of the circle with an “X”

View attachment 1473


For this next step a magnetic compass or a drawing protractor and a straight edge is required.

View attachment 1476
View attachment 1477


Draw a line from the center ”X” of the circle to the outside line of the circle. Then draw another line 120 degree from the first line from the center “X” of the circle to the outside line of the circle. Then draw another line 240 degrees from the first line from the center “X” of the circle to the outside line of circle. Label the lines A, B & C where they intersect the outside circle. Now it should look like this.

View attachment 1474

Now measure the distance from “A” to “C”

View attachment 1475

The measurement should be about 48” (converted to scale would equal 480 volts) depending on how accurate you draw and measure. The measurements from “A” to “B” and “B” to “C” should be about the same.

There are more complex mathematical equations but this is the simplest and a neat thing to do for someone new to the trade.

Carry On!

For those of you that haven't been around too long this is a rerun of the best of John. :thumbsup:

And this will work for most of all 3 phase voltages. :whistling2:

 

hahaha the best of john...
yeah but i did get a kick out of that. never seen it before.

 

Glad you liked it...but can you do it?

I did on lunch break on a large jobsite one time and most of the electricians on the job didn't understand it....:no:

 

Great explanations guys thanks, I’m getting a pretty good idea of the relation between the line current and the phase current.

120 / 208
277 / 480
347 / 600

But to get back to what Raider1 said, is the peak voltage really 170v in a 120v nominal supply line? If my service provider makes sure the voltage is never higher than 127v how can it reach 170v?

Wouldn’t be the other way around, the peak value supplied is 120v therefore :

Efficient Value (RMS) is only 85v = 120v/1.414 or 120v x 0.707