guitarboyled

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?

william1978

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william1978

Welcome to the forum.

wildleg

http://www.electricalknowledge.com/f...sp?FORUM_ID=11

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nick

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

william1978

Well said Nick.

Richard Rowe

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

guitarboyled

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)?

drsparky

Bingo

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"Pay attention son, this is for your own good." - Foghorn Leghorn

running dummy

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

running dummy

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.

guitarboyled

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)

nick

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

guitarboyled

Thanks great site

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

Any recommended reading?

guitarboyled

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?

raider1

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

Toronto Sparky

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.

guitarboyled

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.

alimaj

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.

guitarboyled

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