Why did North America not use 220v

[jrustles]

Originally Posted By: Vikas
I thought the two legs were 120 degrees apart.


only in a three phase system, the type used in commercial and industrial

 

[leeharvey418]

Isn't the RMS calculation independent of phase, though? So the two hot leads don't necessarily need to be 180 degrees out of phase, they just are that way...

 

[Dave Sherman]

Originally Posted By: leeharvey418
Isn't the RMS calculation independent of phase, though? So the two hot leads don't necessarily need to be 180 degrees out of phase, they just are that way...


A center tapped 240 volt transformer will have the 2 legs 180 degrees out of phase, always. True, if you were only interested in 120VAC, then it would not matter how far out phase they are, but since your house usually needs 240 for things like central air, electric ranges, and electric clothes dryers, then they have to be 180 degrees.

 

[PandaBear]

Originally Posted By: Dave Sherman
Originally Posted By: PandaBear
Originally Posted By: KenO
Basically, because Edison was a hard-core egomaniac, and a complete douchenugget. And tried to 1-up Tesla (and was usually wrong in the end, nearly every single time) at every chance he could.


Let me guess, you watched The Oatmeal's cartoon didn't you?


Whatever gave you that idea? Douchebaggery, I tell you.

http://theoatmeal.com/comics/tesla

 

[jrustles]

Originally Posted By: PandaBear

http://theoatmeal.com/comics/tesla



Nice, great read

 

[badtlc]

Originally Posted By: Vikas
I thought the two legs were 120 degrees apart.


Only in a 3-phase service.

 

[badtlc]

Originally Posted By: Dave Sherman


Well, you're partly correct. Yes, you have 240 volt single phase coming to your house. However, if you look at each hot leg with respect to neutral, the two legs are 120 volts and they ARE 180 degrees out of phase with each other. If you could look at each leg with a two-channel oscilloscope referenced to neutral, you would see 120 VAC on one trace, the other would also be 120 VAC, but inverted (i.e., 180 degrees off) from the first.


That appears because you are changing the polarity. The 120V legs are created from the same, single continuous coil. It is impossible for them to have different phase angles.

Again, 120 at an angle of zero plus 120 at an angle of 180 = 0. The math doesn't lie.

 

[Vikas]

Why are you "adding" the two voltages? Remember the term "potential difference" way back from middle school? You need to subtract!

 

[badtlc]

Originally Posted By: Vikas
Why are you "adding" the two voltages? Remember the term "potential difference" way back from middle school? You need to subtract!


nvermind. thought this was a reply to me.

 

[SHOZ]

220v wye 3 phase is the worst.

 

[Dave Sherman]

Originally Posted By: badtlc


That appears because you are changing the polarity. The 120V legs are created from the same, single continuous coil. It is impossible for them to have different phase angles.

Again, 120 at an angle of zero plus 120 at an angle of 180 = 0. The math doesn't lie.


Nope, the math doesn't lie. 120 at angle 0 PLUS 120 at angle of 180 = 0, just as you said. Last I checked, 120 + (-120) = 0. The potential DIFFERENCE between L1 and L2 (i.e., 120 at angle 0 MINUS 120 at angle 180) is 240 volts. Don't believe me? Here:

cos(0) = 1
cos(-pi) = -1 180 degrees off

(120 * cos(0)) - (120*cos(-pi)) = 240

Yes, the polarity is opposite, which is identical to having a sinusoidal waveform which is mathematically 180 degrees off. You're right that the magnetic flux waveform in the transformer certainly didn't change phase, but the two waveforms (with respect to neutral) ARE 180 degrees out phase, nonetheless.

 

[leeharvey418]

I could never keep it straight whether one leg was operating between +120 and 0 V, and the other leg was operating between 0 and -120 V, or whether they both were operating between +120 and 0...

Then I took physics in college, and I learned that it doesn't really matter. Once you take the root mean square of the voltage wave, it comes out the same either way.

 

[badtlc]

Originally Posted By: Dave Sherman

Nope, the math doesn't lie. 120 at angle 0 PLUS 120 at angle of 180 = 0, just as you said. Last I checked, 120 + (-120) = 0. The potential DIFFERENCE between L1 and L2 (i.e., 120 at angle 0 MINUS 120 at angle 180) is 240 volts. Don't believe me? Here:

cos(0) = 1
cos(-pi) = -1 180 degrees off

(120 * cos(0)) - (120*cos(-pi)) = 240

Yes, the polarity is opposite, which is identical to having a sinusoidal waveform which is mathematically 180 degrees off. You're right that the magnetic flux waveform in the transformer certainly didn't change phase, but the two waveforms (with respect to neutral) ARE 180 degrees out phase, nonetheless.


Not to rain on your parade but you are using the false assumption that one leg is at 180 degrees phase angle. That is not true. Since you have access to an oscilloscope, you can prove this to yourself. Use red and black leads. Attach the red lead to one end of the scope and black to the middle. Then move BOTH leads to maintain proper polarity direction. This requires moving the red lead to center tap and the black lead to the end of the coil.

Your scope will show two 120V sine waves in phase. If they were not in phase, they'd subtract from each other (magnitude wise) and cancel each other out. They are in phase and as such will add to each other (magnitude wise) and produce a 240V. When you are measuring end to end, you are not subtracting one voltage from the other.

I'm not sure why you are having a hard time understanding this as you appear to be a smart enough person with regards to the math but you don't seem to understand how to apply it properly.

 

[badtlc]

Originally Posted By: leeharvey418
I could never keep it straight whether one leg was operating between +120 and 0 V, and the other leg was operating between 0 and -120 V, or whether they both were operating between +120 and 0...

Then I took physics in college, and I learned that it doesn't really matter. Once you take the root mean square of the voltage wave, it comes out the same either way.


Correct. With AC there is no + or - voltage leg since the voltage varies between the same positive and negative peaks equally. You can switch between a hot and neutral leg on single phase applications.

 

[kschachn]

OK, help out this BSME who thought he didn't sleep through physics. I understand that if the waveforms are 180 degrees out of phase the voltage would be zero. But are the waveforms shifted in time then? Or they are shifted respect to ground?

Nevermind... Like this?

 

[SHOZ]

Here's another view.

How Does a Transformer Work?

 

[kschachn]

OK, that's good too. Maybe I'm just too stuck on the waveforms when I shouldn't be.

Originally Posted By: SHOZ
Here's another view.

How Does a Transformer Work?

 

[leeharvey418]

Originally Posted By: SHOZ
How Does a Transformer Work?

It goes "HMMMMMMMMMMMMMM".

I actually got that joke from my Physics lab TA in college. He got his doctorate in EE at the end of that semester.

 

[exranger06]

This is what the waveform looks like:

Wire 1 is one hot leg, Wire 2 is the other hot leg.

 

[Dave Sherman]

Thank you, Exranger 06, your graphic is exactly what I'm trying to illustrate, and kschachn's graphic is correct.

Sorry badtlc, but your oscilloscope example won't show two sine waves in phase, and kschachn's graphic shows why. One leg is going more negative as the other leg is going more positive. If they were in phase, you would get zero volts, not 240.