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-   -   240v? how does it work? (http://www.diychatroom.com/f18/240v-how-does-work-9601/)

gooch 07-04-2007 01:51 AM

240v? how does it work?
 
i am a 22 year old 02 journeyman electrician. i have been in the trade for like three years and some months and have been a journeyman for like a month. being a journeyman im a little embarrassed asking this, but could some of the experts explain to me how exactly 240v works with without a neutral. how does the current flow without a path to ground. i know it has something to do about the two phase legs being out of phase with each other and cancelling out or something like that. this question plagues my mind and if someone could explain it in detail that would be great. thanks alot.
-gooch

ben23 07-04-2007 07:06 AM

well im australian, and we have single phase 240v here for general purpose lighting and power, but we have multi-phase supplies (415V) available in most homes for A/C etc. If you measure between the two 120V phases u will measure 240v, there is 240v between phases, just like we have 415V between phases. I am not completely sure how it works, it has something to do with the potential.

JohnJ0906 07-04-2007 08:15 AM

Gooch, what voltage is, is potential between 2 points. Whether one is "neutral" or not doesn't matter. There are several forums out there for electrical pros, where you can pick up a lot of knowledge. (I'll PM you)

slakker 07-04-2007 11:55 AM

Basically, a neutral is 0 volts and so is ground. A typical home has 2 phases, each 120 v AC as measured relative to neutral/ground. One way to imagine it is to think of one phase as fluctuating from 0 to +120 v and the other phase fluctuating from 0 to -120 v (as it is AC it's not really minus 120v it's just 120v). But if you took the 2 phases and shifted them by 180 degrees (so shift the sine wave) you'll see that as one AC circuit goes to +120 v, the other one goes to "-120 v", hence your 240 v difference between the 2 phases...

Similarly with 3 phase circuits where the sine waves are shifted by 120 degrees hence you can set up "Y" or "delta" configurations and get 208 v.

I hope I didn't butcher up that explaination... :wink:

tribe_fan 07-04-2007 12:10 PM

I think this is the theory -( I'm not a pro - but took a class a long time ago )

If you look at the pole - you will see a pair of wires on top - this voltage is " a lot", but I'm not sure exactly what it is.

From there it will go a transfomer - which may be on the pole, or on the ground. ( sometimes they used to be buried ?) These transformers "step" down the voltage and feed multiple houses. The input is "a lot" and the output is 3 wires - one of them is a nuetral. The other two have a potential of 240. There would be 120 between the nuetral and the other wires. Thes are the wires that come into the house. The Ground is supplied by a grounding rod.

Take John's advice and learn your trade, you won't regret it !!

dmaceld 07-04-2007 04:19 PM

The quickest and easiest way to understand how 2 120v circuits add up to 240v is to find an explanation of "center tap transformer." The center tap is connected to ground and each side is 120v to ground and 240v to each other. Neutral is the designation for the ground level conductor in a 2 wire or 3 wire circuit. Ground generally means a separate conductor that physically connects to an earth ground. From meter to first panel box they are the same. From the panel box to all the circuits in the house they are separate.

And no, they are not out of phase with each other. To understand why, find a discussion of single phase versus 3 phase power.

Not to be harsh, but I am appalled that whatever training program you have been in has allowed you to reach the level you have without being grounded in the fundamentals of electricity. Please find a technical school, or community college, where you can take a course or two in electricity. You need it.

slakker 07-04-2007 04:53 PM

Quote:

Originally Posted by dmaceld (Post 51515)
And no, they are not out of phase with each other. To understand why, find a discussion of single phase versus 3 phase power.

Actually, in a centre tap transformer, the two 60 hz sine waves are 180 out of phase of each other...

dmaceld 07-04-2007 09:00 PM

Quote:

Originally Posted by slakker (Post 51519)
Actually, in a centre tap transformer, the two 60 hz sine waves are 180 out of phase of each other...

Well, yeah, but there's only one sine wave between the two end taps! Actually it's not quite that simple. Think about which way the current flows through the center and the two legs. Let's look at a balanced load first. When current is going into leg A, it is coming out of leg B, and vice versa, and no current travels through the neutral. Now if the load is unbalanced, the normal case in real life, say with leg A having the greater load, when current is going into leg A, some it comes back from leg B, and some from the neutral. In the reverse, some current goes to the transformer through the neutral, and some through leg B. It all comes back from leg A. So is the current flow really 180 out of phase between legs A & B? And of course, voltage pushes the current so it's following the same sine wave. Not as straight forward simple as we would like it to be, is it? :)

My intent was for him to get it clear in his mind what the general meaning is when you use the word phase in relation to electrical distribution, as in single phase vs. 3 phase.

What he also needs to get clear in his mind is you need the neutral to have 120v. Take the neutral out of the equation and you have 240v.

Stubbie 07-04-2007 10:36 PM

Don't you just love people with ego's the size of Jupiter! Last time I met somebody that thought he was so intelligent that he could decide who needs to go back to school I was gutting a panelboard for replacement that the intelligent person undersized in the spec drawings. If I remember correctly he even tried to point his finger at us tradesmen for a misread. That is of course till we showed him the print. Then of course it became a misprint. Oh for want of wannabees. I'm surprised we didnt' get bombed with credentials and licenses.
Now... I must say that I'm appalled that a person portraying himself as a knowledgable professional would be so inept at describing single phase 3 wire residential distribution so that gooch can understand it. Frankly that was a pitiful effort and I can only surmise that it was to impress and not teach.

BTW I would suspect gooch got the answer he asked for elsewhere where the internet police wasnt watching.

Stubbie

dmaceld 07-05-2007 10:26 AM

Quote:

Originally Posted by Stubbie (Post 51560)
... would be so inept at describing single phase 3 wire residential distribution ... I can only surmise that it was to impress and not teach.

Sorry if I came across as pontificating. I've been accused of that in the past occasionally. I have often encountered people who have progressed along the training route, be it school, technical trade, computer skills, some phase of engineering, or whatever, only to have missed learning some of the very fundamental principles upon which their knowledge or skill is based. This is what mystified and concerned me about gooch. How did the system allow him to achieve the position where his license entitles him to perform electrical work anywhere and for anyone he desires, single phase and three phase, and miss some of the basic knowledge about 3 wire single phase? A journeyman license does confer a lot of privileges and authority, does it not? It also signifies a high level of knowledge about the trade, does it not?

Please enlighten me. How is my description of 3 wire, 120/240 power inept? It does come from the wiring scheme of the transformer on the pole, right?

Stubbie 07-05-2007 01:18 PM

I doubt if Mike Holt himself could enlighten you. In one sentence in your previous reply you have proclaimed yourself able to judge school programs, computer techs, engineers and the skilled trades. Are you someone we should be familiar with in the professional world? I don't think I will expend the energy explaining anything to someone so well versed.

This journeyman asked a simple question, he didnt ask for an analysis of his learning program, how smart you thought he was nor what school you think he should attend. I have an extreme dislike for people that come on the internet deriding others in order to get a pat on their back. It is unfortunate gooch had to have someone like you drop in and steal his thread to argue with everyone trying to help him with his question.

BTW the technical term for the 180 degree offset in the waveform between the ungrounded conductors at the transformer is called a "phase shift".

If by chance gooch comes back to this thread I'll spend my time with him.

Stubbie

slakker 07-05-2007 01:39 PM

Quote:

Originally Posted by Stubbie (Post 51651)
BTW the technical term for the 180 degree offset in the waveform between the ungrounded conductors at the transformer is called a "phase shift".

Thanks for the correction to my inaccurate vernacular...

Stubbie 07-05-2007 02:49 PM

If you were to tell me that the two ungrounded conductors in a 3 wire residential supply were 180 degrees out of phase I wouldn't correct you. I would know what you meant, it is common for us to refer to it that way. I actually can't remember the last time someone referred to it otherwise. Instructors may take exception to calling it out of phase but in the field we just call it "out of phase". :wink:

Stubbie

johnny331 07-05-2007 04:23 PM

(with no arrogance :))

Read up on Kirchhoff’s law,

This is a simple center tapped transformer, like whats on the pole...

There are two circuits and current is constantly changing direction (60hz) but the one always flows the same way as the other (see the arrows), so just think of it as flowing constantly in one direction for now...

In theory, If the two legs had equal loads, the neutral's currents would cancel eachother out, eliminating the need for a neutral all together.

In reality, things are never totally balanced, so you need a neutral/ground to keep things balanced.


Remember, when looking at any diagram, current is a vector meaning it has it's magnitude, but it also has a direction.
So if you had 20A on the top circuit, and 15A on the bottom, there would be a 5A difference on the neutral. You have to measure everything relative to one point/direction

(ignore the little sine waves in the photos, they arent correct, they should be flipped between the red/blue arrows)

http://i74.photobucket.com/albums/i2.../untitledt.jpg

http://i74.photobucket.com/albums/i2...tx440/blue.jpg
http://i74.photobucket.com/albums/i2...ntx440/red.jpg


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