Understanding the unbalanced load. - Page 2 - Electrician Talk - Professional Electrical Contractors Forum

waco · 06-26-2009, 05:10 PM

Quote:

Originally Posted by p_logix

He goes by his own book.

Is there problem with the values on my book? You are aware that powers of ten are added when values are multiplied, so yes, 3Kw multiplied by 1 Kw is 3 Mw, or three times ten to the sixth. Got it? There will be a quiz.

You're funny.

raider1 · 06-26-2009, 05:11 PM

Quote:

There is no such thing as an unbalance in the 120 volt loads of a single phase, 240 volt system. They don't even know one another exist.

Please, explain this in greater detail with supporting evidence.

Chris

waco · 06-26-2009, 05:23 PM

Quote:

Originally Posted by InPhase277

The laws of physics don't need you to agree. If what you are saying is true, then the current in the neutral would be the sum of the currents in each leg. In reality, this isn't so. The neutral carries the difference between the two loads, so some current from each load must travel through the other load.

There is no need to argue this when a test is easily done. I have done it many times for my apprentices. All you need is two 100 W lamps and a source of 240 V, and a clamp on ammeter.

I doubt that you have since the test you describe would require some awfully sophisticated measurements. However, the current flow can be analyzed by realizing the two "hot" legs are 180 degrees out of phase and their current is never additive on the neutral leg.

I just read Chris' post and so I'll add this: There is no valid technical meaning for "unbalanced" in the context of a single phase, center tapped transformer source such as that which powers single phase services. It can be useful where a 240 volt load burdens one "leg" with 120 volt loads (such as motors and timers on stoves and dryers,) but that is not a multiwire branch circuit.

The best way to explain the center tapped sources used in residential services is to envision the secondary with a center tap at the extreme of one cycle excursion. One end of the winding will be fully positive and the other end will be fully negative and the center tap will be negatove with respect to the positive end and positive with respect to the negative end, but zero with respect to itself and ground. Use arrows to demonstrate current flow, negative to positive and notice that current cannot flow both ways on the neutral at the same time, so the neutral is serving as a return leg for half the transformer while the other half is using that half and not the neutral as a return path.

If that defines "unbalanced," then so be it, but to me, a 240 volt load across the secondary defines "balanced" and any 120 volt loads "tapped" at the secondary create an unbalance, but they are part of a three wire 240 volt circuit, not a multiwire 120 volt circuit.

Been a very long time since I taught this stuff, so excuse me if I don't get too excited about it.

But, I really don't care that much. The circuit theory of a center tapped transformer proved one of the most difficult for students to grasp, I guess because the center tap is positive with respect to one end while at the same time, negative with respect to the other end.

But, I really don't care. I'm somewhat surprised by the insults. Guess I mistook this for a board of professionals.

5volts · 06-26-2009, 05:38 PM

Quote:

Originally Posted by waco

Is there problem with the values on my book? You are aware that powers of ten are added when values are multiplied, so yes, 3Kw multiplied by 1 Kw is 3 Mw, or three times ten to the sixth. Got it? There will be a quiz.

You're funny.

Quote:

Originally Posted by waco

Is there problem with the values on my book?

paul d. · 06-26-2009, 05:38 PM

waco sure is sensitive.

**480sparky** · 06-26-2009, 05:58 PM

Quote:

Originally Posted by paul d.

waco sure is sensitive.

Call him want you want. I have monikers that are MINE!

brian john · 06-26-2009, 06:14 PM

Quote:

Originally Posted by waco

I have probably forgotten more electronic and electrical theory than you have ever known, but, if insults are all you have left, feel free.

I know you are very smart and INSULTS are all I have left.

Quote:

Fact is, my ego doesn't demand I be right at all and I'm completely at ease with the uncertainties of the trade and I still learn something new every day. But not about meggers and not about center tapped, single phase sources.

No your ego does not demand you be right, but in your own convulated way it drives you to stir the pot.

raider1 · 06-26-2009, 06:24 PM

Quote:

It can be useful where a 240 volt load burdens one "leg" with 120 volt loads (such as motors and timers on stoves and dryers,) but that is not a multiwire branch circuit.

Actually it is, read the definition of branch circuit, multiwire in Article 100 of the NEC.

Any feeder that contains a neutral conductor is a multiwire circuit.

So the neutral conductor of a 240/120 volt service will only carry the unbalanced current between the two ungrounded conductors.

Chris

InPhase277 · 06-26-2009, 07:00 PM

Quote:

Originally Posted by waco

I doubt that you have since the test you describe would require some awfully sophisticated measurements. However, the current flow can be analyzed by realizing the two "hot" legs are 180 degrees out of phase and their current is never additive on the neutral leg.

Do the test. Wire two 100 W lamps in series and connect the ends to 240 V. Then take a neutral and connect it to the center point. Clamp your ammeter around the neutral. If the two lamps are equal wattage, there will be zero current in the neutral. If you disconnect the neutral with the circuit energized, you will notice no change in the brightness of the lamps.

On the other hand, if you connect two lamps of different wattage like this, you will measure a current on the neutral that turns out to be the difference of the currents in the two hot legs. So if you had 3 amps on A, and 2 amps on B, then the neutral would carry 1 amp, which is exactly equal to the amount of UNBALANCED current from the two unequal loads.

Like I said, this is readily testable. We're not talking about something mysterious here. You can set up an experiment in five minutes on your work bench and prove it. You are arguing not with just men on this forum, but with fundamental laws of nature and over 100 years of PROVEN electrical theory. Do the experiment.

Quote:

But, I really don't care that much. The circuit theory of a center tapped transformer proved one of the most difficult for students to grasp, ...

No wonder they didn't get it. The guy teaching the class didn't get it either.

ralpha494 · 06-26-2009, 07:13 PM

Waco, here's a quote from Miami Bob that you might like.

"We're not arguing. He's arguing. I'm just being right."

InPhase277 · 06-26-2009, 07:22 PM

Quote:

Originally Posted by waco

Is there problem with the values on my book? You are aware that powers of ten are added when values are multiplied, so yes, 3Kw multiplied by 1 Kw is 3 Mw, or three times ten to the sixth. Got it? There will be a quiz.

You're funny.

And here is a fundamental flaw in your understanding of units and multipliers. I' not going to delve into the multiplier argument again, but I will say that if you multiply kW x kW, you will not get MW. You will not get anything even related to watts. The unit you will get would be in dimensions of "square ohms", whatever that is.

waco · 06-26-2009, 07:26 PM

More insults. The demonstration you use is for a 240 volt circuit with loads in series. Proves nothing relative to the theory involved, but I suppose it makes an impressive demonstration.

I realize the NEC defines multiwire circuits as stated. My point means to address a fine point in theory, one which many find confusing and one which the NEC has possibly confused as well.

The guy teaching the class was carefully supervised by the school administrators who were also marines, so I guess the guy teaching the class got it well enough.

If you would like, I will explain why your demonstration fails to make the point I think you wanted to make, but the key is in the definition of the terms. Point is, I don't really much care.

How we view the definitions doesn't change a thing about the theory and how we understand the theory doesn't change a thing about how it works.

bobelectric · 06-26-2009, 07:30 PM

In theory,3 wire circuits are more efficient, more cost effective,as in using conduit as the ground path.With the evolving technolgy,such as G.F.I.s and Arc fault code requirements,I pull a 4wire or 2 separate runs.

InPhase277 · 06-26-2009, 07:37 PM

Quote:

Originally Posted by waco

More insults. The demonstration you use is for a 240 volt circuit with loads in series. Proves nothing relative to the theory involved, but I suppose it makes an impressive demonstration.

Exactly. Which is precisely what a multiwire branch circuit is. The loads are in series across 240 V, but the center is wired to a neutral.
I've attached an image drawn by Stubbie.

Quote:

The guy teaching the class was carefully supervised by the school administrators who were also marines, so I guess the guy teaching the class got it well enough.

Oh, well, I guess I forgot that Marines wrote the book on electric circuits. The laws of nature are firm, unless a Marine says otherwise.

Quote:

If you would like, I will explain why your demonstration fails to make the point I think you wanted to make, but the key is in the definition of the terms. Point is, I don't really much care.

Well, if you can muster it, I'd love to hear it.

Quote:

How we view the definitions doesn't change a thing about the theory and how we understand the theory doesn't change a thing about how it works.

You're damn right on that one.

5volts · 06-26-2009, 08:07 PM

Quote:

Originally Posted by waco

been a very long time since i taught this stuff, so excuse me if i don't get too excited about it.

Quote:

Originally Posted by waco

the circuit theory of a center tapped transformer proved one of the most difficult for students to grasp......

..

...

**480sparky** · 06-26-2009, 08:10 PM

Quote:

Originally Posted by p_logix

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Careful posting photos like this: You'll get branded as a photo faker.

nick · 06-26-2009, 09:19 PM

Well read this all the way http://books.google.com/books?id=sxm...esult&resnum=5

brian john · 06-26-2009, 09:49 PM

Quote:

Originally Posted by 480sparky

Careful posting photos like this: You'll get branded as a photo faker.

That image was taken from a episode i saw. It is real

waco · 06-26-2009, 09:59 PM

Obviously, few here are prepared for conversational discussion. I should repeat, the theory was never in question. The definition was in question. Try as I will, I can't imagine any 120 volt residential loads being "balanced."

Yes, three wire circuits are more efficient, but there are far too many sensitive devices whose use is mandatory for us to keep using them.

InPhase277 · 06-26-2009, 10:42 PM

Quote:

Originally Posted by waco

Obviously, few here are prepared for conversational discussion. I should repeat, the theory was never in question. The definition was in question. Try as I will, I can't imagine any 120 volt residential loads being "balanced."

Yes, three wire circuits are more efficient, but there are far too many sensitive devices whose use is mandatory for us to keep using them.

Now the tune has changed. Whereas before, there was no such thing at all as unbalanced current, now it is a matter of imagination.