single phase and the Nuetral

[jwhit]

I have been reading in the basic electricity book that the reason on a 3 wire 240vac system that theres no current on the neutral is because the two phases are apposing each other and cancelling out the voltage on the line. In other words one phase is at zero when the other is at full voltage. The problem I have with this theory is L1 and L2 are not 180 degree out of phase with each other? Is this a split phase circuit and does it require using L1 and L3 to work? I am confused.

 

[charlie b]

. . . the reason on a 3 wire 240vac system that there's no current on the neutral . . .

There is current on the neutral, if the loads are not balanced between the two phase conductors.

Suppose, for example, you have a 3-wire circuit supplying lights. For simplicity, let us say that on Phase A (I think this is the same as your "L1") you have two fixtures, each having a single 120 watt bulb. Let us say that on Phase B (I think this is the same as your "L2") you have one fixture, and it has a single 120 watt bulb. When any single bulb is turned on, it will draw 1 amp from a 120 volt source.

If you turn on one of the Phase A lights and you turn on the Phase B light, the loads are balanced. One amp of current will flow from the source along L1, go through the Phase A light fixture, then go to the point at which the common neutrals for the fixtures are connected to L3. From there, current goes ("backwards") through the Phase B light fixture, and finally goes along L2 back to the source. There will be no current returning to the source via L3.

That's what happens on the first half-cycle. On the other half cycle, current goes from the source via L2 to the Phase B light, and follows the path I describe above, in the reverse sequence.

Now turn on the second Phase A light. Load is no longer balanced between the two phases. You will get the exact same 1 amp flowing in the exact pattern I described above. But in addition, you will get one more amp flowing from the source, through the Phase A light, and back to the source via the neutral (L3). So if you used a clamp on ammeter, you would see 2 amps on L1, 1 amp on L2, and 1 amp on L3.

. . . is because the two phases are opposing each other

That is true.

. . . and canceling out the voltage on the line.

I am not sure I understand what you mean here.

. . . In other words one phase is at zero when the other is at full voltage.

Not true. They both hit zero at the same time, and they both hit peak values at the same time. But one is hitting a positive peak at the same time the other is hitting a negative peak. In other words, when they hit zero together, one is about to go positive and the other is about to go negative. That is what is meant by saying they are in opposition.

The problem I have with this theory is L1 and L2 are not 180 degree out of phase with each other?

But L1 and L2 are indeed 180 degrees out of phase with each other. So why does the theory not work for you.

Is this a split phase circuit and does it require using L1 and L3 to work? I am confused.

If you don't have the neutral wire, and if the loads are perfectly balanced (as described above), then you would not notice a problem. But lights get turned on and off all the time, so the load will not stay perfectly balanced.

In my circuit, suppose you don't use the neutral wire. Suppose that only one light is turned to the "On" position. You will not have a complete circuit, and the light bulb will not illuminate. But if you turn on all three lights, then you will get only 80 volts across the pair of Phase A lights (so they will be dim) and 160 volts across the single Phase B light (so it will be very bright, and it might explode).

The neutral wire must be present, so that it can return the unbalanced current back to the source.

 

[jwhit]

The phase difference between L1 or phase A and Phase B is that 180 Degrees? I thought the phase difference between phases was 120 degrees.

So the neutral acts as the return path when the loads are not balanced because the opposing currents are not equal.

 

[69boss302]

Are you confusing things a little. In any three "phase" system, the "phases" are 120 electrical degrees apart. You have three wires A,B, and C. Then you have three "phases" A-B, B-C, and A-C. Each of these will have 120 VAC between the wires and the "phases" will be 120 electrical degree's apart.

You essentially have two live leads when you use a 120VAC three phase system. That is why it is undesirable. To isolate anything you have to use two pole switches (because you must open all non grounded conductors). Makes equipment bigger, larger potential for getting electrocuted.

Edit:
I think I'm the one that's confused. I was going to just delete all this but that would seem inconsiderate and bad forum eticet and I don't want the forum police on me (electricmanscott)

If you are taking 120 VAC off of half of a 240 VAC line yes as long as there is another load on the other half of the 240 then the neutral carries nothing. But as Charlie said, you have unbalanced loads and other things that influence the current on the neutral.

Dang, I can't even spell well enough for the spell checker to correct me.

 

[charlie b]

A 3-wire 240 volt system is called a "single phase" system. It really has two phases, and they are 180 degrees apart. That causes them to look like the same phase, when they are connected in the normal way.

 

[eric stromberg]

Charlie,
I have a little conceptual difficulty with the statement that the two ungrounded conductors are 180 deg apart. I like to think of them as being the same phase. Here's my line of reasoning.

The secondary of a transformer is a coil of wire around a core. For the sake of simplicity, let's say there are 240 turns (coils) in the secondary. This gives us 1 Volt per Turn for this transformer. As the magnetic flux travels through the core, these turns are each excited to the tune of 1 Volt. There is a center tap in this secondary. A circuit that is connected from one end to the center is connected to 120 turns. It sees 120 Volts: 1 Volt per turn. If a circuit is connected to the ends of the windings, it is connected to 240 turns. At 1 Volt per turn, this gives 240 Volts. All of these turns are in phase with one another. The center tap merely accesses half of the available voltage.

Eric Stromberg, P.E.

 

[roger]

When addressing a single Phase system we are better off using the term "Legs" in lieu of "Phases", it will simplify visualizing the 180 deg's

With that said, here is a graphic of a single phase neutral carrying the imbalanced current.

Roger

 

[charlie b]

I have a little conceptual difficulty with the statement that the two ungrounded conductors are 180 deg apart. I like to think of them as being the same phase.

Person 1 says, "The grass is green." Person 2 says, "No, the sky is blue." Which person is right?

Saying the two sine waves are "in phase" or saying they are "180 degrees out of phase" is a matter of convention. First, you pick your starting point. Next, you define which direction of current you chose to call "positive." You and I have merely picked different conventions.

 

[Pierre C Belarge]

For the non-engineer type in our industry, we need to keep this in working order as simply put as possible. Roger's post with the illustration makes this a lot easier to understand.

I try to teach the different types of systems that are in use today just for this reason... having illustrations makes it visual and helps with the conceptualization of it very well.

 

[eric stromberg]

Charlie,
Well said.

I thought about this last night after i wrote the response. Here's another way of looking at it. Imagine that each coil of the transformer is a battery. Imagine a string of batteries end to end. If you start in the middle, and call it Zero, if you look down the string one way you'll see a positive voltage with respect to the starting point. If you look down the other way, you'll see a negative voltage with respect to the starting point.

It is simply a matter of convention. But it is still only a single winding. It cannot be out of phase with itself.

Eric

 

[Pierre C Belarge]

Eric
If each leg is not "out of phase" with each other, than how do we get the voltage potential from one leg to another?

 

[eric stromberg]

Pierre,
Charlie's right. It all depends on your starting point.

If you start at one end, and call it zero, when you get to the middle you'll measure 120 Volts. When you get to the other end, you'll measure 240 Volts.

If you start in the middle, if you look one way you'll see 120 Volts. If you look the other way, you'll see 120 Volts. (We can call this second voltage negative because we are looking in the opposite direction).

If i had a graphics program, i'd draw some diagrams and post them.

Think of a simple graph, slope of 1. Imagine the line starting at the origin and proceeding to 240,240. It is the same line, the value of it is 0 on one end and 240 on the other end. At the midpoint, the value is 120. Now, imagine the same line, but move it so that the midpoint is at the origin. If you go to the right, the end of the line has a value of +120. If you go to the left, the end of the line has a value of -120. It is the same line. It is completely straight, and the slope is consistent for the whole distance of the line.

Eric 8)

 

[charlie b]

If i had a graphics program, i'd draw some diagrams and post them.

No need. Just look at Roger's diagram.

Let me label three points on that diagram. You will note that there are nine purple arrows, in three rows and three columns. Let me call the top row, left arrow (on leg 1, pointing to the right) "A." I will call the center row, left arrow (on the neutral leg, pointing to the left) "B." Let me call the bottom row, left arrow (on leg 2, pointing to the left) "C."

Now, when you want to talk about the relationship between the voltages in the two legs (the 120 volt stuff, not the "leg 1 to leg 2" voltage of 240V), you can compare:

• Voltage "A-B" with Voltage "B-C," or
• Voltage "A-B" with Voltage "C-B," or
• Voltage "B-A" with Voltage "B-C," or
• Voltage "B-A" with Voltage "C-B."

Two of these comparisons will show the pair of voltage signals being in phase with each other. The other two comparisons will show the pair of voltage signals being 180 degrees out of phase with each other.

I will leave it as a homework assignment to determine which is which. :wink: 8)

 

[eric stromberg]

Charlie,
If a person is well versed in the inner workings of transformers, he/she can get by with referring to things in whatever manner gets the point across. But when trying to understand the concepts, wording is very important.

In the part of the electrical world that i see, transformers seem to be very misunderstood pieces of equipment.

If the same magnetic flux excites all the coils in the secondary at the same time, how can some coils be 'out of phase' with other coils in the same secondary? This is impossible. All the coils of the transformer rise and fall in voltage at the same time. Get a dual-trace scope and put the probe shields on one endpoint of the secondary coil. Now, put probe A on the other endpoint and probe B on the midpoint. You will see two sin waves starting at the same point, crossing zero at the same point, ending at the same point; perfectly in phase with one another. The only difference you'll see is that sin wave A has twice the amplitude that B has.

Of course, if you put the probe references on the midpoint and probe A on one end and probe B on the other end, you'll see what you describe. Two sin waves that appear to be in opposition to one another. But, as you also described, the only reason one looks backwards (180 deg out of phase) is because this is the way we connected the scope leads.

You're, of course, correct in everything you say. I just don't like the phrase "out-of-phase" because i think it leads people down the wrong mental path and i think it inhibits the proper understanding of what is really going on.

Eric, just-a-little-out-of-phase, Stromberg :wink:

 

[dlhoule]

Well, I am one who does not understand everything about xfmrs or much of anything for that matter. Can one of you very knowledgeable people explain to me just how a Scott Transformer works.

 

[charlie b]

If the same magnetic flux excites all the coils in the secondary at the same time, how can some coils be 'out of phase' with other coils in the same secondary? This is impossible.

Not at all. The only thing you would have to do is to wind two secondary coils in opposite directions (i.e., left-hand wind and right-hand wind, or if you prefer, CW and CCW).

But you don't even have to do that, because . . . ,

Of course, if you put the probe references on the midpoint and probe A on one end and probe B on the other end, you'll see what you describe. Two sin waves that appear to be in opposition to one another.

. . . that is how we normally build single phase transformers.

A common version of a single phase, 120/240 volt transformer will have a single core, a single primary winding, and a single secondary winding that is center-tapped to ground. The same flux from the same primary is imposed on both halves of the secondary winding, and has the same influence on each. We connect the neutral to the center tap, and a hot leg to each end. But we measure voltage of one leg as from neutral (i.e., zero reference voltage) to one end of the secondary winding. Then we measure voltage of the other leg as from neutral (i.e., zero reference voltage) to the other end of the secondary winding. If you show both of these on the same oscilloscope, you will see two sine waves 180 degrees apart.

 

[eric stromberg]

dlhoule,
"Of this, i have no knowledge."

Quote from Star Trek III, The search for Spock. :roll:

Eric

 

[johnny watt]

When we hook split phase up to loads in our homes we hook the same side of the load up to neutral regardless of which leg we get the 120V from. So it follows ? that is the convention to use when comparing the two voltage signals to judge them as in phase or not.

Definitions cause me a lot of confusion. If it takes two phasors with two separate phase angles to describe the voltages then I would have guessed that that should be called 2-phase. Of course, such is not the case.

Another naming convention that is confusing stems from the following.
You here the terms phase to neutral and phase-to-phase. But when you use the A phase and the B phase on a 208V load it is still called single phase

 

[rattus]

From another post:

It depends on where you connect the common of your phase meter.

120V /180 <----------N---------->120V /0

This is similar to the 3-phase wye phasor diagram.