Current Flow 3

[BigJohn1]

First note, I'm an electrician and not an engineer, so if these questions are a bit low-brow for this site, I appologize.

However, I'm having a diffricult time finding decent explanations for why current behaves the way it does. First things first, I don't understand how neutral current can exist. My understanding is that when a difference in potential exists, current will flow. If you have a service supplying 120 volts to a load and that load utilizes those 120 volts completely, how then is there any potential left to cause neutral current to flow back to the service?

Also, in a system supplying a balanced load my understanding is that current flows back along the legs of the opposing phase(es). For the sake of simplicity I'll use a single phase, 240V system where the two hot legs are 180 degrees out of phase. If there is current flowing back along the opposing legs, am I to assume that because this current is 180 degrees out of phase with the leg it is returning on, the return current would simply be subtracted from the available power on that leg?

Finally: Often, the flow of electricity is likened to the flow of water, how close is the actual relationship of the movement of electrons to the movement of fluid? A voltaic potential will exist because of an difference in electrons between two points, and current will flow, much like the a difference in water levels will cause water to flow. Does current really "try to return to the source (generator)" and and this is because the generator is forcing electrons out one pole, as such leaving a void to be filled by incoming electrons, so to speak? Does this explain why the current does not flow to ground on a transformer that is ungrounded?

Any help at all would be greatly appreciated. If you don't feel like trying to answer my long-windedness, any suggestions of books or web pages or anything that might have more in depth explanations of electrodynamics would really be great.
Thanks a bunch. -John
(For those willing to help, I have no objection at all to mathematics, but no understanding of calculus.)

 

[dpc]

John,

You're quite correct - a potential difference must exist whenever current flows. For a typical 120V circuit, there will be a small voltage drop in the neutral circuit from the load all the way back to the source transformer. But the impedance of the wiring is small, so the voltage used in this portion of the circuit is small as well. So the load does not "use up" all of the 120V. Some of the voltage is used to drive current through the impedance of the wiring.

The analogy between current flow and liquid flow is sometimes carried a little too far. Individual electrons do move, but they move very slowly along a wire during current flow. For a current to flow, there must be a complete conductive path.

Hope that helps. A quick search on Google should give you lots of useful links on basic electrical circuit theory.

Good luck!

 

[busbar]

Consider conductor spans {phase and neutral} as fixed resistances/impedances. As current though the spans increases, so does voltage drop. Aside from harmonic content, an imbalance in phase-to-neutral voltages or load phase currents will cause an increase neutral current.

Characteristics of voltage drop can be demonstrated with some {~150-foot} coils of 14AWG building wire, a hair dryer and a voltmeter. At the coil ends, the per-conductor voltage difference can be directly measured. If left to run for awhile, heat generation in the coils will be noticeable. Ohm’s law can be applied to the measurements to approximate power dissipation and ohmic impedance in the long conductors.

The grounded-neutral conductor feeding the hair dryer could be viewed as a neutral in a 4-wire wye circuit, with “zero load on two phases.”

 

[alehman]

I hope this doesn't need any calculus.
To answer your 2nd question, envision a sine wave on one phase (X1) increasing from 0, while at the same time a sine wave on the 2nd phase (X2) begins decreasing from zero. This is the 180 degree separation. If you assume (as most people do) current flows from positive to negative you can imagine current will flow from X1 to X2 during the first half cycle.

Also as X1 begins to incraese, current will also flow from X1, through loads connected line-to-neutral and return via the neutral (which is at roughly 0 volts). This current will be in phase with the current flowing into the line-to-line load and therefore adds to the total X1 current.

Similarly as X2 begins to decrease, current will flow from the neutral (which is positive with respect to X2 for the first half cycle), through the load and back to X2. The currents for the line-to-neutral loads therefore cancel each other in the neutral.

Hopefully this doesn't just add to the confusion.

 

[bigbillnky]

John,
Hello, good questions for an apprentice. First ignore BJC, he seems more interested in criticizing than helping you. I myself am not an engineer either, but a licensed master electrician and I was misled by journeymen when I was an apprentice, so I understand. Back to your questions. First you must understand that Ohms law applies to purely resistive circuits, and makes no consideration for inductive reactance or capacitive reactance. Inductive reactance causes current to lag voltage, and usually has a greater effect on current than capacitive reactance. Capacitive reactance causes current to lead voltage, and almost all AC loads are inductive. This causes current to be out of phase with the voltage. Therefore, a load that exhibits inductive reactance and is connected to one phase will cause the current that it draws to change phase angles in relation with the current flowing in the other phase. This results in a higher neutral current flow due to the imbalance and change in phase angle. Next, voltage is never completely used up by the load, it must always have enough potential to overcome the slight resistance in the conductors returning to the transformer. Remember, voltage drops thru a load, but current is constant(the same) in any part of a circuit. Finally, a better analogy than flowing water is marbles in a tube. The physical aspects more closely resembles the properties of electrical circuits. I would recommend "The American Electricians Handbook", ISBN 0-07-013936-9. Good luck and all the best. bigbillnky

 

[jbartos]

Suggestion to EPAC (Electrical) Aug 8, 2003 marked ///\\
However, I'm having a difficult time finding decent explanations for why current behaves the way it does. First things first, I don't understand how neutral current can exist.
///Do not feel lonely. There are many others who do not understand neutral current existence.\\ My understanding is that when a difference in potential exists, current will flow.
///Yes. Correct. Start right here. The current can flow in the neutral if there is a potential difference between some points in the neutral conductor, e.g. between a panel neutral bus and loads connected via this neutral at some junction box or sub panel, e.g. lights, fans, etc.\\ If you have a service supplying 120 volts to a load and that load utilizes those 120 volts completely, how then is there any potential left to cause neutral current to flow back to the service?
///Recall Kirchhoff's Laws. One is for currents, Kirchhoff's Current Law (KCL), stating that all currents flowing to one point must equal to zero. Alternately, all summed currents flowing to one point equal to zero, e.g. I1 + I2 + I3=0. Similarly for voltages, Kirchhoff's Voltage Law (KLV), along one circuit: the sum of all voltages in the circuit is equal to zero, e.g. V1 + V2 + V3 = 0.
The 120V potential to the load is not utilized completely. If the switch is on, the 120V potential will stay across the load, e.g. bulb, and Vbulb + Vpanel=0 by the Kirchhoff's Voltage Law (KVC). Then, the Kirchhoff's Current Law (KLC) justifies the flow of current in the neutral since the circuit must be closed, (switch must be closed) for the current to flow in the closed circuit.\\
Also, in a system supplying a balanced load my understanding is that current flows back along the legs of the opposing phase(es).
///Yes, correct for delta connected three phase load, as well as wye connected three phase load.\\ For the sake of simplicity I'll use a single phase, 240V system where the two hot legs are 180 degrees out of phase.
///Here is the problem. Two legs are not 180 degress out of phase. If this were so, there would be two neutral currents returning to the panel and to the transformer. Therefore, the neutral conductor would have to be twice as large as either hot leg, which is not the case. Utility brings triplex to the building of the same conductor size. Therefore, the hot leg voltages are in phase and the current in the neutral may cancel to zero, if the loads are exactly the same at some point, e.g. one 100W bulb is connected between one hot leg and neutral and the second 100W bulb is connected to the second hot leg and neutral. If the neutral is cut, the both 100W bulbs will light on 120V+120V=240V, e.g. they will be connected in series to 240V. By the KVL Vbulb1+Vbulb2=120V + 120V = 240V.\\ If there is current flowing back along the opposing legs, am I to assume that because this current is 180 degrees out of phase with the leg it is returning on, the return current would simply be subtracted from the available power on that leg?
///This is a very good question. If the current is flowing back in the opposite leg, it is the current which may be flowing from the neutral or from the other leg, since the transformer is center tapped and the winding is not wound in opposite direction to each leg from the neutral point (center tap) to avoid the neutral conductor to be twice as big as the each leg conductor. Just recall the triplex runs to buildings, e.g. houses. Also, the 180 degree out of phase, would lead to 120V - 120V = 0V between legs according to KVL. This is not true, since there is 120V + 120V = 240V between two hot legs. Therefore, the 120V voltages between hot legs and neutral must be in phase.\\\

Finally: Often, the flow of electricity is likened to the flow of water, how close is the actual relationship of the movement of electrons to the movement of fluid?
///It is approximately close. Electrical current is material based, it has a matter substance in terms of electrons, neutrons, and protons. All move and flow.\\ A voltaic potential will exist because of a difference in electrons between two points,
///Yes, correct.\\ and current will flow,
///Yes, correct.\\ much like the difference in water levels will cause water to flow.
///Yes, approximately so.\\ Does current really "try to return to the source (generator)"
///Yes, indeed.\\ and this is because the generator is forcing electrons out one pole, as such leaving a void to be filled by incoming electrons, so to speak?
///Somewhat, but not quite. The generator creates the potential difference across its terminals. This potential difference will cause the current flow, by KCL, when the circuit is closed, i.e. there is some load connected to the generator terminals.\\ Does this explain why the current does not flow to ground on a transformer that is ungrounded?
///Yes. It does. The circuit is not closed; therefore, the KCL holds true, and the current does not flow to the ground (except some small current caused by leakages, that are possible to see on the high voltage insulators during rainy weather).\\

 

[BJC]

EPAC
I wasn't meaning to be condescending, I ask some legitamate questions that you still did not answer.
As an engineer most of the projects I design are built by electricans. The majority of them are good and would know the answer to what you ask. My economic well being depends on good craftsmen who do good work. The best designs in the world can be messed up by "wiremen" who got their ticket running romex in track houses.
I have taught in apprentice classes. I have never been condenscending, I have always had good students that were willing to learn. Classes are always fun when you have good eager students. There are a lot of enigineers who post on this forum who rely ( wheather they know it or not) on electricans to turn a design into reality.
We usually cover the question you asked in about 20 minutes. jbartos did a good job, you can imagine how well he could do with a white board. It's not that hard.
Getting to be an electrican can be difficult. You can apprentice with the IBEW, go to a trade school, service ( the navy turns out very good electricans). Most I have know would know the answerr to what you ask.

Bigbillnky-
Over the last 35 years I have seen electricans change from people who got in the union because their daddy was to a better educated, highly trained workforce. Most have the equivalent of an AA degree and many have additional training. There's a lot of equipment out there that didn't exist 30 years ago. Being an electrican in many states requires continuing education. Things are changing fast, you can
t stand still. Good electricans ( and well paid ones ) are a good value, there worth what they get paid, remember nothing cost as much as cheap help. I would think you would be concerned about the "dumbing down" of the trade.
I'm concerned about it, hence my questions.

 

[DanDel]

EPAC, I am an electrician and an engineer, and it is good to hear questions from someone who is willing to admit that they don't understand something, and can phrase their questions in a logical manner.

First off, just because there is a difference in potential, there is not necessarily current flow. An open switch will have 120V across the contacts, but no real current is flowing. dpc's explanation for your first question is accurate enough.

Your second question about balanced load current returning in the neutral of a 240V single-phase circuit has one flaw; you are confusing current flow with power flow. You know that, in an AC circuit, the current flow alternates at 60Hz. The instantaneous current(the current at any instant of the sine wave) in your neutral conductor is always 180° out of phase with the line current. That is why your clamp-on ammeter will show zero amps when it is used on both conductors. The current flows through the load, and the majority of the voltage is dropped across the load, and that's how the power is used(the current flowing through the load, multiplied by the voltage drop across the load, equals the power used by the load).

Third question; yes, the current must return to the source. This is why you can't get a bulb to light up by connecting one terminal to the '+' side of one battery, and the other terminal to the '-' side of another battery. There must be a complete circuit for current to flow, and, yes, that is exactly why current does not flow to ground on a transformer that is ungrounded.
The main difference in theory between the flow of water and the flow of electricity is that water flows at different speeds to account for changes in current, while electricity always flows pretty much at the speed of light, and the amount of electrons flowing will change for a change in current. As far as a load, you can imagine a water wheel turning by the flow of water. The more current flowing, the more force on the wheel, and the more work the water wheel can do, just like electrical current through a motor.

Don't forget, and I'm sure some other posters will tell you, that these are the simple electrical theory explanations for your questions. I won't confuse you at this point (and I hope others won't continue to, either) by throwing in more advanced concepts that you will, I'm quite confident, understand later. Good luck and post back with any more questions or if you need any more clarifications.

Why don't you think about taking college courses on these subjects, you might find out that you'd be happier as an engineer, after all.
I certainly am.

 

[Jerry123]

Try

http://www.sea.siemens.com/step/downloads.html

Read Basics of electricity Parts 1 and 2. It may give you some insight.

 

[jbartos]

Suggestion: Visit

http://www.cinergycge.com/Business_Services/service_request/electric_service_manual/default.asp

etc. for more info; especially, the section 6

 

[SidiropoulosM]

EPAC: a star goes to you for not being afraid to ask simple questions and for not being intimidated by certain snobs. I am an engineer with a Master's degree and often find that an attempt to answer the most basic questions can challenge my own understanding.

 

[alehman]

I second SidiropoulosM's comment. Well formulated basic questions can be a valuable discussion. I dislike many of the recent one sentance questions which are clearly not thought out or are lacking obviously needed information.

I'm not sure BJC intended to be as harsh as he came across in his first post.

 

[BJC]

alehman
Iwasn't trying to be harsh on the guy. He started out as an electrican and then went to an apprentice real fast. We never have heard if he was a first year or what ever.
The group is "electrical power engineering" more than any other group( say computer or electronics) we rely on skilled knowledgeable craftsmen to get our designs turned to reality. It's a team effort. Over the years I have learned a lot from electricans. You'll hear lots of engineers say or write things about electricans ( that htere dumb, that the union is a mafia, they make rules to slow up and make work.), My experience is just the opposite. Labor costs are a minor part of most jobs. The big cost is equipment and materials. Poorly trained and inexperienced labor can break a job and cause it to go over budjet and over schedule.
I have a couple of more jobs to do before I get out of the business and when EPAC posted his question I want to check out the state of the craft in general and a particular area. tage

 

[alehman]

BJC - I agree with your last post. I am an electrical engineer with a BSEE and all coursework completed toward a Masters. I have ten years as a consultant and commissioning agent. I've seen a huge range of knowlege and diligence among contractors. Most I think have the best interest of their customer in mind. Obviously EPAC's questions were basic, but honestly they made me stop and think for a minute or two. Your concerns about the "dumbing down" of the trade are valid. You can argue that an apprentice should have a handle on this level of theory, but I think it is important for people such as EPAC to have a forum such as this where questions can be asked without fear of humiliation. It appears someone has red-flagged your first post and EPAC's reply.

 

[BigJohn1]

BJC,

I wasn't try to deceive anyone when I said I was an electrician. To me that term just describes people in that field, Apprentice to Master. Just like I know there are different types and probably different grades of engineer but I can't imagine that when someone asks you "What do you do?" You would respond with "Oh, I'm a second year hydrodynamical engineer with a BS in fluid mechanics." I think a lot of guys would say "I'm an engineer."

To answer your question I am a first year apprentice, I got into the trade in D.C. (Local 26) primarily because I've always been interested in the theory.

My experience is that simple explanations given to me usually don't hold up when you start looking closely. That's why I came here and asked the simple questions, because I was hoping to get relatively simple answers that would still be applicable even at very complex levels. My ultimate goal is to be able to paint a picture of how this [electricity] all works that would effectively explain it to someone who hasn't much of a clue about it but would still be a completely functional description to them by the time they were getting there PhD in electrical engineering.

Working around Washington I learned an incredible amount about putting in quality, good-looking, highly functionable installatons, but very little about the theory. I recently moved to northern NJ and have yet to get into the trade up here, but have seen a whole lot of shoddy work: Liquid-tight flex and PVC are used everywhere for everything. Maybe it's a trade-off and I'll find everyone up here knows their theory front-and-back, but cares very little about the quality of the job they do.
-John

 

[lyledunn]

EPAC,
I think you hit a bit of a raw nerve! I work with many excellent engineers who will often admit that basic theory has slipped away as they progressed in their respective specialised areas.
A young apprentic once asked me the following; a bulb is connected to a circuit which is 10 metres long. A similar bulb is connected to a circuit which is 10 kilometres long. If the switch in each circuit is depressed at exactly the same time which bulb will light first?
I can tell you that the range of answers offered by the above mentioned engineers quite aptly demonstrated big gaps in their understanding of basic theory. Nonetheless, they still remain the best of engineers in my book!
You will find that I too will ask the most basic of questions in these forums. I dont apologise because no matter how fundamental the question you will always tend to get an answer that will make you consider the subject in a different light!
For goodness sake, look at the engineering brains available to the US and they can still manage to blackout 50million people!
Best of luck in your career.

Regards,

Lyledunn

 

[rbulsara]

Epac:

Among all the above responses, Only dpc's respnose (the first response) is concise, accurate and to the point, with all due respect to others.

Your water analogy is very good. In fact mathematcial equations for network solutions for fluid mechanics and electricl network soultion are almost identical. Without going on tangent let me use the water analogy. Imagine (draw on paper) a closed water loop, with a pump and a turbine connected in series. The Pump represents a voltage source (generator) and the turbine is the load. The pump has to generate enough head (voltage) to drive the turbine and also over come the friction losses in the pipe (wire resistance). After passing through the turbine the water 'do' return to the pump albeit at very low head.

And yes the 'current' do have to return to the source one way or the other.

You are also almost correct in saying that in a balanced ( 3 phase) system current in a phase return through other phase. Except that it effectively returns through both the other phases (not one particular phase). Math works out such that at any given moment, the 'net' current value remains the same in all 3 phases of a balanced system.

Refer to any Basic Electrical AC Circuit theory book for more information.