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| Subject - AC Voltage | |
| David Hyatt | You have a black white and ground in a circuit. AC circuit. Does the voltage travel back and forth on the black or does it ever travel back on the white? Just trying to picture the flow of electrons in an AC circuit. I think this will help me learn more in my studies if I could see the circuit work. |
| JimmyDee | quote: Yes it travels back an forth in both. Think of the circuit as a circle that has to be completed. Part of that circuit is the source and part is the load. The source of electrons leaves the source, travels through the wire to the load and then returns to the source out the other side of the load and completes the circuit. Break the circuit anywhere to install a switch which, of course will shut the load and current flow off. Jim |
| ricky | The voltage on the black wire alternates between+120volts and-120 volts in a sine wave. If you start at 0volts,go up to +120 volts, then back down to 0 volts, continue down to -120 volts,then back up to 0 volts, that is one cycle. 60 cycles per second. The white wire is connected to earth ground and center tap of the transformer. The electrons, positive or negative, try to get back to 0 volts thru the load. They flow thru the grounded conductor until they reach the ground. Conventional current flow is pos to neg, however, electrons are negativly charged and actually flow neg to pos. It helps to see a sine wave on an oscilloscope. With a dual trace you can compare the phases. |
| tony thrower | Voltage does not travel. It is the pressure that causes electrical current to flow through a complete electrical circuit.In any electrical circuit you must have a load cnnected between two points of the electrical that have a difference of potential between them. In a 120 volt A.C. circuit these two points are: A.Ungrounded "hot" Usually the black wire,however it can be any color other than white, gray, or green. B.Grounded neutral or the white wire. Once during each half-cycle the "hot" wire will be at the positve potential. Exactly at this same time the neutral is at the same potential but it is negative in polarity. Since electrical current flows from the negative potential to the positive potential, during this half cycle, current is flowing from the neutral (-) through the load component back to the "hot"(+). During the next half-cycle the polarity of the power source alternates. The "hot" is now negative and the neutral is positive. At this time the electrical current is flowing from the "hot" through the load Back to the neutral which is positive. This same alternation will occur during each cycle of an A.C. electrical system. I hope this is a somewhat clear explanation. But what can you expect frm an electrician from Alabama. "ha" |
| tony thrower | Sorry about the typing errors in the first paragraph.The third sentence should read,in any electrical circuit you must have a load connected between two points of the electrical system that have a difference of potential between them. |
| David Hyatt | Yea, guys this is helpful, sometimes you have to see what you are talking about. Thanks
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| JimmyDee | quote: Tony thrower, Do you know that you have an icon on the top of your post that if you ckick it, you can edit your post. I do it all the time and sometimes one will write something that you have second thoughts about and can delete it as well. I edit probably 1/3 of my posts because of errors. Also if you click on the link in my signature, there is a free spell check that works great for posts. Its free and all it takes is a right click of the mouse. Jim |
| CozyGuy | The often confusing part is 'neutral', especially the notion that it's connected to 'ground'. This would seem to imply that the ground wire is somehow not really needed. In fact, the way to understand it is to envision the transformer on the utility pole. The utility transmits power at a relatively high voltage. This is because you can use much smaller wire with significantly less loss at higher voltages. This could be done with DC power, but then you'd have to somehow make it a low enough voltage to be safe for consumers to use; hard to do with DC. AC creates magnetic fields as it oscillates from 'high' to 'low'. These magnetic fields in the primary winding of a transformer induce voltages in the secondary windings. The secondary has a center-tap for a reference, and the windings on each side are the hot leads. The center tap is neutral, and each of the others are 180 degrees out of phase. In other words, when one is at +120V, the other is at -120V. These three wires from the transformer constitute what's know as single phase power. Understanding that, it's now possible to see the rest. Typically, the center tap is connected to an earth ground. However, electricity must always make a complete circuit, so the flow of current can be seen to be from either of the 'hot' leads to either each other or to the transformer center tap. A path for current flow may be established through the earth rather than wires. That is, if you do not have a ground wire bonding the various parts of your system, and instead just ground your equipment directly (grounding rod, water pipe, etc), current may travel from the 'hot' leads through the equipment, into the earth ground, and back through the earth to the ground referencing the transformer center tap to complete the circuit. This can result in injury or death if a human is part of the circuit. That's the purpose of a ground wire; it causes all grounds to have a low-impedence path back to the center tap on the transformer to complete the circuit, rather than having sufficient potential to form a circuit though your body from the equipment case. So, we know what ground is- what's 'neutral'? Well, it's called neutral because it is. Ideally, you'd balance the two legs in your breaker box. That is, you'd have close to the same amperage flowing through each of the two hot leads from the transformer. When they pass through the load and are coupled to the neutral return, they are still 180 degrees out of phase. In effect, they cancel each other out, and very little current flows through neutral under those conditions. If there is no amperage at all on one leg, the most through neutral will be that which is flowing through the other. That's why neutral does not need to be twice the size of the two hot leads, even though it 'returns' them both to the transformer. Incidentally, 1 phase 220 uses the two hot leads at the same time. Since one is max in one polarity while the other is max in the opposite, the max potential between the wires is 220 volts. In this case, no neutral is needed. Hope this rounds out the discussion a little. |
| Electricman |  Hats off to CozyGuy Great Post
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| Scott Vickrey | I think David is fishing for another diagram. Unfortunately I'm not at home so I can't build one. This is a good idea for an image and when I get home on sunday night I'll create one. Unless someone else does it first.
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| David Hyatt | Glad to see you are still alive Scott, yes a diagram will be nice. |
| Scott Vickrey | Okay, Okay, I'm busy believe me! Here's the image I promised.  Don't forget you can include any of the images from Images and Drawings Collection in your post if they become relevant again. |
| David Hyatt | I knew you could do it Scott. Thanks. |
| Chris Clement | The voltage actually goes higher and lower and "averages out" to 120. About 170 volts peak both ways. Not being a smartass here. My experience is from the other side of the meter (Florida Power retiree). |
| wireman | Is this why my New Greenlee digital LED display tester always shows wild fluctuations beyond 120 before it shows the average voltage reading? |
| Scott Vickrey | Chris your transmission knowledge is extremely valuable to the distribution electrician. Most of us can't take time to go build power lines to round out our knowledge of the entire electrical system. You are absolutely correct Chris and your the first person to mention this on this forum. The 120 volts we are so used to is the Root Mean Square or as it is more commonly stated The RMS voltage. Most meters read RMS. However if you where to hook up an oscilloscope you would see that the sine wave is larger than the 120 volts. The extreme measure of voltage in an AC circuit is peak voltage. The RMS of an AC voltage is .707 of Vp. We know AC circuit voltages by their RMS or effective working voltages. Here's an interesting and helpful tip for the new electrician that I wish I would have known when I was a starting electrician. Most digital multi-meters are very sensitive, so much so they will read induced voltages on a dead circuit run close to a live circuit. Anyone who has been reading voltages for any period of time has fallen into this trap. I'm sure you all went through this (and tell me if you did) while trying to troubleshoot a 120 volt circuit with a digital multimeter you read 70 or 90 something volts on a circuit that and you wondered if the circuit was energized or dead. This is an induced voltage. Even most flukes will read this voltage. This is actually a credit to the accuracy of the meter because this voltage is present. The problem is that this voltage is not a working voltage and could not even operate the smallest of loads. If you use a meter that actually pulls a load on the circuit (like a wiggy or a simpson) you will read zero volts. I worked at a plastic plant that kept 20 company and 20 contractor electricians on pay role and It was stressed company policy not to use a digital multimeter for troubleshooting 120 volt AC circuits. You can actually read 120 volts on a wire that is disconnected from it's power source. Touch it to ground and it will read zero volts (don't do this at home folks). I hope I saved at least one person from this quandary. This is why I bother. |
| karlwayne | My cousin called me this morning, said his panel in the barn had 180V on one leg and 60V on the other. when I got there, he took his digital to demonstrate, with all the breakers off, he had 120/240. My knopps picked up 240 but nothing to the neutral. Overhead service,copper to aluminum, got a ladder to go up and look,mechanical connector - wires broke off when I grabbed it. |
| Chris Clement | Karlwayne - That is probably a broken neutral, a very nasty but common problem. With no load, it will balance but when you turn anything on, it flops around. Kill it now and investigate! |