Visually understand how AC works

[RichyL]

Ok so I am not a teacher, in fact I am a student, reading the categories though I figured this might be the best place to post as the instructors category is based on finding ways to help students understand electrical concepts. My question relates to if there is any visual (videos or diagrams) resources that show how electricity moves, specifically in 120/240 volt applications. In a straight 240 load with no neutral, (such as a water heater or electric furnace) I am led to believe the current travels in a circle that rotates. Such as moving from one hot on the breaker to the load then back through the other current carrying conductor towards the panel, then coming back to the load again from the opposite direction. There is no neutral load so I'm assuming the ground is only used to carry fault currents back to the neutral main to trip the breaker. That is what i got out of it anyway. As far as 120 volt goes (a basic 15 amp receptacle) or a range that has 2 hots and a neutral I'm a little confused. Is a 120 volt receptacle still considered alternating current since there is only 1 hot wire? My understanding of this is it goes in a circle from breaker to the hot to the neutral then neutral main to carry the balanced load. To me it sounds as if this is direct current. Any videos pics diagrams or explanations would be greatly appreciated. I did not see an electrical theory topic on the forums, so if there is a better place to post this please let me know. Thank you much
Rich

 

[ibew441dc]

I recommend Mike Holts electrical theory set. It might clear some questions up for you. It helped me a lot with some basics.

 

[Denis]

google images sine wave scope
\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/ modified sine wave made the lines sharper

/-\_/-\_/ modified sine wave stretched out the peaks

------------------------------- modified sine wave used diode rectifier 60 cycle DC

find yourself a scope and play around
pick up a heathkit!!!!!

 

[tallgirl]

Ok so I am not a teacher, in fact I am a student, reading the categories though I figured this might be the best place to post as the instructors category is based on finding ways to help students understand electrical concepts. My question relates to if there is any visual (videos or diagrams) resources that show how electricity moves, specifically in 120/240 volt applications. In a straight 240 load with no neutral, (such as a water heater or electric furnace) I am led to believe the current travels in a circle that rotates. Such as moving from one hot on the breaker to the load then back through the other current carrying conductor towards the panel, then coming back to the load again from the opposite direction. There is no neutral load so I'm assuming the ground is only used to carry fault currents back to the neutral main to trip the breaker. That is what i got out of it anyway. As far as 120 volt goes (a basic 15 amp receptacle) or a range that has 2 hots and a neutral I'm a little confused. Is a 120 volt receptacle still considered alternating current since there is only 1 hot wire? My understanding of this is it goes in a circle from breaker to the hot to the neutral then neutral main to carry the balanced load. To me it sounds as if this is direct current. Any videos pics diagrams or explanations would be greatly appreciated. I did not see an electrical theory topic on the forums, so if there is a better place to post this please let me know. Thank you much
Rich

The difference between 120, with the neutral, and 240, without the neutral, is the range of voltages for each of the conductors.

For a 120 volt receptacle, the hot conductor starts off at 0 volts, goes 120 volts, turns around and goes back down towards 0, down towards -120 volts, turns around and goes back towards 0 again. The neutral conductor, all this time, has a constant voltage of 0. So, the difference between the hot and the neutral alternates between 0 and 120 volts.

For a 240 volt receptacle, the first hot conductor starts off at 0, goes up towards 120 volts, turns around, goes back towards 0, then goes down towards -120 volts, turns around, and goes back towards 0. At the same exact time, the second hot conductor starts off at 0, goes down to -120, back to 0, up to 120, then back to 0 again. So, the difference between the two hot conductors alternates between 0 and 240 volts. When L1 is at 0, L2 is at 0. When L1 is at +120 volts, L2 is at -120 volts, and when L1 is at -120 volts, L2 is at +120 volts.

In both circuit types, current moves from high voltage to low voltage. So, in a 120 voltage circuit, when the hot conduct is greater than 0, current flows from the hot conductor to the neutral, and when the hot conductor is less than 0, current flows from the neutral conductor to the hot conductor. This creates the circle that was described to you. In a 240 volt circuit, when L1 is greater than 0 volts, current flows from L1 to L2, and when L2 is greater than 0 volts, current flows from L2 to L1.

What happens in a 3-wire circuit where you have L1, L2 and neutral (N)? The current flows the same way -- from the higher voltage conductors to the lower voltage conductors. The difference is that there is a neutral conductor that always has a voltage of 0. So, if the current on L2 is 0, and the current on L1 is 1 ampere, there is no path for the L1 current to return through L2, so the L1 current returns using N. If the current on L1 is 0, the opposite happens. The L2 current returns using N.

What happens when there is a load on L1 and L2? Remember that current always flows from high voltage to low voltage. However much current is on L1 will return using L2 depending on how much current is on L1 and L2 and the difference in voltages. The difference in current between L1 and L2 will have to use N. So, if L1 has 1 ampere and L2 has 2 amperes, when L1 is greater than 0 and L2 is less than 0, one ampere will come from L1 and flow from L1 to L2 and one ampere come from N and flow from N to L2. When the voltages make it half way through the cycle and both are 0 again, the current changes direction. Now, there are 2 amperes coming from L2. One amp from L2 goes to L1, and the second amp goes to N. As you can see, there is always one amp on L1 -- it either comes from L1 through the circuit breaker, or from L2. There is also always one amp on N -- it either comes from the panel or from L2 as the difference between L1 and L2. And there is always 2 amps on L2 -- it either comes through the circuit break, or from N and L1.

Makes sense? Got questions?
----
Note: Don't any of you who know what's wrong with the numbers correct me about RMS voltage. As soon as the OP understands this, I'll explain RMS.

 

[RichyL]

Thanks for the explanation tall girl. I am trying to understand this concept, I have never heard the term negative voltage before, so this is the best I can explain how I'm understanding this, however dumb it may sound. I may be wrong, but here it goes.

I picture voltage as a man running. The left arm is L1 and the right arm is L2,
the heart is the load. When my right arm goes forward i have 120v. At the same time my left arm is going back just as far as the right arm is forward which is -120 giving me 240 v to my heart. That is for 240v load no neutral

For 240 with neutral such as an electric range with a timer on it, My right arm goes forward slightly more than my left arm goes back and the uneven load is going through the neutral. ( I dont have a metaphor for neutral:grin: )

in 120 volt I am running with one arm. My opposite arm that is not moving is neutral. The arm is going back in forth in equal measurements.


As far as amps go I am still trying to visualize that. I believe my neutral scenario may be based on amperes instead of voltage. Please comment on how wrong I am :smile: or an explanation based on my running scenario if possible lol

 

[tallgirl]

The analogy I learned years ago is to water.

"Voltage" is pressure. So, positive voltage is positive pressure, and negative voltage is negative pressure (suction, or a vacuum).

"Amperage" is volume. More amps is like more gallons per minute.

Alternating current would be like a pump that pumps water out at one instant, sucks it back in at another instant, then pumps it back out, repeating the cycle.

Negative voltage is easy -- all you need is for something to "suck" the electrons backwards, instead of pushing them forwards. When electrons move "forward", a negative voltage is created (electrons are actually negative charges). When electrons move "backwards", a positive voltage is created.

It turns out that a rotating magnet within a coil of wire can do precisely that. As the magnet moves closer to the coil, electrons move one way. As the magnet then moves further away (because it's going around in a circle and the coil is staying put), the electrons change direction. This is the principle of the generator used by almost all electric generating facilities -- something acts against a turbine (like a giant fan) causing the shaft of the generator to turn. The rotating magnets (or rotating coils and fixed magnets) then create an electric current that changes at the same rate as the generator shaft rotates.

If you have some leftover bell wire, such as from wiring a door bell or telephones, you can wrap it around a toilet paper tube ten or twenty times and connect each end to a DC voltmeter. Then, get a magnet, such as from a magnetic pickup, or a magnetic tipped screwdriver, and move it in and out of the toilet paper tube quickly. You should see a voltage on the voltmeter.

 

[RichyL]

THank you. That is a good analogy never heard it put that way before. So when the voltage reaches 0 volts that is the point where it turns around and goes the opposite way? Still a little confused about how the neutrals are carrying the unbalanced load. Thanks for your patience i'm a little slow, I'm kind of a visual learner which makes it hard lol:grin:

 

[tallgirl]

THank you. That is a good analogy never heard it put that way before. So when the voltage reaches 0 volts that is the point where it turns around and goes the opposite way? Still a little confused about how the neutrals are carrying the unbalanced load. Thanks for your patience i'm a little slow, I'm kind of a visual learner which makes it hard lol:grin:

The voltage "turns around" at the positive and negative maximums. At the zero crossing point the voltage is moving in the same direction -- 10v, 5v, 0v, -5v, -10v, etc. It's at the maximum that it "turns around" -- 100v, 110v, 120v, 110v, 100v. And the same at the negative maximum.

As regards the neutral carrying the unbalanced current, imagine you have a pair of pumps. When one pumps "out" the other pumps "in". When the first pumps "in" the other pumps "out". If both pumps are pumping the same amount of fluid, the fluid just goes back and forth between the two pumps. But if one pumps "out" more than the other pumps "in", the fluid has to go somewhere. And if one is pumping "on" more than the other is pumping "out", the fluid has to come from somewhere. In an electrical circuit the fluid is electrons and the "from somewhere" is the neutral conductor.

 

[DavidTu]

How does all of this compare to 240v systems in Europe? Do they also have a neutral conductor or is this something special because we are based on 120v?

 

[RichyL]

Thanks I understand now. I ordered the Mike Holt NEC exam prep book off this site and the pictures were very helpful. It is just like you explained it although i think seeing the pictures helped it click in my mind. Especially the diagrams of the AC generator with the polarity changes from the electromagnets revolving around the conductor loops. I was confused about the neutral conductor because i wasnt aware that the neutral is not a neutral but a grounded conductor in non multiwire 120 volt circuits. The thing i'm trying to figure out now is how is a load/current only established on the circuit by an ammeter when you have something plugged in , Is there always 20 amps running through a 20 amp circuit, and the meter is only reading the power used out of that 20 amps? Do I ask too many questions?:grin:

 

[brian john]

I can post waveform snapshots if it would be of help

This is a load coming on line single phase snapshot

or this a bit more than basic 3 phase 4 wire current sine wave with a harmonic load.

 

[RichyL]

why do electrons only flow through the circuit when a load is plugged in? What is drawing them there since the resistance of a load is higher than the resistance of the circuit itself, since higher resistances are supposed to oppose the flow of current. From what i understand the resistance of the load is what regulates how much current will be going through the load but i dont understand how plugging something in will attract current when there is no current flowing through it to begin with, unless i am misunderstanding and there is always 20 amps flowing through a 20 amp circuit that is going back and forth stagnant and the ampere reading you get from a meter is telling how much more power must be drawn from the main to keep 20 amperes in the circuit

In the 120volt sinewave it is showing there is only amperes present when the load is plugged in. I'm sorry if im not making sense just a little confused, thanks for helping me understand:smile:

 

[Smart $]

why do electrons only flow through the circuit when a load is plugged in? What is drawing them there since the resistance of a load is higher than the resistance of the circuit itself, since higher resistances are supposed to oppose the flow of current.

Electrons (charges) can only flow when there is a conductive path between two points having a charge differential (aka electromotive force, aka voltage). Electrons cannot flow through an incomplete circuit, such as a receptacle on a 120VAC system. Resistance of an incomplete circuit is infinite ohms, even though the wire connecting the receptacle to the 120VAC system has, comparatively speaking, near zero ohms. There has to be a load plugged into the receptacle (and turned on) before there is a complete circuit.

From what i understand the resistance of the load is what regulates how much current will be going through the load but i dont understand how plugging something in will attract current when there is no current flowing through it to begin with, unless i am misunderstanding and there is always 20 amps flowing through a 20 amp circuit that is going back and forth stagnant and the ampere reading you get from a meter is telling how much more power must be drawn from the main to keep 20 amperes in the circuit

You are confusing voltage with amperage. Analogous to a closed-loop water line, voltage would be pressure, amperage would be the flow rate, and valve would a switch or a means to open and close the ciruit (note: regarding the terms "open" and "closed", a switch is functionally opposite that of a valve).

 

[RichyL]

Electrons (charges) can only flow when there is a conductive path between two points having a charge differential (aka electromotive force, aka voltage). Electrons cannot flow through an incomplete circuit, such as a receptacle on a 120VAC system. Resistance of an incomplete circuit is infinite ohms, even though the wire connecting the receptacle to the 120VAC system has, comparatively speaking, near zero ohms. There has to be a load plugged into the receptacle (and turned on) before there is a complete circuit.

Ok this makes sense now, when the load is plugged in i.e. a alarm clock for instance the hot and grounded conductor become connected together inside the alarm clock creating the opposite charges needed for electron flow. This would explain why a grounded conductor in a 120v circuit is only hot when there is a load on it? Is this correct?

 

[Smart $]

Ok this makes sense now, when the load is plugged in i.e. a alarm clock for instance the hot and grounded conductor become connected together inside the alarm clock creating the opposite charges needed for electron flow. This would explain why a grounded conductor in a 120v circuit is only hot when there is a load on it? Is this correct?

Well, no. In a sense the ugc and the gc get connected inside the alarm clock, but it is through the load having higher resistance, or impedance. IOW, they do not connect directly to each other. Perhaps your understanding is improving, but your terminology leaves quite a bit to be desired :smile:

Additionally, the opposite charges?more correctly stated as opposing charge, or better yet as charge differential?are (is) not created inside the clock, but rather at the source of energy, which locally, for the issue at hand, is usually a transformer on the premises or otherwise nearby. The clock's innards simply complete the otherwise incomplete circuit.

Furthermore, while the gc may have a measureable voltage to ground on it while the circuit is conducting, it is never considered to be "hot" unless there is a fault condition, such as a disconnected splice in the gc, unintentional or not.

 

[RichyL]

Ok so if I am not mistaken, which i probably am :grin: Resistance completes the circuit by adding a charge differential to the power source. The resistors create heat and friction when the current flows through them which use energy.

 

[George Stolz]

The voltage is always there, load or not.

The load completes a circuit, and the power source continues to do what it was doing the moment before a load was connected. The only difference is that now there is a load doing work.

This would explain why a grounded conductor in a 120v circuit is only hot when there is a load on it? Is this correct?

There is never 120V present on a grounded conductor, unless you break the conductor and measure between the two points of the break.

 

[RichyL]

Ok so the nuetral is always 0v. The hot is always being pushed by voltage 120/0/to-120v. The resistance factor actual uses up the electrons, by heat and friction. The grounded and grounding conductor are not connected to the hot wire of the circuit and only carry fault current in non mwbc's.

As far as mwbc:

As regards the neutral carrying the unbalanced current, imagine you have a pair of pumps. When one pumps "out" the other pumps "in". When the first pumps "in" the other pumps "out". If both pumps are pumping the same amount of fluid, the fluid just goes back and forth between the two pumps. But if one pumps "out" more than the other pumps "in", the fluid has to go somewhere. And if one is pumping "on" more than the other is pumping "out", the fluid has to come from somewhere. In an electrical circuit the fluid is electrons and the "from somewhere" is the neutral conductor.

My question is if a hot and a neutral in a mwbc are never connected how is the current flowing through it to carry the unbalanced load.

Thanks again to all the posters responding to help me grasp these concepts, I think I am getting close.... you have a lot more patience then the guy i work for, who seems intent on not giving out any information and trying to keep me at a certain level.
Rich

 

[roger]

My question is if a hot and a neutral in a mwbc are never connected how is the current flowing through it to carry the unbalanced load.

There can be no unbalanced current flowing if more than one ungrounded conductor is not connected to the neutral.

Voltage across the load(s) must drop to zero after all the impedances of the circuit are served.

What leaves the source in the way of current flow must return to the source.

Try to find some information on Kirchoff's Voltage and Current laws


Roger

 

[George Stolz]

Richy, one concept that I use when thinking about single phase circuits is this: Current is leaving L1 looking for L2 (and L2 looking for L1), and only takes the neutral as a last resort.

Using your fluid analogy: picture two lakes, and a drainage ditch between them. The north lake has a dam on it's south end, and the south lake has a dam on it's north end, the dams face each other.

When you connect an electrical load to a 120V receptacle, you're opening the dam a little bit on, say, the north lake. If the gate on the south lake is shut, then the water from the north lake can't get into it. The water flows down into the drainage ditch and goes into the river (the neutral conductor).

When there are five amps of 120V load on line 1, and five amps of 120V load on line 2, then both "floodgates" pointed at each other are open the same amount, and allow the same amount into each other. Nothing has to flow into the ditch, because that's not the goal of our water in this analogy.

That's why I kinda dislike water analogies.

At any rate, this natural desire for the whole house's system to be a happy 240V system (instead of a pair of disinterested 120V systems) is the reason why a MWBC works - if you have 10A on L1, and 8A on L2, then only 2A will flow on the neutral every time.

The current that you'd expect to flow on the neutral (as in 10+8 = 18A above) takes a right turn and runs right through the other phase's load to get to the other phase (to get 2A instead of 18A).

Don't know if that helps at all, but I had fun, and that's what counts.