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  1. #1 AC 
    Forum Freshman KYPOWERLIFTER's Avatar
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    This is a pitifully simplistic interrogatory, but here goes.

    Is this roughly how an AC circuit works?:

    An AC generator supplies an electron flow to start the circuit. These electrons then flow down a single wire to say a household fusebox/breaker. From there they flow to a wall plug and then which has two prongs, one 'hot' and one 'neutral'. Then the electrons pass through the 'hot' wire and through a device and return on a separate wire (although bundled in a cord with the hot wire) to the 'neutral' plug. The neutral plug wire then completes the circuit by going down a separate wire and eventually back to the AC generator. Parenthetically, I realize that the current switches polarity 60 times per second and that I have ignored transformers, capacitors, etc. But is this crudely correct?

    Edit: it appears, after viewing this: http://hyperphysics.phy-astr.gsu.edu...ic/hsehld.html that I may have displayed further ignorance...

    So, it's two 'hots' and a neutral that come into the home, with two 'hots' going to the breaker and the neutral going to a 'neutral tie in box'. It still appear from the diagram that the THREE wires go back to the AC supply generator? So, are the three wires not to be thought of as having electrons flowing to and from but more properly thought of as all with a back and forth electron flow? Or is it just the 'neutral' wire that completes the circuit? Convoluted, but I am confused.


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  3. #2  
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    I think the 2 hots are not actually both going to an outlet at once, instead thay are there to provide the OPTION of having double the voltage difference (i.e. a North- American house has 2 120V lines, allowing a difference of 240V and the ability to power something like a dryer if need be.)

    I think your were otherwise right that current flows through the hot and into the neutral to complete a circuit.

    I think thats why.

    Hope it helped,
    Gmano


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  4. #3  
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    If your are trying to trace the path of the electrons in the generator, they never get any farther than the primary windings of the main step-up transformer at the generating station. The current flowing in the transmission lines is induced in the seconary of the transformer by magnetic flux. In reality, the electrons probably never make it from the generator to the tansformer. Since it is alternatig current, they just sit there wiggling back and forth.

    Likewise, the current flowing in your house goes no farther than the secondary of the transformer on the utility pole on your street.

    In the USA we use center tapped 240 volt transformers to supply the house current, with 120 volts between each "hot" and the center tap which is called "neutral." In Europe,they just use 240 volts, not center tapped.

    Each 120 volt circuit in your home carries the load current in the "hot" and neutral conductors back to the service entrance, but if the current in the two hot legs in your house are balanced, the neutral current going back to the pole transformer is zero.
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    Forum Freshman KYPOWERLIFTER's Avatar
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    Thanks to each of you...

    Even if it's just 'in thought' and very much simplified, I do not understand how there can be a circuit without a return path. The electric field flows; but where to? Or does the field merely flow back and forth down a one path (wire). But that's not a 'circuit'? I do not doubt the veracity of the second reply, I just am a bit confused, still.
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    The current does flow in a circuit. Consider the transformer feeding just one 120 volt circuit in your house. The current flows from one terminal of the transformer to the breaker panel, through the load, back through the neutral, through the center tap of the transformer secondary and through the windings of the transformer. That's a complete circuit.

    Now what if you have another circuit of equal current powered from the other leg of the transformer. It is 180 degrees out of phase so the two currents flowing into the neutral cancel. But what is really happening is that the current flows down one hot leg of the transformer to the first load, back through the circuit 1 neutral, then to circuit 2 through its neutral, then via circuit 2 hot leg and back to the transformer. Voila, a completed circuit.

    Also, I hope you are not getting confused about electrons and current. The current flows around the circuit instantaneously (well, actually at light speed) but the electrons themselves don't move far. They just push other electrons ahead of them. (Somebody will probably nitpick that last sentence.)
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    Forum Freshman KYPOWERLIFTER's Avatar
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    Quote Originally Posted by Harold14370
    The current does flow in a circuit. Consider the transformer feeding just one 120 volt circuit in your house. The current flows from one terminal of the transformer to the breaker panel, through the load, back through the neutral, through the center tap of the transformer secondary and through the windings of the transformer. That's a complete circuit.

    Now what if you have another circuit of equal current powered from the other leg of the transformer. It is 180 degrees out of phase so the two currents flowing into the neutral cancel. But what is really happening is that the current flows down one hot leg of the transformer to the first load, back through the circuit 1 neutral, then to circuit 2 through its neutral, then via circuit 2 hot leg and back to the transformer. Voila, a completed circuit.

    Also, I hope you are not getting confused about electrons and current. The current flows around the circuit instantaneously (well, actually at light speed) but the electrons themselves don't move far. They just push other electrons ahead of them. (Somebody will probably nitpick that last sentence.)
    Hello Harold:

    I understand completely. You are demonstrating the complete circuit from the standpoint of the transformer (I'm assuming you mean the transformer on the pole). But alas, the circuit from the original generator of power must also be a COMPLETE circuit. correct?
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    Quote Originally Posted by KYPOWERLIFTER
    Hello Harold:

    I understand completely. You are demonstrating the complete circuit from the standpoint of the transformer (I'm assuming you mean the transformer on the pole). But alas, the circuit from the original generator of power must also be a COMPLETE circuit. correct?
    Yes, it does. The generator is actually 3-phase but let's just talk about one phase. The current goes from one terminal of the generator winding, through the primary winding of the transformer, and back to the other terminal of the generator winding. This current loop is contained within the generating station. The secondary winding of the step-up transformer carries the power out onto the grid. The secondary circuit is electrically isolated from the primary; i.e. there is no physical connection. It is linked only by magnetic induction.
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    Forum Freshman KYPOWERLIFTER's Avatar
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    Last question, I promise

    Are you saying that the secondary winding of the step up transformer is not ALSO a closed circuit? (By answering "yes" you would be stating that the secondary winding of the step up transformer that delivers the power to the folk is NOT a closed circuit.

    If it is the case that it- the secondary winding of the step up transformer that delivers the power to the folk- is not closed then its mysterious to me...
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    Quote Originally Posted by KYPOWERLIFTER
    Last question, I promise

    Are you saying that the secondary winding of the step up transformer is not ALSO a closed circuit? (By answering "yes" you would be stating that the secondary winding of the step up transformer that delivers the power to the folk is NOT a closed circuit.

    If it is the case that it- the secondary winding of the step up transformer that delivers the power to the folk- is not closed then its mysterious to me...
    The secondary loop is closed, at the power plant.

    However their are other loops and transformers in between the power station and your house. The last transformer near your house that feeds your house and probably a few other neighbors in a suburban style area. Does have an open secondary. Until you connect something to it.




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    William McCormick
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    Forum Freshman KYPOWERLIFTER's Avatar
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    Then I suppose I have it!

    It is neither useful nor accurate to think of an AC circuit as a "circuit" in the sense of a closed loop but more like a linear system with various loops throughout that provide 'power'. To try and reason that there must be a return 'wire' to the original generator that proceeds from the 'terminus' of the system is inaccurate.

    Hopefully I'm clear on it now? I appreciate the help throughout. This seems like a good group of volk here. I have always lost site of the forest for the trees in electricity. I recall getting through a couple of low level physics classes in college by just working through the problems while never really thinking a lot about the practical reality of AC systems. No idea how, I managed that...

    BTW: William, it appears that my interests are quite simpatico with yours.

    Regards to all who offered help.
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  12. #11  
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    Quote Originally Posted by KYPOWERLIFTER
    Hopefully I'm clear on it now?
    The question mark tells me you are still a little fuzzy. Maybe this example will help. You have a power outage and set up a home emergency power generator. No problem understanding the current flow there, the current goes right from the generator, through the loads (whatever you plug into it) and returns to the generator. You want to power your neighbors house as well, but it is too far away. The power cable you would need would be too heavy. So to save line losses, you use a step-up transormer, let's say from 240 volts to 480 volts. Then at your neighbor's house you need another transformer to step it back down to 240.

    You have three current loops now, to get power to your neighbor. From the generator through the step-up transformer primary back to generator. Another loop from the step up transformer secondary to the step-down transformer primary returning to the step up transformer secondary. Then a third loop from the step down transformer secondary, through your neighbor's load circuits and back. That's all that's happening, on a larger scale, with the utility power distribution system.
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    Holy shit, Harold!!!!! I've got it!

    Here in Western Kentucky, I am very familiar with that analogy. (Terrible ice storm of few weeks back. It was like we were all troglodytes. Demonstrated how terribly easy it is to cripple our society... just take water and power. Many older folks really suffered.)
    Le silence éternel de ces espaces infinis m'effraie.

    -Pascal
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    Yeah, my power was out for a week when the remains of Hurricane Ike went through. That one probably hit you too.
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    So, do the electrical devices we plug into the wall each have their own transformers as well? I've often been confused about this.

    My other confusion is how it works that I can plug multiple devices into the same outlet and they all get the same amount of power as if they were the only device plugged into that outlet (or they seem to).

    I'm guessing that puts me at a truly novice level. I like what you've described so far, though. If I understand right, each stage of an AC electrical system is a closed circuit. The transformers, through which current passes on one side, each begin their own, new completed circuit on the other side, and somehow, through all of this, you only draw as much power out of the system as what the devices you plug into the wall use up.

    It's just weird that the total voltage available doesn't drop or increase depending on how many devices you plug into it. From what I can tell, that seems to happen in DC circuits (at least if they're connected in series instead of parallel). Why not in AC circuits?
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    Quote Originally Posted by kojax
    So, do the electrical devices we plug into the wall each have their own transformers as well? I've often been confused about this.
    If they are designed to operate at less than line voltage, yes.

    My other confusion is how it works that I can plug multiple devices into the same outlet and they all get the same amount of power as if they were the only device plugged into that outlet (or they seem to).

    I'm guessing that puts me at a truly novice level. I like what you've described so far, though. If I understand right, each stage of an AC electrical system is a closed circuit. The transformers, through which current passes on one side, each begin their own, new completed circuit on the other side, and somehow, through all of this, you only draw as much power out of the system as what the devices you plug into the wall use up.

    It's just weird that the total voltage available doesn't drop or increase depending on how many devices you plug into it. From what I can tell, that seems to happen in DC circuits (at least if they're connected in series instead of parallel). Why not in AC circuits?
    Every device you plug in to an outlet is conectted in parallel to the other devices. Also, there are fuses or circuit breakers that will trip if you try to draw more current than the wires of your house can safely carry. For any given circuit, this is much less than the current that can be delivered by the power lines entering your house.

    Even then, a brief spike in current draw which lasts for too short of a time for the circuit breaker to react to can effect other devices on the circuit. Such a brief surge could cause lights on the same circuit to momentarily dim.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  17. #16  
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    Quote Originally Posted by kojax
    ..somehow, through all of this, you only draw as much power out of the system as what the devices you plug into the wall use up.
    That is not quite true. A transformer does have an efficiency associated with it and does not put out quite as much power as it consumes. Those a-c adapters for electronic devices like cell phone battery chargers are just transformers with a rectifier built in. They will use up some power just sitting there plugged in without your cell phone connected. That's why President Obama wants you to unplug them when not in use. Yes, that's part of his energy policy.

    It's just weird that the total voltage available doesn't drop or increase depending on how many devices you plug into it. From what I can tell, that seems to happen in DC circuits (at least if they're connected in series instead of parallel). Why not in AC circuits?
    A-c is no different than d-c as far as that-s concerned. You wouldn't normally connect things in series. The voltage does not quite stay the same when you increase the load because there is a voltage drop through the resistance of your house wiring. You will notice the voltage drop if you try to run a large motor with an extension cord that is too long.
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  18. #17  
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    Quote Originally Posted by Harold14370
    Quote Originally Posted by kojax
    ..somehow, through all of this, you only draw as much power out of the system as what the devices you plug into the wall use up.
    That is not quite true. A transformer does have an efficiency associated with it and does not put out quite as much power as it consumes. Those a-c adapters for electronic devices like cell phone battery chargers are just transformers with a rectifier built in. They will use up some power just sitting there plugged in without your cell phone connected. That's why President Obama wants you to unplug them when not in use. Yes, that's part of his energy policy.

    It's just weird that the total voltage available doesn't drop or increase depending on how many devices you plug into it. From what I can tell, that seems to happen in DC circuits (at least if they're connected in series instead of parallel). Why not in AC circuits?
    A-c is no different than d-c as far as that-s concerned. You wouldn't normally connect things in series. The voltage does not quite stay the same when you increase the load because there is a voltage drop through the resistance of your house wiring. You will notice the voltage drop if you try to run a large motor with an extension cord that is too long.

    Any transformer has, start ohms and run ohms. That is what makes transformers so dangerous to hook up. Unless you use the proper current limiting devices in a power distribution panel. Or a procedure or switching system to connect them.

    If you were to connect a transformer directly to another line transformer, directly without a limiting device. Like the main fuses or breakers. You could do some damage to yourself and the line transformer supplying power.

    Some welders tell some pretty drastic tales of hooking up welders directly to pole power. I was told never try that at home, by guys that have disintegrated tape measures in the shop, and much worse and lived to tell about it.

    Part of the reason is the start ohms for a transformer could be, 0.9 ohms or lower. The line or pole transformers could supply you with as much as 75,000 amps. For a cycle. Or a lot more if the transformer explodes during your experiment.

    Especially if you accidentally tap the connection rather then to cleanly connect it.

    Once the transformer is correctly started, it may create as many as 10,000 ohms, not under a load. And draw almost no watts.


    Sincerely,


    William McCormick
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