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Question about Alternating Current

  1. Jul 21, 2014 #1
    I'm a bit confused on some of the fundamentals of Alternating Current, if anyone could help me out it would be awesome.

    1) In what form is the mechanical energy provided to an alternator to produce the electrical energy? Does someone sit at each generator and spin a hand-crank (joke)?

    2) Is any Direct Current applied to produce AC? Or is it simply the mechanical energy that causes the armature to spin which in turn induces a current?

    3) How does AC ever actually get anywhere due to its back-and-forth motion? How is it transmitted from power grids to homes? I know that no actual electrons travel from the plant to homes, so how is the energy received?

    4) After watching a couple videos, I am still confused on how the spinning of the armature is initially started.

    5) Why is 3-phase AC (120 degree offset) the optimal setup as opposed to 4 or 5?

    I realize this is a lot to ask, any help is much appreciate.
    Thanks!
     
  2. jcsd
  3. Jul 21, 2014 #2

    russ_watters

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    Welcome to PF!

    These questions sound like homework, but even if they are not, you need to show some effort in trying to find the answers yourself. You'll learn better that way. So do some research, make some guesses and we'll help from there.
     
  4. Jul 21, 2014 #3
    It's not homework, just my own curiosity and I have done some research. I will try to explain where my thinking is at:

    From this video () at roughly 4:54 lets say the armature is rotating clockwise with its left hand side moving upward. Based on the RRH and how the brushes are connected to the armature, this would mean current is flowing negative to positive thus shows a positive voltage reading. When the armature completes a half rotation from this position and the now right hand side (previously left) is rotating downward thus the current is flowing from positive ring to negative ring so voltage is negative? If this is correct, doesn't that mean the current in the armature is always in the same rotational direction and the actual "alternating" part is what is coming out of the rings?

    I guess I'm confused on where the current is actually alternating.
     
    Last edited by a moderator: Sep 25, 2014
  5. Jul 22, 2014 #4

    dlgoff

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  6. Jul 22, 2014 #5

    NascentOxygen

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    You could use a windmill to supply the grunt needed to rotate the armature. Or a water-wheel.
     
  7. Jul 22, 2014 #6

    psparky

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    BTW....what is the speed of electricity? We know that a 60 htz generator does one cycle (positve and negative) in 1/60 of a second....

    But that does not tell us the speed it goes or the distance it travels.

    Is it the same as the speed of light??????
     
  8. Jul 22, 2014 #7
    its usually 2/3rds the speed of light or so. it varies by the material.

    of course, that is the speed of the electric field. the actual speed of electrons is much much lower.
     
  9. Jul 22, 2014 #8

    psparky

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    That actually would be fast enough to go back and forth to the power plant....would it not?

    According to your 2/3rds speed of light.....

    Electricity would travel about 1,000 miles in one half of a sin wave at 60 hertz. That's definitely fast enough to go back and forth to the power plant.

    Which brings up another point....the electrons or whatever don't go actually go through the transformers....instead they build a rotating electric field, or change in flux, which pushes current thru the secondory or primary depending how you look at it. Fascinating considering you could easily go thru 5 to 10 transformers before even getting power....or giving power back...oh boy.
     
  10. Jul 22, 2014 #9

    Averagesupernova

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    As Mr. Sparkle has pointed out, the electrons do NOT travel anywhere close the speed of light. In fact I don't think they even travel close to the speed of walk. Consider a small pipe filled with marbles. If you push another marble in on one end a marble on the other end falls out immediately. Each marble is traveling very slow compared to the effect from one end to the other.
     
  11. Jul 22, 2014 #10

    dlgoff

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    Yep. For copper, the electrons drift velocity is a few millimeters per second.

    micohm.gif

    Microscopic View of Ohm's Law

    My analogy would be a 3 meter piece of Rebar (reinforcing bar) with one person at each end. The "load" person places his end of the bar on his forehead and the "generator" person taps his end with a ball-peen hammer. (note: this is a thought analogy)
     
  12. Jul 22, 2014 #11
    Thanks for the replies, so I understand now how the AC is generated however am still a little confused on how it gets to our homes from the plant. Since no electrons actually end up moving from the plant to houses, in what form is the energy brought in?
     
  13. Jul 23, 2014 #12

    NascentOxygen

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    Electron motion.
     
  14. Jul 23, 2014 #13

    sophiecentaur

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    I do not see your problem (assuming you have read the replies). Nothing has to 'move' from A to B, at all in order to transmit energy. The piston in a car engine has no net motion but 200kW of power can be transferred through the reciprocating motion from the burning fuel to the crankshaft. The rate at which Power is transferred from your feet to the ground when you are cycling is not governed by the speed of the chain - you can use a whole range of gears to do the same speed. You need to look further than just the moving electrons - we already decided they actually go nowhere in AC (even their peak displacement is actually microscopic).
     
  15. Jul 23, 2014 #14

    psparky

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    Ok Sophie, I understand your points, is there any truth to the electric field traveling at 2/3 speed of light?

    And if so is that rotating speed or traveling distance speed...or both?

    BTW, a top fuel dragster or funny car fueled by nitro methane make up to 10,000 HP (Blown, 500 CID reciprocating pistons with no net distance)....thats about 7.46 MEGA watts!!! Enough to propel a 2,000 lb vehicle from 0 to 330 mph in 3.75 seconds on rubber tires! If you never seen this live, go do it. It's so different live than on TV. The sonic wave these vehicles put out is unbelievable. If you dig power, this is the place to be.
     
  16. Jul 23, 2014 #15

    sophiecentaur

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    The EM wave will travel at around that speed when 'tethered' in an electrical circuit. That will tell you roughly when to expect the bulb to light after you turn the switch. lol
     
  17. Jul 23, 2014 #16

    jim hardy

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    oops i see AverageSupernova already used same analogy - apologies extended for repetition.
    I'll leave this anyway - it might amuse (or hopefully help) someone.


    In simplest terms , and for simplicity, let's think for a moment about DC:

    You can think about power transmission as loading a pea shooter. One pea at a time.
    Once the tube is full, every pea you push in one end makes an identical pea pop out of the other end.
    Immediately. With no delay. (Well, actually with infinitesimal delay because the force of your "push" gets transmitted through each individual pea at the speed of sound in peas.)
    But the point is it's not the same pea.
    Any particular pea plods down the peashooter at the pokey, phlegmatic pace of one pea length per push.

    But that's the secret: the delay between pushing on the pea at one end and feeling that push on that pea's identical twin at other end
    is very short.
    Speed of "push propagation" through the medium is quite high, speed of "pea propagation" is quite low.

    AC is just pushing the peas back and forth , so none ever make it from one end to the other they just shuttle back and forth one pea length..

    Power is transmitted by the push. That's what travels at a goodly fraction of c.

    A word of caution:
    Use these oversimplified analogies to make your mind accept the laws of Ohm and Kirchoff.
    Then communicate in those terms - there's good reason for using precision in terminology .

    Lastly: almost all folks nowadays speak in terms of positive charge being what moves, and virtually all textbooks do too, so form that habit as soon as you can. It'll save you lots of trouble as you progress in your studies.


    Have fun.

    old jim
     
    Last edited: Jul 23, 2014
  18. Jul 24, 2014 #17

    psparky

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    Thanks Jim.

    That clears it up 100%.
     
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