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Three-phase power explained using water/electricity-analogy?

  1. Sep 25, 2010 #1
    Basic electricity is often explained using the so called water/electricity analogy. This works, but I've never understood three-phase power. Can this also be explained using somewhat the same analogy?
     
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  3. Sep 25, 2010 #2

    russ_watters

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    I don't see how it could be.
     
  4. Sep 26, 2010 #3
    I do. Imagine three pipes of water. The power is alternating, so the flow is reversed every time the sign is. For a positive current water is being sucked away from the device. For a negative current water is being pushed towards the device (counter-intuitive because of the charge of electrons, which are the things that move). A three phase design is one in which you offset the timing of the three pipes so that a little bit of water is always flowing towards the device through one or two pipes and the same amount of water is being sucked away from it through one or two of the others.
     
  5. Sep 26, 2010 #4

    Averagesupernova

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    I was going to suggest something similar but I would stick with simpler circuits using a water analogy. After circuits are understood without the water, 3-phase can be explained.
     
  6. Sep 27, 2010 #5
    It seems harder to understand with imagining water from three pipes terminating at a null point.
     
  7. Sep 27, 2010 #6
    @ Blenton: Fortunately blood vessels have the equivalent to a fluid diode. It prevents blood from flowing backwards through your body as your heart pumps. Connect each of the pipes to a main line with a diode for water coming out of the device. For water going in you need some sort of rotating port that will cover either one or two of the pipes at the same time and make it rotate in phase with the fluid. Hopefully that's easier to imagine.
     
  8. Sep 28, 2010 #7

    sophiecentaur

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    This approach seems very convoluted. Water can, at a stretch, give you a very basic idea of DC flow (to 'get you feet wet' on the subject LOL) but WHY????? try to extend the analogy to AC? Multiphase systems involve phasors and an idea of vectors and they are used for generators and motors, which rely on fields and phase differences. The whole thing is hard enough without getting involved with an analogy which is flawed from the start.
     
  9. Sep 28, 2010 #8
    @sophiecentaur: You're not wrong, but the weak point I'd be going after is magnetism. Electricity is very easy to understand as a transverse wave, and the analogy is almost perfect under ideal conditions without taking into account laminar flow, superconductivity, chemical reactivity, etc. The really very basic point at which the analogy breaks down though is magnetism. There is no simple analogue to a magnetic vector created by a moving charged particle. And those are tough to understand intuitively, hence the reliance on an analogy (even so flawed).
     
  10. Sep 28, 2010 #9

    sophiecentaur

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    This water analogy thing is never far from the forum. I just think that, except for the most 'noddy' appreciation of electricity, it can be a total snare and delusion for the unwary. I think that you just can't expect to get much of a feel for electricity "intuitively"; it's just not that sort of beast.
    Just as with Quantum Theory, if you think you've got it, you probably haven't.
     
  11. Sep 28, 2010 #10

    Averagesupernova

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    EVERY analogy is flawed. They only work so far. Some analogies are better than others and water in a hose is soooooo common that it helps most people.
     
  12. Sep 29, 2010 #11
    Water flow in a pipe does have an analogy for magnetism around a current carrying wire, At least in the transmission line sense. The momentum of the moving water causes similar effects to magnetism around a conductor. For example when a flow in a pipe is quickly shut off at the output end the water momentum causes a spike in pressure at the valve. The pipe bang called water hammer. This is a direct analogy to a voltage spike that occurs when a current is interrupted. The magnetic field collapse causes a voltage spike in an analogous way. There is also a capacitive analog, a pressure tank. Three phase power is analogous to a multi cylinder positive displacement water pump. One piston would give a pulsating pressure output, multiple cylinders on an offset crankshaft would reduce these pulsations in a way similar to multi phase electrical power This can also be done with a capacitor, pressure tank.
     
  13. Sep 29, 2010 #12
    You seem to have very strong opinions on the use of this analogy. Rather than address those, I think I'll refer you to this book on catastrophic interference: http://books.google.com/books?hl=en...=onepage&q=catastrophic interference&f=false"

    The general theory, at least as I understand it, is that simplifying systems to accommodate sequential learning methodologies might be the wrong answer for back-propagation networks. It creates catastrophic interference, which is demonstrably undesirable. The use of analogy helps by creating a cognitive stepping stone from which we can draw similarities to and differences from the analogue without detracting from the overall retention and recall.

    But the O.P. wasn't asking for opinions. He was asking for the provided analogy.
     
    Last edited by a moderator: Apr 25, 2017
  14. Sep 29, 2010 #13
    @drummin: Hey, that's neat. I should have been more specific: magnetism as it relates to interference or inductance. Parallel pipes will not draw or induce current from parallel lines, and perpendicular pipes will not behave analogously either.
     
  15. Sep 29, 2010 #14

    sophiecentaur

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    Well, there's flawed and there's flawed.
    As a way into the idea of charge flow, water flow is ok but too many people hang onto it way beyond its applicability. From some of the posts on this and all other threads about it, I can see that there are some really staunch supporters. With enough 'religious' fervour, it's possible to stretch and stretch an analogy until it fits a bit better. But, once you get to that pont, it will just serve to confuse the beginner even more than the more rigorous method using R,V,I etc..

    All analogies have a massive Caveat associated with them and must be used very carefully when extrapolating beyond them. The ultimate analogy is, of course, Mathematics. How many times does the beginner do a sum and rush off with the inappropriate sign of square root, a hidden 'divide by zero' or a wrong choice of quadrant and come up with a seemingly reasonable answer that is nonsense?

    People who rely, solely on the waterflow analogy (and that potentially includes anyone who got no further than GCSE) are going to fall over when they try to extrapolate away from this too familiar and too comfortable model. I'm not sure that it actually "helps" many people. I think it has the potential to them a legacy for a lifetime of misconceptions.
     
  16. Sep 29, 2010 #15
    I agree completely - all you need to do is scan through some of the "Similar Threads For..." listed below. As I see it, the 'water analogy' is something you tell a 10 year old to help him get past the notion that you can't 'see' electrons moving. After that, what's the point?
     
  17. Sep 29, 2010 #16

    sophiecentaur

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    A biblical parable is an analogy and how many members of the forum would insist that parables are reliable models under all circumstances? Some stories of ancient religious origin may (and do) have very strong emotional appeal but should they be used as the sole basis for making decisions?
    Stands back and waits for tirade. . . . .
     
  18. Sep 29, 2010 #17

    sophiecentaur

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    Good and interesting point - and I looked at some of that reference. I think I made sense of it!!?
    From that and what you have written I can see that analogies are useful (of course they are) it's just that they can easily get out of hand (to wit the water flow analogy) because they are, in fact, never superceded or resolved for many people. No one helps them past the 'stepping' stone and they think they have actually arrived. Reading statements from 'champions' of an over-stretched analogy will only lead them further into misconceptions because they start to think it really must be reality.

    In my experience, the water analogy is one which the 'weaker' (in general) teachers of Science tend to hang onto longer than those with a better understanding (in general). That's my problem.

    And since when did threads stick to what the OP wanted? Where would be the fun in doing that?
     
    Last edited by a moderator: Apr 25, 2017
  19. Sep 29, 2010 #18
    Just have to say how I appreciate how this thread really got more than the few responses I was hoping for. I'm reading every word :)
     
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