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I just want to understand 'phase'

  1. Feb 27, 2014 #1
    Can someone explain a little of the physics behind 'phases' I can't get a grasp on what is actually happening. Current - I can understand that electrons are moving. Voltage - okay, no problem different charge here than over there. But then you throw in phases...and stuff is cancelling out.....where did it go? I can do the math, that's not the problem, but I must have missed the lecture day when the broke it all down. Thanks in advance.
     
  2. jcsd
  3. Feb 27, 2014 #2

    jim hardy

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    Some catching up to do, eh?

    A mind-picture might help....

    It is useful to take a protractor and graph paper and plot sin(theta) for theta=0 to 360 degrees,
    then recognize that your (indeed any) sinewave can be represented by
    the vertical component of the radius of a rotating circle.
    That rotating radius is a "phasor"

    See animated phasors here.
    http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/impedance/HTML/acbasics.html

    phase is simply the angular distance between two phasors that are rotating at same speed.

    Don't get behind in this course for they'll throw concepts at you as fast as you can absorb them. It's nearly impossible to march double-time.
     
    Last edited: Feb 27, 2014
  4. Feb 27, 2014 #3
    We're talking about power correct? Think about a graph of two currents that are equal but 180 degrees apart. They would be a mirror image of each other about the x-axis. If one current is positive then the other current is negative by the same amount. They would add up to 0 so they cancel each other out.

    Two currents are easier to visualize but the same thing happens when you have 3 currents that are equal but 120 degrees apart from each other. If you graph them you will see that at any time their sum will be 0.

    This is why you will measure 0 amps on the neutral of a service that has balanced loads. When one phase is pushing current the other two are pulling current by the same amount. When the loads are unbalanced, where one phase current is not the same as the others, then you will see some current returning on the neutral.

    Note that we're talking about wye 3-phase power where the neutral carries all the return current. In a delta system the load imbalance will cause current imbalance in the 3 feeder wires. One or two feeders will carry the return current for the other wires and because real wires have real resistance it will cause a voltage imbalance too.

    All this may or may not help you understand what is happening physically but don't neglect the phasor diagrams. I'm sure phasors are going to be on your test.
     
  5. Feb 27, 2014 #4

    tiny-tim

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    hi tempneff! :smile:
    in DC current, the electrons go in one side of the light bulb and out the other

    in AC current, the electrons just jiggle about inside the light bulb, and go hardly anywhere

    so the current is sometimes to the left, and sometimes to the right, and is sometimes zero (loosely speaking, all the electrons stop moving for an instant) … so a graph of the current looks like a sine curve, and the phase tells us how far round the curve we are at any particular time

    in AC current, in a resistor R, the voltage is always proportional to the current (V = IR), so the graphs looks the same, and the phases are the same

    in AC current, in an inductor L, the voltage is always proportional to the rate of change of the current (V = LdI/dt), so the graphs are shifted, and the voltage is always 90° ahead of the current

    in AC current, in a capacitor C, the current is always proportional to the rate of change of the voltage (I = CdV/dt), so the graphs are shifted, and the voltage is always 90° behind the current

    (i might have got all that the wrong way round! :redface:)

    and in AC current, with a general mixture of resistors inductors and capacitors, the phase difference is more complicated (but constant), and isn't exactly ±90°
     
  6. Feb 27, 2014 #5
    It always helps me to visualize a concept. So the analogy of in phase would be I pull on a rod and the other end of the rod follows the X displacement exactly. If I pull on a spring (you have to imagine one of the right strength) then the displacement on the other end of the spring is coming, but its behind; the faster I pull the further behind it is; conversely if I pull slow the spring stay in phase. Then a dashpot (car shock) if I pull hard the following end follows exactly, but if I pull slow it will take a while to catch up. I theory the energy is not lost, its just in the process of catching up.
     
  7. Feb 27, 2014 #6
    Thank you all, this was very helpful. I guess I got caught up in the math and forgot that we are simple alternating current.
     
  8. Feb 27, 2014 #7

    nsaspook

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    It's useful to think of harmonically changing electrical energy as a circle where the instantaneous value is a point on that circle that changes with time (frequency) and amplitude (voltage/current and polarity).

    If we plot that rotating point as the center of that circle moves forward in time we see a wave and the phasor relationship.


    So, if we see two waves we can now see a phase relationship between the two values if we think about the value position moving up/down each of the lines range of harmonic motion as seen on the first visual.
     
    Last edited by a moderator: Sep 25, 2014
  9. Mar 5, 2014 #8

    sophiecentaur

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    Phase is phase is phase, in power, audio or RF contexts.
    Power Engineers, being Engineers, use their own shorthand and a 'phase' can refer to a 'wire' (carrying a signal of a particular phase wrt the others). Having said that, the phases of a three-phase system are not necessarily separated in phase by 120° - so the 'phase' can have a 'phase'.
    No wonder people get phased by it all. Using and reading about the system will help you grow accustomed to what the word refers to in each instance.
     
  10. Mar 5, 2014 #9

    jim hardy

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    To make it visual -

    When you ride your bicycle, observe the pedals. They rotate 180 degrees out of phase, a half cycle.

    This time of year the sun is low and your bike casts a tall shadow,,, so -
    observe in your shadow the valve stems or the little rectangular reflectors in your spokes.
    If they're at top on both wheels at same instant they're "in phase",
    and as you ride you'll notice their phase shifts because the front and rear wheels take slightly different paths in the turns.
    Equinox is almost upon us and shadows are shortening every day so try this experiment soon.

    We all go through various phases in our understanding.....

    (hat-tip to Sophie)

    old jim
     
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