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When do we get peak voltage in a 3-phase generator?

  1. Oct 17, 2012 #1
    Think Im having a brainfart here, so I need a tad bit of help.

    Sorry, imageshack rotated the image for some reason:
    http://img690.imageshack.us/img690/5629/img1380as.jpg [Broken]
    (http://img690.imageshack.us/img690/5629/img1380as.jpg [Broken])

    I cant really understand anymore when do we get the peak voltage? When the rotating magnet is the closest with its pole tip to the inductor, or when the tip of the magnet is 90 degrees to the pole, like in the picture?
    It seemed like "common sense" that we get the peak voltage when its tip is closest to the inductor, but once I started thinking about it, I got confused when I noticed that when the tip of the magnet is nearing the inductor, the inductor creates a magnetic field that works against the change, which in return creates a certain direction voltage. Once the tip of the magnet reaches its closest point and starts moving away from the inductor, the inductor creates a magnetic field and a voltage that tries to preserve that same magnetic field. Which is pointed in the opposite direction to the voltage/magnetic field in the first situation.
    It would mean that when the tip is closest to the inductor, the voltage is decreased to 0 at that instance, and after the tip passes, the voltage, and therefore the currect change direction.

    Now Im not sure my logic is correct here, because I've always thought it was the opposite way. Is there a brainfart somewhere there in my logic? Would appreciate if someone could point it out.
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Oct 17, 2012 #2
    I don’t think there’s much wrong with your analysis.

    The picture you downloaded is just very basic. If you had a generator like that your resulting waveform would be highly irregular, it wouldn’t give a nice sin form. All you would get is some sort of spike when a pole nears a coil. Like you said the voltage changes sign when the tip is closest. If we call that point 180 degree, then the max voltages of this generator will be at 170 and 190, instead of 90 and 270 degrees. Generators and motors used for 3 phase mains power have more than one coil per phase and the magnetic poles are of a different shape. There’s a lot more to it.
     
  4. Oct 17, 2012 #3
    In a three-phase system three alternating currents of the same frequency reach their instantaneous peak values at different times....120 degrees out of phase.

    lots more here:
    http://en.wikipedia.org/wiki/Three-phase_electric_power

    Double click the bottom of the first diagram for a nice schematic:

    Hawkins Electrical Guide - 3phase Elementary 6wire.jpg
     
  5. Oct 18, 2012 #4
    That doesnt add up. If the voltage changes sign at 180-point, then the max value has to be 90 degrees from it, as you can read from the graph. So as the wave makes one oscillation per one magnetic rotor rotation, the max value seems to be reached at 270 and 90 degree points (as we called that closest point 180-point). Am I wrong here?
     
  6. Oct 18, 2012 #5
    The graph reads 90 degrees, but that’s for a perfect generator. The generator displayed is only very basic, just to give a novice an idea of 3 coils and a rotating magnet. Real life generators are more complicated.

    If you would build a generator such as the one displayed you wouldn’t get the nice diagrams where the max voltage is at 270 and 90 degrees.

    Why don’t you build this simple generator, put an oscilloscope at the end terminals and see what you get. You only need one coil, the others will behave similar. Make everything roughly the same ratios. It’s fun and educational. And you can prove me wrong.
     
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