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Homework Help: Drawing 3-Phase Voltage Waveforms

  1. Mar 26, 2013 #1
    1. The problem statement, all variables and given/known data

    Draw and label the 3-phase voltage waveforms where VAN= 347∠0° Vrms, VAB= 600∠30° Vrms, Frequency: 50Hz, Volts/Division 200, Time/Division: 2 ms

    2. The attempt at a solution

    Im really not sure how to tackle this one. My prof hardly went over this in lecture, but gave this out as an assignment (wtf). Does anyone how to go about starting this?
  2. jcsd
  3. Mar 27, 2013 #2
    So for starters do you know how to convert from the frequency domain to the time domain? It might help get you in the right direction if you convert your readings.
  4. Mar 30, 2013 #3
    3 phase systems in the most classical sense are provided by 3 sinusoidal voltage sources with:

    (1) the same peak values
    (2) which alternate at the same frequency
    (3) which are displaced in time by 1/3rd of a cycle or 120 degrees

    I would expect that you have three voltages which are out of phase (if you want to do the polar plot with the rotating vectors).

    So if you had my three classical voltage waves - lets call them Va Vb Vc
    and lets say that they are all oscillating at the same frequency, with a magnitude of 5V, but are out of phase.

    then Va = 5<+120
    Vb = 5<0
    Vc = 5<-120

    To convert from frequency to time domains in a really proper way is to move from a fourier transform to a laplace transform. However I think this may not be needed in your case.

    Were you to plot out the three waveforms that I have described you would see the following:

    So what you (probably) need to do is:
    1) work out the phase difference (displacement along the time axis of your waves.. which ones lead and lag which other ones)
    2) assume they are all operating at the same frequency (which they most likely are)... and understand the relationship between frequency and wavelength.. which can help you draw waveforms on the squares
    3) use the voltage and time/division to space out your plot squares.
  5. Mar 30, 2013 #4
    I think the information you have put up at present only describes 2 phases of your 3 phase plot.

    VAN= 347∠0° Vrms, VAB= 600∠30

    Unless the third phase is somehow "implied" or assumed to be at 0. Ie that the VAN phase happens at exactly the same time as this phase. Those two phases definitely have different magnitudes.. ie different heights on your plot if it is voltage against time... or different vector lengths on your rotational plot..

    What exactly has he asked you to plot voltage against? :)
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