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Homework Help: Buck Converter Questions (DC-DC Converter)

  1. Sep 26, 2012 #1
    1. The problem statement, all variables and given/known data

    See first figure for lab attachment

    2. Relevant equations

    3. The attempt at a solution

    Hello all,

    I recently did a lab on buck converters and I have a couple a questions.

    See the figures attached for lab and my work.

    For Experiment 1:

    The measured quantities for part b) were,

    [tex]I_{ave} = 0.2A \quad I_{min} = 0.09A \quad I_{max} = 0.28A \quad Ripple_{pp} = 0.19A[/tex]

    These values differ slightly from my theoretical calculated values, did I make any mistakes or are there any discrepancies?

    Also, the Imin I calculated was negative, is this a problem? Also, I'm not sure if I am correctly the average current correctly, can someone verify?

    The new switching frequency we measured for part c) was,

    [tex]f_{swnew} = 300Hz[/tex]

    The theoretical frequency I calculated was 240Hz. Is this a large discrepency? Did I make any mistakes in the calculation?

    For Experiment 2:

    For part a),

    The measured quantities for 500 rpm are,

    [tex]V_{t} = 32.8V \quad I_{a} = 0.82A \quad \text{Duty cycle }= 32.8\%[/tex]

    The measured quantities for 1000 rpm are,

    [tex]V_{t} = 59.81V \quad I_{a} = 0.89A \quad \text{Duty cycle}= 59.81\%[/tex]

    The measured quantities for part c) are,

    [tex]I_{ripple,p-p} = 1.27A \quad I_{ripple,avg} = 0.79A \quad f = 292.51Hz \quad \text{Duty cycle }=32.09%[/tex]

    When we preformed this we did not observe any oscillations on the shaft speed but I'm not quite sure why.Is it simply because the switching frequency was high enough such that the motor wouldn't have time to react to any of the sudden changes in voltage?

    Can someone clarify why that would be the case?

    Thanks again!

    Attached Files:

  2. jcsd
  3. Sep 28, 2012 #2
    Bump, still looking for some help!
  4. Sep 28, 2012 #3


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    Staff: Mentor

    Sorry, I got confused by the post yesterday so didn't reply. Let me try to make some headway on the Buck part of the questions.

    The assignment sheet seems to specify 150Hz for the switching frequency, and you are asking now about 240Hz and 300Hz. Can you please clarify that?

    And you say you are worried about your Imin being negative, but you show Imin = 0.09A. Can you clarify that please?

    When you say you measured values different from your predictions, did you take all the non-ideal numbers into account in your prediction? Did you measure the actual value of the inductor and the Vf of the diode across your flywheel current range, and use those numbers in your calculations? How about the voltage drop across the IGBT?
  5. Sep 29, 2012 #4
    For Experiment 1,

    In part b) we dialed the frequency to 150Hz and measured the values in the table, the measurements we observed are in my original post. I then calculated the theoretical values (see 2nd figure attached), which slightly differed.

    In part c) it says to adjust the frequency to obtain a ripple of 0.1A, the observed measured frequency was 300Hz, but when I did the theoretical calculation (again, see 2nd figure attached) I obtained a frequency of 240Hz.

    Becuase that Imin is the value we measured in the lab. If you look at the 2nd figure attached that contains all my work for calculating the theoretical values, where I had solved for a negative Imin. (-0.0513A)

    No we did not, but obviously neglecting some of these non-ideal numbers will account for some of the discrepency between the measured values and the calculated values, I just want to know whether I am executing the calculated values correctly, and that my there is some agreement between the calculated values and measured values. (It need not be exact)
  6. Sep 29, 2012 #5


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    Staff: Mentor

    Sorry, I'm not able to read the 2nd attachment. Maybe that's part of my confusion.

    Can you do a higher resolution scan of your 2nd attachment? Or maybe take several pictures closeup of different parts of it...?
  7. Sep 29, 2012 #6
    Here is all my work attached, parts b), c) and d).

    Attached Files:

  8. Oct 1, 2012 #7
    berkeman are you still checking in on this thread?

    I've worked out all the details regarding the calculations for Experiment 1, but I am still curious as to why we did not observe any oscillations on the shaft speed of the motor.

    Thanks again!
  9. Oct 1, 2012 #8


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    Staff: Mentor

    What numbers did you get for part 2c? Did you feel the shaft with your hand, or just listen for any oscillations?
  10. Oct 1, 2012 #9


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    Science Advisor

    Were you able to independently verify the value of the inductance you used. To specify "an inductance of 300 ohms" seems to be a fairly dubious thing to do when using a square wave (or pulse train).

    I mean to specify a load resistance of 300 ohms and a time constant of xx ms would have been a reasonable thing to do. To specify a load resistance and inductance of 300 ohms, no so reasonable in my opinion.
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