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  1. #1
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    Six-Pulse Rectifier Capacitor Value

    Hi guys, I am a beginner-intermediate engineering student and I am currently developing a six-pulse three-phase full wave rectifier with output capacitor parallel to a purely resistive load. The problem I have had is that the equation (which I deduced) that I use for the calculation of capacitance as a function of ripple factor (rf) does not coincide with the simulation ripple factor.

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    The equation is:

    C=1 / (6*4√3*R*f*(rf))

    To obtain the equation, I based an analysis on a single-phase full-wave rectifier with parallel capacitor (in a book) and I made a similar analysis with the three-phase case.

    The values ​​I have for my rectifier are: VLLrms=141.42 , f=20Hz , R=8 ohm , rf =0.03=3%.
    that gives me according to my formula: C=5mF.

    I'm checking the ripple factor with Matlab, using the formula:

    rf=((VoutRMS/VoutDC)²-1)^½ or √((VoutRMS/VoutDC)²-1) in Matlab simulation rf = √((190.9/190.7)²-1) 4.58%

    Matlab simulation : VLLin(ab)=yellow , VoutDC =blue.

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    I don't know what happens. suddenly:

    The formula is not correct?

    Am I checking the ripple factor wrong?

    All aids are welcome and appreciated, Thank you very much

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  2. #2
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    Re: Six-Pulse Rectifier Capacitor Value

    Hi,

    First:
    Please keep on standard signal flow on schematics = left to right.
    Yours is reversed. Thus for us your schematic rather looks like a VFD schematic:
    Left: DC input, DC supply capacitor, H-bridge switches, 3 phase load. (Right)

    Some thoughts about your problem:
    With a 1 phase rectifier, resistive load, no capacitor:
    The expectable signal goes from 0V to sine_peak to 0V to sine_peak to 0V within one fullwave.
    One diode pair is active from 0 to 180° the other pair 180° to 360° of input sine. (Worst case)
    The worst case peak to peak voltage ripple is: 100% × sine_peak

    But a three phase rectifed signal never goes down to 0V. Not even close to it.
    One diode is active from 90 to 120° of V(LL) only. (Worst case)
    The worst case peak to peak voltage ripple is: 13.4% × sine_peak (LL)

    The current of a capacitor is directly proportional to dV/dt.
    Thus let's analyze the dV/dt of a single phase vs 3phase rectified signal.
    Worst case dV/dt of a 3phase rectified signal is only 50% compared to a single phase rectified signal with same peak voltage.

    But three phase rectified signal has 6 fold line frequency, while single phase rectified ripple frequency is 2fold line frequency only.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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  3. #3
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    Re: Six-Pulse Rectifier Capacitor Value

    I have discovered that graphical description is very helpful if you have questions.

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    A single sine wave after rectification (full wave) becomes twice the frequency. Half wave rectification will remove the negative half and keep the top half. Just ignore diode drops etc for the time being.

    A three phase supply is basically a single phase shifted by 120 and 240 degrees. They will have three peaks in one cycle. After rectification you will have six peaks in one cycle. Now see at the graph:

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    You can now see the min and max values of the result voltage from the graph. You can easily calculate the exact value.

    And this is without the capacitor.

    With capacitor and load the calculations become messy. But you get the idea.


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