# Ripple factor & Complex Imag. 1-75



## slowhokie (Oct 2, 2012)

Complex Imaginary Vol I problem 75 asks to find the ripple factor of a 3 phase DC rectifier. The formula they use for the solution is r = (Vrms/Vavg) * 100 (actually, they write Vac for Vrms and Vdc for Vavg) for a ripple factor of 103%. But I thought ripple factor r = sqrt((Vrms/Vavg)2-1) * 100, a formula which they actually use in Vol IV problem 25. So why the different formula in the first problem?


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## danderson (Oct 3, 2012)

I'm not sure if this is the answer, but vol. 1 question 75 gives you the supply AC voltage, not the rms voltage across the load which is what vol 4 question 25 gives you. Both give you average voltage across the load.

Like I said, I don't know if that is the answer or not, but I would think you would have to solve the two problems differently given the difference in supplied data.


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## Kovz (Mar 26, 2015)

I am having trouble with the same issue as the original post. I have two ripple factor formulas and I'm not sure when to use them. Can someone please elaborate?


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## jgharris P.E. (Mar 26, 2015)

I did the first CI test Monday and I ran across this problem and would like to hear some clarification of when to use the two different formulas


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## Kovz (Mar 27, 2015)

Bump...

Two equations for Ripple Factor. When do you use each one? Someone has to have some insight on this...

r = (Vrms/Vavg) * 100

or

r = sqrt((Vrms/Vavg)2-1) * 100


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## MyBeardAndMe (Mar 28, 2015)

I'm not sure, but I would guess that you use Vrms/Vavg when Vrms is the supplied rms voltage and sqrt((Vrms/Vavg)2-1) when Vrms is the rms voltage across the load. That's the only difference I see between the two problems.


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## Kovz (Mar 30, 2015)

MyBeardAndMe said:


> I'm not sure, but I would guess that you use Vrms/Vavg when Vrms is the supplied rms voltage and sqrt((Vrms/Vavg)2-1) when Vrms is the rms voltage across the load. That's the only difference I see between the two problems.


I think you're right. In another thread while searching for Ripple Factor, I came across a post directly from Complex Imaginary. Thank you MyBeardAndMe. You have explained it better. I'll be sure to mark my notes as to when to use each formula.



Complex Imaginary said:


> I don't believe this problem contains an error. As explained above, there are two ripple factor formulas. The difference in the two formulas is which Vrms are you going to use: the rms value of the AC supply components; or the rms value of the outputted rectified waveform. In this particular problem, the waveforms are both given at the load, which means you have to use the formula of the latter.
> 
> Josh


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## MyBeardAndMe (Mar 30, 2015)

You're welcome. Glad I made the right assumption. I too made note of it in my binder. Thanks for finding the response from CI.


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