PF vs frequency in 3phase diode rectifier

[jad.mak]

i have an assignment on PF vs frequency in 3phase diode rectifier the DR want me to draw a graph and stuff but I am not sure how 2 do it or if it does affect the PF.
thank you

 

[uart]

You'll probably need to give a bit more information about your task before you get much help.

What type of load are you allowed to assume?

Do you understand displacement factor and distortion factor and their relationship with power factor?

 

[jad.mak]

i have a 500 V L-L rms 3 phase diode rectifier that supply a load of 5mH and 10 ohm the initial source frequency is 1000 hz. my task is to study the relation between PF of source and source frequency. according to my book the PF= Is1/Is cos(φ) but i dnt think the frequency can affect this. so i dnt know what to do

 

[uart]

according to my book the PF= Is1/Is cos(φ)

That's correct. The ratio of fundamental to rms (Is1/Is) is called the distortion factor and $\cos(\phi)$ is called the displacement factor. They both combine to cause the average power supplied by the mains to be less than the apparent power.

but i dnt think the frequency can affect this. so i dnt know what to do

.

Ok let's look at the case where f is very large. With the RL load (time constant 0.5ms) it's clear that if f is large enough then the load current is sensibly constant. This is a simple case to analyze, the line current is quasi square and the displacement factor is unity. All you need to do is find the ratio of fundamental to RMS for the quasi-square.

As f is reduce eventually we'll reach a point where the approximation of constant load current will not be appropriate. Now we get to the harder part. We'll still have something like a quasi-square wave but the tops of the wave won't be completely flat. Presumably this will somewhat alter the ratio of fundamental to RMS. It may even alter the displacement of the fundamental (though I haven't yet analyzed it myself in detail) and so the PF may indeed be a function of frequency.

So start with the quasi square (large f - const current) case. After that it's time to really test your transient analysis skills and find a time domain expression for the current in the more general case. Once you know the exact shape of this "modified top" quasi square wave then you can calculate the displacement and distortion factors).

 

[jad.mak]

ok thank you i will try to work with what u said and if i have any questions i will ask thank you so much hope i have enough time because this is due to tuesday.

 

[uart]

Hi jad.mak, I just made some calculations and while the PF does vary a tiny bit with frequency it's only very minimal and a pain in the butt to calculate. I got a 0.1% change in PF as frequency varied from zero to infinity!

Are you SURE this is the question you were asked to look at? Are you sure it wasn't power factor versus the firing angle in a controlled rectifier or something like that they asked for?

 

[jad.mak]

well yes that's the question I am so pissed i didnt find anything at all i was working on it all day and i didnt even find this change can u send me by email or smth this calculation. i have this question for diodes and thyristors rectifiers but i didnt know for diodes anything and i think the thyristor will be the same problem!

 

[uart]

Just making sure, it's a 3 phase full bridge rectifier, correct?

You are allowed to assume ideal components?

To see how little the PF changes with frequency consider the case where freq becomes very large in relation to R/L and therefore the load current is approximately constant. Did you get an answer for this case?

Now look at the case where freq goes to zero and we can assume the load is purely resistive. Now the load current waveform is just the same as the voltage waveform on the DC side. Can you sketch the current waveform on the AC side. You just need to find it's RMS value along with the in phase component of the fundamental (the first cosine term of it's Fourier series).

Can you make a start on these two cases?

 

[jad.mak]

it's 3 phase full bridge rectifier well in calculation i have 2 assume its ideal to give the difference and than i should show my result on simulation on multisim. ok i will start with those to cases the first one is a general case because the teacher said we always assume that the current is constant for high frequencies. the second case sounds complicated but i will try it because he didn't teach us anything about it but i will read in the textbook and see