Fundamental filter questions

[exskool]

Hallo guys..

I hv very basic questions which need your help to clarify them..

Let say that I hv a LC filter, tuned at 8kHz with THDv and THDi limit to 5%, configured for 3-phase system (400Vrms,562VAC,50Hz,10A)

a) if there is a spike of i.e. 60V, how does the filter actually deal with it (absorbed, suppress, cutoff,??)..

b) if absorbed, is it means only 562V is allowed on all the time? how about spike less or more than 60V?

c) what is the gain of filter in bode plot tells us since the gain is negative in the working frequency (8kHz)? is it means it will suppress extra voltage on the system (noise/spike) ? how about if it only able to suppress only 5V at certain frequency but the spike is actually 10V?

d) if the filter is design to specific THD limits, is it means that it will suppress harmonics starting from 2nd/3rd harmonic number to maybe the 17th or only works on certain harmonics number only

e) is it ok to limit the THDv and THDi < 5% ? how about make them much much lower i.e. 0.05% ?

I hv googling a lot but still cannot reach to any concrete answer..

Thanks for reading those questions and many thanks for answering them!!

 

[MacGyverS2000]

homework?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[exskool]

no..kind of filter design project..

although I hv computed the components' values, still I not quite understand about how it works..

btw, thanks a lot

 

[Skogsgurra]

Not possible to answer your questions. You have to understand what a filter does.

There are no such things as an LC filter being able to suppress 5 V but not 10 V. It all depends of the frequency contents of the spike, a short spike has more HF components than a broad spike with slow edges.

Also, you cannot specify allowed THD, only attenuation in a frequency range. There are no filters where you can specify 5% or 0.05% distorsion. You really have to learn about filters before asking about them. Or, at least, ask more realistic questions.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

"Also, you cannot specify allowed THD..."

Not strictly true, at least not once wound components with saturable cores get involved, but I think you are a long long way from being concerned about distortion introduced by the filter itself.

There are some good filter design books available; in your position I would start by reading one or two of them. Most decent analog electronics texts have a section on filters too. I'd probably reach for 'The Art of Electronics' for a good general grounding in analog design.


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If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

Scotty,

I agree that the OP is probably not ready to discuss distorsion intruduced by nonlinearities in the filter and a 'distorsion limit' set in some imaginary software.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[exskool]

hi..now i hv a better understanding on the filter..thanks to all comments which make me to think properly..

forget previous questions as I hv another question..

i suppose to have a filter working at fs..so far, when i read articles, the cutoff frequency should be between input freq (50Hz) and fs(8kHz)..

ok, now i hv cutoff freq at 2kHz, and at fs, the gain is -20dB = 0.1 voltage ratio (Vout/Vin)..

so, if i hv Vin as 400V, the Vout is 40V..is it right?

but isn't it that the output must be as close to input at working freq?

or is it the Vout is relative value, not an absolute value?

 

[Skogsgurra]

What does 'hv' stand for? High Voltage?

Please, try to use ordinary English. You are not sending SMS here - or are you?

Why are you saying that gain is -20 dB at cut off frequency? It usually is not. If you are talking about an LC filter, you cannot say much about gain before you have defined the filter. If you are talking about an RC filter, which I think you are not, then the slope is -20 dB/decade while the gain at cut off is -3 dB.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

Hi Gunnar,

I think you should stop reading this thread. Think about your blood pressure!


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If we learn from our mistakes I'm getting a great education!

 

[exskool]

hv = have

"i hv cutoff freq at 2kHz, and at fs, the gain is -20dB = 0.1"

-20dB at fs(this is my freq which I work at)..

better slow than never...

 

[Skogsgurra]

I am taking pills for that. Thanks for caring...

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[exskool]

at least look at my background(computer programming)...

may be it is so clear as sunlight to you and my questions look so dumb..

just drop some answers, that should be enough..

 

[Noway2]

"just drop some answers, that should be enough."

I don't know about the rest of you (regulars) here, but I find statements like this to be really rich. A newbie comes along, makes their FIRST post, doesn't follow the rules of the forum, and then starts getting rude and nasty when they don't get a simple answer handed to them. Forget the fact that they clearly don't understand the topic they are asking about and appear to have near zero interest in understanding it.

Personally, I found the thread interesting. It sounds like he is working on a harmonic filter for a power supply or UPS. I was going to respond in a manner to help this guy, but when I read his last post, I changed my mind.

 

[exskool]

ok..thanks you for all comments...

i accept the fact that i was rude to you all..

i am really sorry and wish you a nice weekends..

fyi, i have found the answers for my questions..

bye..(hopefully a peace and happy ending)

 

[Skogsgurra]

That is great, exskool. I am somewhat puzzled. You say that you found the answers. It would be very interesting if you shared the answers with us - 'cause we don't have a clue. We didn't even understand what the questions were.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...