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LC filter for 3phase rectifier


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#1 Kumara Swamy.K

Kumara Swamy.K

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Posted 19 August 2003 - 04:46 AM

Hi,
I have designed a 3phse input and 155V/220A output rectifier. I am unable to calculate the filter inductor and capacitors to reduce the ripple on the output DC. In my case when measured the ripple current is around 70A approximately. What shall be the calculated ripple current. How many capacitors are required to be used in it.

Any one can help me out.

Thanx..
Kumar.

#2 marke

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Posted 19 August 2003 - 09:55 AM

Hello Kumara Swamy.K

Welcome to the forum.
Please post your questions once only. Multiposting just splits the replies and makes it more difficult to follow the thread.

When I have designed applications such as this, it has generally been for use in inverter front ends and the approach that I have taken is to select the capacitors, not on the capacitance, but on the ripple current rating. If you are not careful, you can focus on the no of farads and forget the Amps. My process is usually to study the applicable capacitors, get the prices and establish the lowest cost per Amp ripple current, then use a multiple of that capacitor in a series parallel arrangement to give me the required voltage and current rating. There are many theories on what you should do, but I have seen many designs where the ripple current rating is too low and the capacitors last only for a year or three.

I always design the filter inductor to minimise the harmonic current on the input to the rectifier. To do this, you need to select the inductance such that current will flow on each phase for 120 degrees per half cycle (Maximum that you can get with a passive system) and I would select for "critical" inductance at around half rated load current so that the harmonics are minimised down to half load.
It does depend on whether you are using DC or AC inductors.

Best regards,
Mark.




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