LC Filter for Three Phase & Single Phase Thyristor DC Converter

Without so much as a single datum, how can anyone answer?

My circuit was a battery charger of 24V DC output, 10A max; Input supply to the rectifier is fed from a 750VA 230/45V AC, 50Hz transformer. I want the ripple to be limited to 3%. The L value i have selected is 3.14mH & C value is 10000MFD. My doubt is up to what limit the L & C can be increased and what is their impact on the rectifier efficiency?

Use PSpice to simulate your circuitry.

just a reminder that dB is a generic relative term, and varies in its definition across the full spectrum of how it is applied. sometimes 20dB represent a factor of 10 increase (-20dB would be 1/10 of the original), whilst other times 20dB would be a factor of 100 increase, and -20dB would be 1% of the original.

The links are not clear which definition is being used, so we assume to power ratio definition so that -20dB will represent 1% of the original signal as ripple. The calculator gives a pretty good result based upon that definition, but who knows if in fact it is right?

The impedance of a battery under charge is so low that ripple elimination is mostly carried out by the battery itself!

The L and C can be increased without limit in terms of value, however with increasing value comes increasing size: in the end it is a judgement call - limitations of space inside the equipment enclosure can have a greater bearing on this decision than anything else.

A battery charger of that rating can usually be purchased at various automotive spares outlets (names withheld), so there is rarely any need to design.

Without any liability

The value for a smoothing capacitor is 2000 uF for each ampere current (rule of thumb). In your case you need a 10 000uF capacitor. But it all depends on the allowable ripple current.The voltage level for the capacitor is 45 X sqrt (2) x 1.2 (safety value) = minimum 76V.

Keep in mind to use a 1 phase transformer with a center tap and 2 diodes or if there is no center tap use 4 diodes (bridge rectifier).

A better solution, if you have 3 phase supply, is to use a 3 phase (separation) transformer and 6 diodes.

Another point of attention. Your output voltage must be 24V, if you use a transformer with a secondary voltage 45 Vac, the output voltage will be too high.

Without any liability

The value for a smoothing capacitor is 2000 uF for each ampere current (rule of thumb). In your case you need a 10 000uF capacitor. But it all depends on the allowable ripple current.The voltage level for the capacitor is 45 X sqrt (2) x 1.2 (safety value) = minimum 76V.

Keep in mind to use a 1 phase transformer with a center tap and 2 diodes or if there is no center tap use 4 diodes (bridge rectifier).

A better solution, if you have 3 phase supply, is to use a 3 phase (separation) transformer and 6 diodes.

Another point of attention. Your output voltage must be 24V, if you use a transformer with a secondary voltage 45 Vac, the output voltage will be too high.

I intend to use single phase half controlled bridge rectifier with 2 SCRs & 2 Diodes. The output voltage is 2Vm/pi * Cos(alpha). The output voltage can be adjusted by firing the SCR is suitably.

If i use 10000uF capacitor what shall be my choke value?

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The smaller the fire angle is, the less energy you put in the capacitor, the bigger the capacitor and coil must be.

No idea what the minimum value will be for the coil, it also depends on the ripple current that is acceptable. But the higher the inductivity, the smoother the current will be. Higher inductivity means a bigger coil and more windings. Please use no air-coil, but coil with laminated steel core.

A secundairy from an old transformer is good. But the cupper wire diameter must be big enough to carry the 10A current. (rule of thumb = 3A per square mm cupper surface)

Using a variac is much easier, capacitor and coil can be smaller.

Why not using a SMPS (switched mode power supply), efficiency is much better, current is much smoother, price is probably less.

http://be.farnell.com/tdk-lambda/rws-300b-24/ac-dc-conv-1-o-p-24v-12-5a-300w/dp/2444007.

Interesting that you would go there. Those are great little power supplies, IMHO.

You want to get a copy of the paper titled, "Analysis of Rectifier Operation" by O.H. Schade. It was published in 1943. Yes, 74 years ago. Mr. Schade put a lot of work into those curves. Don't waste his efforts by ignoring them.

Here is a source for more knowledge:

http://www.powerelectronics.com/content/ac-rectifier-bulk-capacitor-selection-analysisspice

and here:

http://www.powerelectronics.com/power-management/analyzing-full-wave-rectifiers-capacitor-filters

Good luck with your quest.

It is fascinating how many Engineers try to design power supplies when there is so much immediately available on the open market that saves one having to do the design.