# desiging filter

#### rajrbr

Joined Apr 8, 2010
6
hi all

i have problem in my load bank

output voltage 65 vdc
current 300A

iam getting 30-40 AC voltage ( ripple) in o/p i tried using C filter ranging from 10000-30000 microfarad 100v but no big diffrence in AC component

instead Dc voltage got increses to 90VDC.

i want to retain 60-65 vdc with minimum Ac voltage , i understand from forum that LC filter would be good for filter application.

could any one help me designing for above voltage and current.

regards
raj

#### thatoneguy

Joined Feb 19, 2009
6,359
For a 300A load, you will need large inductors and capacitors to reduce the ripple.

However, this is going to increase the filtered voltage to over 90 Volts. This is due to the 40V ripple will be the peak DC value, if completely filtered, which, as stated, will be difficult.

What frequency is this power at? Is it created from multiple transformers or phases?

You may be looking at 0.5H Inductors, and near 1F in capacitors if it is line frequency (50/60Hz)

#### crutschow

Joined Mar 14, 2008
34,408
A large inductor and capacitor filter can generate an output voltage equal to the average value of a full-wave rectified sinewave (.637 of the peak value) but you have to maintain a minimum current load on its output or the voltage will go to the peak value of the sinewave. Here's a link that discusses the design of an inductor (choke) filter for rectified voltages.

#### rajrbr

Joined Apr 8, 2010
6
hi thatoneguy

the frequency is 50hz.

i did some calculation to find c value c=I load/ freqency x v ripple for load current of 300A i guess 90000-100000 micro fards ( 0.1fards) is good enough.

and about l around 1 henry will that be ok?since L is in series there wont be any ac component to capacitor there by getting 60-65 desired dc voltage.

hi crutschow
my load minimum would be around 1-2A and max 200A but to be on safer side desided to design 300A

is my calculation of L & C above is ok ? ( 1 henry and 0.1 fards ).

regards
raj

#### thatoneguy

Joined Feb 19, 2009
6,359
How long would the 200A max pulses last?

A 1 H inductor for 300A is going to be rather huge.

This one is 3 Henry for 15 Amps in in the photo, 300 Amps will be a lot larger, even if it is 1/3rd the turns. I'd suggest adding more capacitance, or finding a way to reduce the peak current. This is roughly half the size &diameter of a 55 gallon drum.

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#### Ron H

Joined Apr 14, 2005
7,063
I'm not sure how big a 1H, 300A inductor will be, or how much it will cost, but I'm pretty sure you won't like either answer.
This doesn't sound like a load bank. It sounds like an AC to DC power supply.

#### thatoneguy

Joined Feb 19, 2009
6,359
Using 0 gauge wire, you'd only need an air core coil 3/4 of a mile long, wound with multiple layers, having a final diameter of 16 feet, requiring 7,120 turns around a 1" form.

0 gauge wire is about 1/3 inch in diameter + insulation.

Adding laminations to increase permiability may reduce the number of turns by 2,000 or so, but it would still be rather large. And it could only handle 180A, so you'd need to get 4 of them, 2 in series in parallel with another series of 2 for 1H capable of 250V/340A

For one coil, you'd have to buy 70 miles of cable (using air core), a laminated core would reduce wire use, as I stated.

It's also use 62 tons of copper, in case the first part seemed daunting.

#### rajrbr

Joined Apr 8, 2010
6
hi
thatoneguy

yes 3h would be very huge.

if i use 0.5 henry with 1farad capacitor will my o/p volatge will be 60 vdc?

if i reduce the o/p current say for max of 200A ( insteadt of 300A) can i use 0.5 H and 0.1 /0.2 farads.

raj

#### Ron H

Joined Apr 14, 2005
7,063
This doesn't sound like a load bank. It sounds like an AC to DC power supply.

Is that what it is?

#### thatoneguy

Joined Feb 19, 2009
6,359
The largest choke I can find, other than the one above is for 1H @ 1Amp for a bit over \$200 surplus.

If you tried to put even 2 amps through it, it'd smoke.

#### Lundwall_Paul

Joined Oct 18, 2011
236
It does not sound like a load bank to me either. What is the real load and application?

#### thatoneguy

Joined Feb 19, 2009
6,359
To get down to even a 10 Volt ripple you'd need 4 farads of capacitors, which at the voltage you are talking about, would be a rather large bank of them.

#### rajrbr

Joined Apr 8, 2010
6
hi ron

yes you can call AC / Dc power supply.

60 v rheostas load are connected for AC and Dc load via changeover switch.

hi that one guy yesterday i discussed the same with one of our vendor he suggested to use

4mhenry inductor with 80000 microfards 150v along with 6 ohm( 1000w) bleeder resistor accross cap ,
the bleeder resitor which will ac as load regulation by providing minimum load.
will try in weeks time and let you .