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3 phase rectifier capacitor

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seyyah

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When we rectify a 3 phase 220VAC line we get a max ~538Vdc voltage at the output. So the filter capacitor must withstand 600V i think. But i couldn't find a capacitor which has such a voltage rating at 220uF or 330uF. I'll solder it on a pcb so it must be suitable for that. I always see having 400-450V max rating capacitors in that capacity range. There are very big capacitors say 9mF at KV ratings but they aren't convenient for this job. If they don't exist or they are rare how is this job done? Do they use 2 capacitors in series? Is it convenient? Or are there another techniques?
 

Borber

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538V stays for 380V AC rectification. But rectifying 3x220V will give you only 310V if your reference voltage is at neutral wire potential (starr connection). Another thing is with 380V AC inter-phase voltage rectifying.
Series connection of capacitors is usual thing, only the capacitors must be connected on equal voltage resistor divider.
 

seyyah

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Yes i mean 380VAC.

Borber said:
Series connection of capacitors is usual thing, only the capacitors must be connected on equal voltage resistor divider.
Is this method usually used? Is it the usual way to make the job is done? If so how must we choose the resistors to help the capacitors charged equally, doesn't cause much losses and doesn't reduce the performance?
 

Borber

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Resistor divider is often calculated from few mA to few 10 mA current. All depends on the quality of capacitor insulator, leakage current. Select divider current to be at least 10x leakage current of capacitors.
 

seyyah

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What happens if we don't use resistor divider? As two capacitor will total have more than needed voltage rating and each of them will have more than half of the needed, if one of them has some more voltage drop on it, it won't be a problem i think. What side effects may occur or how does it affect the operation, performance etc?
 

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As electrolitic capacitors usualy have a big tolerance (+30/-20% ) you can find yourselve having a capacitance difference from one to other serialy conected capacitors which can be as high as 50%.If this happends you will find yourselve in serious trouble.( HV capacitors with high capacitance can store much energy and looks nasty whwn one explodes and after that prepare yourselve for another explosion because first one will make short circuit and the second one will not be able to sustain whole voltage).
It is advisable to use resistor devider and dimension resistances for a apropriate power.
Greetings
Dragan
 

Borber

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Resistor divider is needed. Leakage current different for each capacitor and difference can be quite large. Because of that series connected capacitors will have different DC voltage accross them. To make this voltage distribution equal you must place resistor divider. If divider is not used one of the capacitors can have greater voltage of what it is rated for.
 

seyyah

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Ok i understand so i must choose two equal resistors connected parallel to the capacitors. If leakage current of the capacitors is 0.8mA than a current of at least 8mA should flow over resistors. So for 538VDC; two 33k@3W resistor should be connected in series and each of them parallel to the capacitors. Right?
 

Borber

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Yes, you are right. One more tought. Because the lekage current of capacitors change with time, it usually rises, and it can be different for each capacitor, the smaller resistor divider current can earlier cause voltage rating problems.
 

    seyyah

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RATAN ROY

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Hi..........
I gone through the topics u discussed.I am also facing some problem with three phase diode rectifier.i am not that much of expertise in that just learning how to design power supply.My input is 415VAC.Can u please tell me the details how to calculate the proper value of filtering capacitor???here m not using the inductor for filtering purpose.

Another problem is to limit the inrush current.I am plannig to put one resistance before the capacitor to limit the capacitor current.But here also i am not able calculate the proper value and wattage of the resistance.So if any of u cn give me some advice.........that might be very helpful for my goal!!!!!!!!!
 

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