Problems on three phase rectifier design calculation.............

[RATAN ROY]

Hi..........
I gone through the topics discussed on three phase rectifier.I am 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 electrolytic 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!!!!!!!!!

 

[trekkytekky]

The size of your capacitor will be dependent on full or half wave bridge, worst case load and allowable ripple. As to limiting inrush current either a SCR bridge or diode bridge is used in conjunction to a current limiting resistor with a bypass, the bypass being energised when the voltage has stabilised to within 20% of full voltage. By limiting the number of inrush starts per hour and forced cooling the resistor doesn't need to be rated for continous use.

 

[DXNewcastle]

Re: Inrush current limiting.
One inexpensive solution to inrush limiting which has become popular in consumer products is the NTC (Negative Temperature Coefficient) Thermistor. These present a high resistance when cold and quickly drop in resistance after a few moments. In a 3-phase rectifier you would use 3 of them, one in each phase's input.
BUT, in a 3-phase design there would be a risk that they did not share the current evenly, so if you were designing a product for commercial or professional use I would advise a more complex arrangement with a small monitoring circuit which bypassed the current-limiting resistors after the supply had stabilised and which opens all 3 circuits if the supply doesn't stabilise within a defined period of time, this reduces the risk of fire from overheating the bypass resistors.

 

[chuckey]

For a 26kV 6A DC three phase power supply the 415V inrush limiting resistors were .5 ohm and were shorted out after 1 second. They had thermostats above them so if they started to overheat the incoming 415 V was switched off.
Frank

 

[RATAN ROY]

m using hex bridge full wave diode rectifier,output power is 30KW(560V,60A) and considered 20% of output ripple............as per my calculation it's coming arround 1mF,i think my calculation is wrong...............iso i need proper way to calculate.With the same output parameters i need to calculate the inrush current limiting resistance value with proper wattage.I am very much confused for this,I planned to use only single thyristor parallel to limiting resistance (at output of rectifier).......