AC(3 phase)/DC power conversions

Thread Starter


Joined Oct 15, 2014
Hello all,
I have a few basic doubts regarding AC to DC power conversion calculations.
Please correct me if i am wrong at any stage.

Firstly, when we consider single phase AC system,as a rule of thumb we can equate both the powers like this
DC power = efficiency * AC power or more precisely
Vdc * Idc = efficiency * Vrms * Irms, -------(1)
where efficiency is between 75-90%
All good so far

Now coming to a 3 phase system
DC power = 1.732 * power factor * AC power or it can be re-written as
Vdc * Idc = 1.732 * power factor* V * Irms-----(2)

What is V in the above equation,is it the Line to Line voltage or Line to Neutral voltage?I think it is the Line to Line voltage but i would like to confirm it with you guys first.Also how would equation 2 look like if i would want to write it in terms of efficiency?


Joined Apr 24, 2011
How is the PS connected to the line?

If you have a 4 wire / 3 hot + neutral and you connect your unit directly to these4 lines then you are using the line to neutral voltage.

If you have a 3 wire connection just across the 3 lines then it is the line to Lin voltage. Usually this connection is done thru a transformer. The first way can be direct to the line.

So to sum this up, go a schematic?

Thread Starter


Joined Oct 15, 2014
No schematic,i got a bulky weidmuller 3 phase AC to DC power supply at work,and was wondering how the calculations are done.This particular power supply has 3 separate lines of phases ,a neutral and an earth connector.
So if its a 4 wire system,the voltage is line to line ,and if its a 3 wire(without neutral) it is line to line voltage.
Thanks for clearing that up.

About the second part,can equation 2 be written in terms of efficiency,like in the single phase system?
Many thanks