Discussion in 'General Electronics Chat' started by DumbDummy77, Jul 12, 2010.

1. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0
Hi

I need a help to calculate the (3 phase - 400Vac) AC load pull from 300V @ 300A DC power supply unit. Please tell me what formula should I use?

DumbDummy77

Last edited: Jul 12, 2010
2. ### russ_hensel Distinguished Member

Jan 11, 2009
820
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Try to apply the law of conservation of energy. You might correct with some assumed efficiency ( say 80 percent ).

3. ### rvh002@gmail.com Active Member

May 15, 2009
118
2
Very strange question. Do you mean that you want to draw 300A at 300V DC ? How do you get the DC from your 3phase supply? That will also effect the effiency of your calculation. 80% might be a bit high. I would think for high current rectifiers and the required equipment to produce 300A, you are probably looking at closer to 65-70 % (nice cooling etc.)

4. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0

In response to this, I am simply have a DC power supply which requires 3 phase 415Vac input which allows DC power supply to produce 100V @ 120A for our large number of loads. So I need to know what AC will it pull. This is why I ask you guys how to determine it because I am not entirely sure to be honest.

DD77

5. ### rvh002@gmail.com Active Member

May 15, 2009
118
2
Ok, that it is a hell of a lot less than 300V @ 300A. It always help to state the problem as clearly as possible. Your 100V@120A is !2000VA or 12kVa. So assuming a loss and safety factor of 20% you will need 15kVa on the 3 phase side. Devided by 3 that is 5kVa per phase and that means more or less 22A per phase. These are all "close enough calculations" and I would put 30A fuses in the supply and 30A cable to supply it.(Depending on the distance)
The voltage you refer to make me think your single phase supply is 230V AC. That means you could use a 4mm sq 3core plus earth cable up to approx. 25 meters. Longer than that go to 6mm sq cable.
Hope this helps,

6. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,787
973
read the nameplate or installation instructions on the DC supply.. Don't guess when safety and fire are concerned.

7. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,787
973
and why was it 200A @ 200vdc the other day, then 300A @ 300VDC, and now 120A @100VDC....

8. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
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Good point, I am simply trying to get the idea of how calculate the AC load's pulling regardless of what voltage or current Im planning to use.

9. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
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I thought it should be 40A fuse (12kVA/3=40A) The distance should be no more than 15m.

Btw why is it expensive to use three phase instead of single phase?

DD77

10. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,787
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3 times the components...

11. ### rvh002@gmail.com Active Member

May 15, 2009
118
2
How on earth do you get 12kVA/3 =40A.......12kVA/3 =4kVA...that is per phase, on a 230V system that is 4000/230=17.4 A per phase. Correcting that with all the possibilities you end up at about 22A.
So what is your problem? I am beginning to think you know nothing about electricity and are just wasting everybody's time