# The current of anyone of three phases is less more than two others in the AC220V.

Discussion in 'General Electronics Chat' started by ScottWang, Jul 12, 2015.

1. ### ScottWang Thread Starter Moderator

Aug 23, 2012
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The current of anyone of three phases is less more than two others in the AC220V.

One of my friend, he got a special problem that he made a AC220V 3KW power supply, the normal situation is shown in the Fig a, when he do the test in May, whatever the load is resistor or capacitor, everything is ok, the main problem is in the Fig b, when he added the capacitor to test and then the current happened as anyone of three phases will less more to two other phases, it means that even change Ua, Ub, Uc are the same result, this was happened during these days, my friend thought that this problem could be comes from the city power, he asking a man whom retired from the power company to make sure that, the man said that it could be happen, but the power company may not do any change for you, from May to July the power company probably do some change for the power equipments of local area near the place of my friend, now my friend have to do something to conquer this problem, do anyone have any good ideas to solve this problem, thank you.

What he needs maybe is some kinds of current balance circuit, but how do design it?

2. ### WBahn Moderator

Mar 31, 2012
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What does it mean for the current of one phase to be "less more"" to two other phases? Do you mean just unequal?

3. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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Question does not make any sense to me.

4. ### ScottWang Thread Starter Moderator

Aug 23, 2012
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AS the total current is 13A and Ua provided 6A, Ub provided 6A, but Uc only provided 1A, if you change the input line As Ua to Ub, Ub to Uc, Uc to Ua, or some others, the results are the same, always has one phase is the lower current.

it means that the city power can't provide all three phases have enough current to a capacitance load, but for a resistance load is ok.

5. ### ScottWang Thread Starter Moderator

Aug 23, 2012
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The problem could be the power company provide a false three phases power, they could be using two power lines to convert to three phases to caused this problem.

Jul 18, 2013
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Do not understand the question? Is this from a Delta or Star source?
A capacitor provides only momentary load to the bridge at connection.
Also the % ripple for rectified 3ph is <5% before capacitance added.Is there an actual load on the output, if so what relative to the supply capacity?
Max.

7. ### ScottWang Thread Starter Moderator

Aug 23, 2012
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When the power supply has had the problem, my friend was used the basic rectifier and capacitor to do the test, and that was shown in the circuit.
1. When the load is resistor that the A meters were shown that the three meters have similar values of amp.
2. When the load added capacitor that the A meters was shown that anyone of three meter will less more than two others.

The real load are used many of resistors in series and in parallel, the resistor shown in the circuit was represent the load, because the real load must be use the DC, so the capacitor should be added in, but it happened the providing current unbalance problem.

Normally when we designing the circuit, only care about is that our circuit is right or wrong, but this issue was over that, so that's why my friend got a headache.

Jan 15, 2015
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3,000 KW is 3.0 Mega Watts, would that be correct? I also do not quite understand the unbalanced current?

Ron

9. ### ScottWang Thread Starter Moderator

Aug 23, 2012
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Thanks, the 3000KW was my mistake, I already modified to 3KW, the unbalanced current means that normally each phase current should be providing very close to the average current as I = 13A/3 = 4.33A, but now whatever you change the input line and then there is always has one phase current very low comparing to two others.

10. ### tcmtech Well-Known Member

Nov 4, 2013
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What are the voltages of the three lines at no load and at load and what is the size of the capacitor.

If one phase has a little lower voltage than the other two and the capacitors capacitance is fairly large it may be simply skipping over the lower voltage phase.

If all three phase lines are equal voltage and moving the connection around for what phase feeds which input of the rectifier changes which phase becomes the low one then likely the bridge rectifier has a open diode on that input.

Also at a total load of 3 KW's a three phase power source is not really necessary so perfect phase balancing is certainly not needed. Maybe at 30 Kw or more but not at 3.

11. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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Why does one phase current is lower ?

12. ### ScottWang Thread Starter Moderator

Aug 23, 2012
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Ha ha, I can't see that you really read the problem of description.

13. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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No really.
If the current is not balanced then it is diode issue.
But if the phase voltage is not balanced then it is a power company issue or you might be having a bad phase connection in your distribution box.

14. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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771
Un normal (unequal) 3 phase will result in unbalanced phase current

15. ### ScottWang Thread Starter Moderator

Aug 23, 2012
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The problem is in the current not voltage.
No matter how much KW, the problem is really there.

16. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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Measure the phase voltage first and then come back.
Numbers will help to find a solution.

17. ### ScottWang Thread Starter Moderator

Aug 23, 2012
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I have had described that the same power in different date happened different results, and already make sure the problem was came from the mains.

Now my friend make sure that there are two old power supplies he bought, the result of old one is the same with the one he made, and the new one is working fine, because the new one has 3 phases PFC inside, but the old one didn't, so, now we starting to find the way along the 3 phases pfc to solve the problem.

18. ### ScottWang Thread Starter Moderator

Aug 23, 2012
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The problem is not in the voltage, if you want to know then they as 220V, 220V, 230V, now, already make sure the problem was came from the mains, but the power company won't do any change for that, so only thing can do is the power have to using the PFC, the problem is in the Ua, so my friend will try to using some coils to slow down two others.

19. ### tcmtech Well-Known Member

Nov 4, 2013
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1,750
The voltage is clearly a contributor here and I am now under the impression you do not have a good understanding of how three phase works let alone what goes on in a simple three phase bridge rectifier when its under load and there is a good sized capacitor on the DC side.

Disconnect your 230 volt line and I bet the current will transfer from that line to the other one with the low reading. If not then you have a dead phase and the whole thing will quit working.

20. ### ScottWang Thread Starter Moderator

Aug 23, 2012
4,925
777
Did you see the post #7 and the circuit Fig-b.
1. When the load is resistor that the A meters were shown that the three meters have similar values of amp.
2. When the load added capacitor that the A meters was shown that anyone of three meter will less more than two others.

The circuit should be add the capacitor, but when the capacitor add in and then the problem occurred, that's why has this thread.