Discussion in 'Homework Help' started by Cerkit, Aug 28, 2012.

1. ### Cerkit Thread Starter Senior Member

Jan 4, 2009
286
3
Hi. Can someone explain to me how/why a current develops in the neutral if the phase loads are unbalanced. Is there some literature which displays this with the waveforms in each cable P1,P2,N??

I guess I am not getting an intuitive understanding of how this current results. Will there not be a current in the neutral at all times anyway since the currents are out of phase in the first place?

Thanks

2. ### BillO Distinguished Member

Nov 24, 2008
992
139
If the loading is balanced and the phases are at the correct angle, the current flowing through any one phase is exactly balanced by the sum of the currents in the other phases. If either the phase loads are not equal, or the phase angles drift from ideal, a current can form in the neutral as the phases no longer cancel each other out.

Like you said, the phases are unbalanced.

Here is a Wikipedia article on three phase power transmission.

3. ### Cerkit Thread Starter Senior Member

Jan 4, 2009
286
3
Ok that makes sense. So in the scenario of a Delta configuration with no neutral cable, with unbalanced loads does it mean that one or even two of the other phase coils in the generator could be put at risk due to excess current from the unbalanced loads across the third phase??

4. ### mlog Member

Feb 11, 2012
276
36
Just because the currents are out of phase does not mean there is a neutral current.

Consider three currents.

First is 1$\angle$0° A.
Second is 1$\angle$-120° A.
Third is 1$\angle$-240° A.

Each of these currents has the same magnitude (1 A) but is 120° out of phase with the other. Sum the three currents as you would at the neutral. I think you'll find they add to zero.

As an experiment, repeat the process by making one of the three as a magnitude of 2, and you'll see there is a necessary neutral current.

Last edited: Sep 4, 2012
5. ### mlog Member

Feb 11, 2012
276
36
Why would a delta be at any more risk than a wye? Either can supply unbalanced currents. The delta simply has no neutral. I would suggest you work some problems and use the technique of delta to wye conversions.

6. ### Cerkit Thread Starter Senior Member

Jan 4, 2009
286
3
Ok. From a more intuitive perspective when the loads are balanced and the current amplitudes are the same I am assuming that each phase is able to use the other phases as return paths, and when there is an unbalanced load the excess current therefore uses the neutral as the return path?

I guess I wasn't saying that Delta is at more risk but what I was trying to find out is that with Delta it would be the actual phase coils that would be in danger as opposed to the neutral as in the case of the Wye configuration?

7. ### mlog Member

Feb 11, 2012
276
36
You can have an unbalanced load with a delta and by definition not have any neutral current. The phase currents will be unbalanced.

You can have an unbalanced load with a wye minus the neutral connected and by definition not have any neutral current. Sometimes wye circuits have open neutrals.

You can have an unbalanced load with a wye plus the neutral connected, and you should have a neutral current.

Consider the possibility that the neutral could have impedance within its circuit. In this case, any neutral current will result in a voltage difference between neutrals. See how much fun this can be?

8. ### BillO Distinguished Member

Nov 24, 2008
992
139
The situation you describe, for a 3-phase power transmission, is balanced and in correct phase and would not ever be described as being out of phase.

First is 1$\angle$-3° A.
Second is 1$\angle$-118° A.
Third is 1$\angle$-240° A.

Would be out of phase and there would be a resultant current in the neutral.

If the transmission lines and/or generator and/or load were operating at their limits for current and/or power, then yes a delta arrangement is more susceptible to out of phase induced overloading since the current being carried by the phases can be pushed higher than rated current. However, is not to say that wye with a neutral is totally in the clear either, it's just less susceptible and you have to work harder to screw it up.

9. ### mlog Member

Feb 11, 2012
276
36
Did you read my entire post and the original question? I thought the questioner was asking whether different phase angles always resulted in a neutral current. I was attempting to show that it didn't in the case of balanced loads. To someone new to the subject of 3-phase power, the subject of neutral currents isn't always obvious.

To your point that the situation I described was balanced and in the correct phase, that's absolutely correct. I never intended to suggest otherwise. The point I was trying to make was that a balanced 3-phase power source provides 3 currents with each having a different phase angle (by 120 degrees) from each other. So in a sense, each one is "out of phase" with the other if the reference is, say phase A at 0 degrees. 3-phase power is inherently of different phase angles. Perhaps it was a poor choice of words, but i thought I explained it sufficiently in my message. Apparently not everyone got the point.

10. ### BillO Distinguished Member

Nov 24, 2008
992
139
Looks like we're in agreement then.