Neutral Sharing 3 phase 120 volts

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iwire said:
On a ballanced linear circutuits the neutral load would be 0

With non-linear loads the neutral load would be the triplen harmonics.

I really don't know if that would be worse or not with 2 loaded legs or 3.
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even if the load is unbalanced isnt that the reason why you would make shure all hots are on seperate phases so it would cancel out the load on the nuetral
 
Rick Christopherson said:
Not if their magnitudes are similar and phase angles consistent with the 3-phase primary, no. Harmonics are no different that any other signal. They will cancel if they are all the same, and fall within the 0, 120, 240-degree triangle. If they are unbalanced, then they will not cancel. The only time you will have a problem is when there is a signal manipulation on 1 phase but not the others. If all three phases are manipulated in the same manner, then all three manipulations will negate one another.
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Ok, I guess I am in over my head... I was taught that 60 Hz, along with 120 and 240 even multiple harmonics cancel. Third multiple harmonics are additive and do not cancel, and will cause the largest distortion of wave forms
 
roger said:
I'm' sure Bob wouldn't but, I advise you to get off your high horse. It is apparent that you are daring us as well as continuing to be abusive to a number of members.


Roger
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I agree. I see nothing wrong with Bob's posts.
 
Rick Christopherson said:
Not if their magnitudes are similar and phase angles consistent with the 3-phase primary, no. Harmonics are no different that any other signal. They will cancel if they are all the same, and fall within the 0, 120, 240-degree triangle. If they are unbalanced, then they will not cancel. The only time you will have a problem is when there is a signal manipulation on 1 phase but not the others. If all three phases are manipulated in the same manner, then all three manipulations will negate one another.
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Check again Rick. Triplen harmonics add in the neutral of a three-phase system.

Hv&Lv said:
Third multiple harmonics are additive and do not cancel
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You are correct, of course. Much discussion can be found on the additive problem with triplen harmonics.
 
jumper said:
An example of non linear loads is flourescent lights, computers, copiers and such.

MWBC = multiwire branch circuit, 2 or 3 hots(different poles/phases) with 1 neutral.
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thanks im familiar with mwbcs but havnt heard of linear loads thanks for the info
 
Rick Christopherson said:
Not if their magnitudes are similar and phase angles consistent with the 3-phase primary, no. Harmonics are no different that any other signal. They will cancel if they are all the same, and fall within the 0, 120, 240-degree triangle. If they are unbalanced, then they will not cancel. The only time you will have a problem is when there is a signal manipulation on 1 phase but not the others. If all three phases are manipulated in the same manner, then all three manipulations will negate one another.
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what is a harmonic load?
 
liquidtite said:
what is a harmonic load?
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One which the load's current has multiples of the fundamental frequency. For example, 120 and 180Hz are second and third harmonic currents of a 60Hz fundamental frequency.

Non-linear includes not just harmonic currents, but also any current which deviates from the sinusoidal fundamental.

Such current deviation is considered distortion in the broadest sense, while harmonic distortion is a narrower type.
 
iwire said:
On a ballanced linear circutuits the neutral load would be 0

With non-linear loads the neutral load would be the triplen harmonics.

I really don't know if that would be worse or not with 2 loaded legs or 3.
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Say you have a balanced mwbc with identical loads and excessive triplen harmonics such that there was 10A on all 4 wires. That means each leg is contributing 33% to the neutral current (so 67% fundamental). Turn off one leg and the neutral will see 67% fundamental plus 2 times 33%, which amounts to 13.3A of current on the neutral.
 
mivey said:
Check again Rick. Triplen harmonics add in the neutral of a three-phase system.
Hv&Lv said:
Ok, I guess I am in over my head... I was taught that 60 Hz, along with 120 and 240 even multiple harmonics cancel. Third multiple harmonics are additive and do not cancel, and will cause the largest distortion of wave forms
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You are correct, of course. Much discussion can be found on the additive problem with triplen harmonics.
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Yea, I was trying to make the point by asking the question in a more self deprecating way. I didn't want to come off as snarky...:roll:
 
T.M.Haja Sahib said:
Surely,with 3 loaded legs,the neutral load of 3rd harmonics is worse than with 2 loaded legs because the 3rd harmonics in each phase are in phase.
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concerns of a lot of scenarios, I guess are there but what dangers are present I don't know. But what if you load two circuit linear and one non linear. I guess keeping circuits to 40 percent is best. I don't like to load more than 60 continuous anyway. Engineers I believe follow a percentage rule. Concern are higher when others of Maintenance tend to change around wirings. Just cause they get it to work. Not knowing the consequences nor changes they made.

I would like to give example of this.

Was working in large building where 26 non linear fluorescents were installed per phase. a = 12, b =12, c =12. This how it was designed back then with single breakers not tied. Now someone went and just moved breakers from b and c.
And I believe transferred all to A phase all circuits now, and being non linear Neutral Was hard to find first off because they did not mark which was neutral neither. I went thru all to find a High valued. Just to see it detrimental affect on wiring. Was way high but don't recollect value. This is why maintenance should stay away from installs.
 
Rick Christopherson said:
This is not correct, and is actually quite common with single phase loads on a 3-phase system. If you have a motor load on the B-phase and a resistive load on the C-phase, then your neutral current will exceed the ampacity of the neutral. (Forgive me if I have my leading/lagging backward, but the principle is the same.) This is the biggest reason for burning up neutrals in power distribution systems and MWBC's.

What is even harder to comprehend for many, is that if you simply reverse the phases, then the current in the neutral will be reduced for the same loads. This is a situation where phase rotation is important.

View attachment 6788

No. Anything that is balanced will still cancel, including the harmonics. The wording in the NEC is rather poor in this area because it causes people to overlook powerfactor. I don't recall the actual wording at this time, but my recollection of it is that it is misleading because it "suggests" that powerfactor is not part of the issue, when in fact, it is the major contributor to the problem.

P.S. Just for reference, here is the same diagram I used recently for another discussion. It is the same diagram, except it is based on resistive loads.

View attachment 6789
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Thanks was looking for a answer like that.
 
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