Unbalanced load on neutral 3 phase?

[Carultch]

Some ask why two phases and neutral doesn't cancel like it does for systems with 180 degree phase angles. This doesn't really explain why but does show you the result is the same value for the neutral if the two phase currents are the same.

This is why:

You see another example of three vectors that are 120 degrees apart, and adding up to zero when they are all equal in magnitude. It's the same math problem, just a different application.

The two diagonal ropes carry 100 kg of tension (i.e. 980 N), and are supporting a 100 kg weight. Not considering the vector math, you might wonder why the tension in the two upper ropes is 100 kg, instead of 50 kg. After all, each rope should support half the weight. In a way, they do, because the vertical component of the tension in each diagonal rope is 50 kg. The fact that the ropes also pull mostly sideways in this arrangement, makes the tension in each rope twice as much as it otherwise would be, with two vertical ropes instead.

If you cut the left diagonal rope, the weight will no longer be supported in its position. Because a rope that is 120 degrees from the direction of the weight, cannot possibly hold it in place on its own. The weight will swing like a pendulum, until it settles directly below the support on the right. What once was a 120 degree angle, will become 180 degrees.

 

[LarryFine]

And, if the two upper ropes are spread greater than 120 deg., their tension increases even more..

What once was a 120 degree angle, will become 180 degrees.

. . . Turning 208v into 240v.

 

[Fred B]

Been reading thread with great interest as have veeerrry little experience with 3 phase. The last several posts have me thinking that balancing of loads on 3Ph is even more critical than on 1ph as it seems to indicate that an imbalance will or may greatly impact other phases. So is it possible that an imbalance be so great that it could cause damage to equipment on another phase? Can such imbalance caused by equipment (like lighting or other 1ph item) on a single phase leg cumulatively create large enough imbalance to effect negatively other equipment such as 3ph motors?

 

[ptonsparky]

Been reading thread with great interest as have veeerrry little experience with 3 phase. The last several posts have me thinking that balancing of loads on 3Ph is even more critical than on 1ph as it seems to indicate that an imbalance will or may greatly impact other phases. So is it possible that an imbalance be so great that it could cause damage to equipment on another phase? Can such imbalance caused by equipment (like lighting or other 1ph item) on a single phase leg cumulatively create large enough imbalance to effect negatively other equipment such as 3ph motors?

When the imbalance is enough to create excessive voltage drop or differences between phases, it will create a problem for three phase motors. About 2% is an issue.
Lightning is a different ballgame.

 

[Julius Right]

Sorry I am late [as usually].
Let's say the phase voltage system is balanced. That means VRN=120Vϕ0 ;VYN=120Vϕ-120 ;VBN=120Vϕ-240
Now, let's say all power factors between each current and corresponding phase voltage are the same: [cos(ϕ)=0.85].
The angle of cos(ϕ)=0.85 it is acos(0.85)=0.555 rad. or 31.8 degrees.
Then IA=10*[0.85-SIN(0.555)i]=8.5-5.27i
Since VYN lags VRN by 120 degrees then the IY angle will be -120-31.8=-151.8 degrees or -2.65 rad. and IY=13*[COS(-2.65)+SIN(-2.65)i=-11.456-6.145i
The same IB=0.5+16i
The neutral current is the negative value of all current sum:
Ineutral=2.456-4.585i and the absolute =5.2 A
However, if the power factor per phase is different[ Let say A stays 0.85, B will be 0.9 and C 0.8] then Ineutral= 0.3435-3.318i [absolute 3.336].