wye connected loads

[PhaseShift]

I there is a balanced 3-phase wye connected load and there is a neutral connected to wye point, no current will flow through neutral because 3-phase load is balanced.

But what if the currents in the 3-phase wye load are unbalanced for whatever reason? Will there then be current in the neutral connected to the wye point, or will the currents still return on other phases?

 

[Cold Fusion]

---But what if the currents in the 3-phase wye load are unbalanced for whatever reason? Will there then be current in the neutral connected to the wye point,---

Generally yes.

---But what if the currents in the 3-phase wye load are unbalanced for whatever reason?
---will the currents still return on other phases?

It's possible, but it takes some really different power factors on the single phase loads - not something I've ever seen.

cf

 

[Rick Christopherson]

It's possible, but it takes some really different power factors on the single phase loads - not something I've ever seen.

No, it doesn't take anything unusual. It will always be true. The sum of the currents entering a node will always be zero, and that means that some will be entering and some leaving. The imbalanced current in the neutral is that current that cannot flow through the other phases as dictated by Ohm's Law.

Here is another thing to consider. Rarely will you find a wye configured 3-phase motor that is perfectly balanced. So if they are not balanced and there is no external neutral connection, then where does the unbalanced current go? The answer is that the neutral point will move as necessary to force the currents to balance. The voltage of the neutral point will move closer to the winding with the lowest resistance, such that V/R remains equal for all three windings.

When an external neutral is present, the voltage of the neutral point is locked in and cannot move to balance the currents. This is when you will have excess current at the neutral point and current will flow through the neutral wire.

 

[Cold Fusion]

Quote:
Originally Posted by PhaseShift
---But what if the currents in the 3-phase wye load are unbalanced for whatever reason?
---will the currents still return on other phases?

It's possible, but it takes some really different power factors on the single phase loads - not something I've ever seen.

cf

No, it doesn't take anything unusual. It will always be true. The sum of the currents entering a node will always be zero, and that means that some will be entering and some leaving. The imbalanced current in the neutral is that current that cannot flow through the other phases as dictated by Ohm's Law.

PhaseShift -
Could be I was answering a question you weren't asking.

I thought your scenerio was a 4w wye with unbalanced single phase loads, can this be done and not have any neutral current.

Yes it can. It would be extremely uncommon - the luck of the draw would always have neutral current.

This is not the same thing Rick is discussing. His example, the wye connected motor, has no neutral connection - so it's guaranteed - no neutral current

cf

 

[PhaseShift]

PhaseShift -
Could be I was answering a question you weren't asking.

I thought your scenerio was a 4w wye with unbalanced single phase loads, can this be done and not have any neutral current.

Yes it can. It would be extremely uncommon - the luck of the draw would always have neutral current.

This is not the same thing Rick is discussing. His example, the wye connected motor, has no neutral connection - so it's guaranteed - no neutral current

cf

I guess I was asking a more hypothetical type of question since I guess a three phase wye load would not really have a neutral wire, but I was refering to a 3-phase wye connected load.

So in Rick's example with a three phase wye connected motor, if there happened to be a neutral wire connected to the star point would there be current flow through the neutral for unbalanced phase currents?

I know 3 CT's in a residual configuration will sum to zero even with unbalanced current, so I'm trying to see what would happen with a wye connected neutral. If residual CT's sum to zero for unbalance, then I would think that unbalanced wye connection would sum to zero as well and there would be no current in neutral even if wire was present.

 

[Smart $]

But what if the currents in the 3-phase wye load are unbalanced for whatever reason? Will there then be current in the neutral connected to the wye point, or will the currents still return on other phases?

I agree with Cold Fusion. It is possible, however unlikely, that an unbalanced wye load can have 0 connected-neutral current.

The standard formula for calculating neutral current is only accurate under a unity power factor condition.

Of the three individual-phase, neutral-connected wye loads, if you calculate the neutral current magnitude and phase angle of any two of those loads (as if the third load is off at the time), then let [or force] the third load [to] have an equal current magnitude and phase angle as that of the neutral current when it is off, there will be 0 neutral current when all three loads are on.

 

[Smart $]

I guess I was asking a more hypothetical type of question... I would think that unbalanced wye connection would sum to zero as well and there would be no current in neutral even if wire was present.

Try the hypothetical scenario I mentioned in my last post using my Excel Neutral Calculator.

When the file opens, change the Line C current to 0 and observe the neutral current's magnitude and vector.

Now change the Line C current to 8.66A and a PF of 0.866...

 

[PhaseShift]

Try the hypothetical scenario I mentioned in my last post using my Excel Neutral Calculator.

When the file opens, change the Line C current to 0 and observe the neutral current's magnitude and vector.

Now change the Line C current to 8.66A and a PF of 0.866...

O.K I see how power factor will have an effect on neutral current.

So if we have a motor or any other three phase load that is connected in a wye configuration, and we add a neutral do we no longer have a three phase load but rather three single phase loads?

Since we are saying that any unbalanced current would flow through the neutral what about a residual CT configuration. Does current in CT's still sum to zero for unbalanced currents becuase currents return on opposite two CT's? If we added a neutral CT to the configuration then the 3 CT's + N CT would all add to zero?

 

[Smart $]

O.K I see how power factor will have an effect on neutral current.

So if we have a motor or any other three phase load that is connected in a wye configuration, and we add a neutral do we no longer have a three phase load but rather three single phase loads?

Correct.

Since we are saying that any unbalanced current would flow through the neutral what about a residual CT configuration. Does current in CT's still sum to zero for unbalanced currents becuase currents return on opposite two CT's?

I'm not familiar enough with a "residual CT configuration" to answer your question with certainty... but I believe the answer is yes.

If we added a neutral CT to the configuration then the 3 CT's + N CT would all add to zero?

I believe so.

 

[PhaseShift]

Correct.

O.K and since these would now be 3 single phase loads the currents would try to return on the neutral rather than the other three phases but obviously if they are balanced they will cancel at wye point and no current will return on neutral.

So to sum it up an unbalanced three phase wye load will have all the current return on the other phases however the wye point will have a higher voltage to ground. With the wye point grouned the three phase load becomes (3) single phase loads and no current returns on the other phases but rather just cancels at the neutral point with any unbalanced current returning on neutral.

 

[Rick Christopherson]

O.K and since these would now be 3 single phase loads the currents would try to return on the neutral rather than the other three phases but obviously if they are balanced they will cancel at wye point and no current will return on neutral.

So to sum it up an unbalanced three phase wye load will have all the current return on the other phases however the wye point will have a higher voltage to ground. With the wye point grouned the three phase load becomes (3) single phase loads and no current returns on the other phases but rather just cancels at the neutral point with any unbalanced current returning on neutral.

No, this is not correct. You guys originally dismissed my example thinking that I was referring to a 3-phase load while everyone else was discussing 3 single-phase loads. In reality, they are the same.

Oh, as long as I am posting, Smart$ stated, "The standard formula for calculating neutral current is only accurate under a unity power factor condition." This too is incorrect. The circuit analysis is the same regardless of any conditions, unless he is referring to some sort of shortcut formula that I have never heard of.

Contrary to what many people say (including myself), the currents don't literally cancel and disappear at the neutral. That view point is a result of nodal analysis, but when viewed via mesh analysis, the viewpoint is different. All three phases work in unison. If you analyzed the current using a mesh analysis you would see that the current flowing through one phase conductor splits off to the other phase conductors.

The reason why I brought up the previous example was to explain the difference between a fixed neutral voltage versus a floating neutral voltage, and how this impacts current flow at the neutral point. Even though my example used an unbalanced 3-phase motor, the same information still holds true for 3 single-phase loads.

This is a really, really broad topic and I know I haven't explained it very well because there is too much information, but if you ask a specific question about what I have tried to say, that I can answer.

 

[rcwilson]

I know 3 CT's in a residual configuration will sum to zero even with unbalanced current, QUOTE]

Not quite correct. Three CT's in a residual connection have one side of each CT connected together (the neutral or return) and the other side running to the meter or relay for each phase. After the currents run through the phase meters and relays, the three phase wires are joined together to make the residual connection and return to the common point of the CT's, possibly running through a residual current relay (device 50N) on the way.

If the loads are balanced, you will see current in all three phase wires and none in the return lead. Any unbalance will cause current to flow in the return or neutral lead.

 

[Smart $]

I know 3 CT's in a residual configuration will sum to zero even with unbalanced current,

Not quite correct. Three CT's in a residual connection have one side of each CT connected together (the neutral or return) and the other side running to the meter or relay for each phase. After the currents run through the phase meters and relays, the three phase wires are joined together to make the residual connection and return to the common point of the CT's, possibly running through a residual current relay (device 50N) on the way.

If the loads are balanced, you will see current in all three phase wires and none in the return lead. Any unbalance will cause current to flow in the return or neutral lead.

What you say is correct if the definition of balanced load includes both magnitude and vector. If we use only current magnitude, then your statement is incorrect.

PhaseShift's statement would be correct if he used "may" instead of "will" and refers to an unbalanced load as determined by current magnitude only.

So when we say an unbalanced load, are we referring only to the current magnitude, or both magnitude and vector?

 

[Cold Fusion]

--- You guys originally dismissed my example thinking that I was referring to a 3-phase load while everyone else was discussing 3 single-phase loads. In reality, they are the same.---.

Rick -
I certainly didn't discount your unbalanced motor example. It just didn't fit the connection I was discussing.

I thought the OP was discussing unbalanced, wye connected, three phase loading, with a neutral connection and has the loads arranged such that there is no neutral current - and that isn't anything normal.

I translated your example to unbalanced, wye connected, three phase loading, without a neutral connection. Which is not the same connection. However, as you said, "The circuit analysis is the same regardless of any conditions, ---" The math is the same. I agree.

I didn't dismiss your example, I just translated your first response to me (post 3) as, cf doesn't know what he is talking about. After that, I was pretty sure I couldn't help. I just wanted to explain that I was discussing a real narrow set of constraints (my translation of the OP's original Q) and then get out.

I think you have a good grasp on the subject and were/are doing fine - better than I would have. I probably would have insisted on sticking with one scenerio at a time until it was understood.

phaseshift -
When you say. "With the wye point grouned ...", I assume you mean, "With the wye point connected to the supply neutral ..."?

cf

 

[Cold Fusion]

What you say is correct if the definition of balanced load includes both magnitude and vector. If we use only current magnitude, then your statement is incorrect.---

Smart-
It's a 3phase system with unbalanced loads. At least one of the OP scenerios required differences in the magnitude and phase angle displacement or it couldn't happen

---So when we say an unbalanced load, are we referring only to the current magnitude, or both magnitude and vector?

Of course "magnitude and phase angle". These are all vector calculations.

cf

 

[Smart $]

Oh, as long as I am posting, Smart$ stated, "The standard formula for calculating neutral current is only accurate under a unity power factor condition." This too is incorrect. The circuit analysis is the same regardless of any conditions, unless he is referring to some sort of shortcut formula that I have never heard of.

The formula I referred to is as follows:

My statement holds true (i.e. you are incorrect :smile because the formula assumes a 120? phase difference between currents and only the current magnitudes are entered into the formula.

If you can demonstrate, either by your own working example or by that of another's, that the formula is accurate for currents having different power factors (i.e. not 120? apart), I will concede.

 

[Smart $]

Smart-
It's a 3phase system with unbalanced loads. At least one of the OP scenerios required differences in the magnitude and phase angle displacement or it couldn't happen



Of course "magnitude and phase angle". These are all vector calculations.

cf

I referring to vector or phase deviation of current to applied voltage. When we refer to an unbalanced load, most automatically assume an imbalance of current magnitudes only. Power factor (the mentioned deviation) plays an important role in true balancing of loads. You can have unbalanced currents by magnitude only, yet the vectorial sum of the phasors (which include the deviation) is zero.

Example:
Line A, 5A, pf=1
Line B, 10A, pf=1
Line C, 8.67A, pf=0.867

 

[Rick Christopherson]

What you say is correct if the definition of balanced load includes both magnitude and vector. If we use only current magnitude, then your statement is incorrect.

Without magnitude and direction, how would the currents cancel? Without direction, even when perfectly balanced, the currents would just be additive.

Your next quote below helps me understand why you made this statement, but it doesn't make is any less incorrect.

The formula I referred to is as follows:

My statement holds true (i.e. you are incorrect :smile because the formula assumes a 120? phase difference between currents and only the current magnitudes are entered into the formula.

First I have to admit that this gave me a good chuckle. :grin: How could you say "you are incorrect" if this is the first time the "alleged" equation was presented. :grin: It really isn't an equation per se, it is an intermediate step in the solution of neutral current with assumptions made. Those assumptions are so limiting that the equation have very little value, and that is why I had never heard of it before.

Getting back to the topic of magnitude-only currents; based on the stock you put into this equation, I can understand why you made the statements you did, but that doesn't make it right. The mere existence of this equation is based on both magnitude and direction. Even though you are only entering the magnitudes into the equation, the prerequisite conditions for using the equation are where the directions, and hence the vector analysis, come into play.

Yes, I know that you understand how to solve a 3-phase circuit analysis, but I have never heard of anyone suggesting or discussing it from a magnitude standpoint.

 

[Rick Christopherson]

Rick -
I certainly didn't discount your unbalanced motor example. ....

I thought the OP was discussing unbalanced, wye connected, three phase loading, with a neutral connection and has the loads arranged such that there is no neutral current - and that isn't anything normal.

I translated your example to unbalanced, wye connected, three phase loading, without a neutral connection.

Sorry if I left you with the wrong feeling. I didn't mean anything defensive when I suggested that my earlier posting was dismissed. That is not the way it was intended.

The thing in the original posting way back at the beginning (and I think it was repeated somewhere else too) was the confusion of how current flows in a 3-phase wye system. That's the reason why I used the example of a wye configured motor where the neutral connection was both absent and present.

The point of my first posting was to point out how both the current and the internal voltages change, not because there is a new conductor there, but because the center point of the wye is either locked down to a fixed voltage reference, or it floats. The current through the neutral conductor is actually the current which results from moving the floating neutral point to its grounded position. (Did that make any sense?)

I think the original poster is confusing the commonly repeated phrase that the currents cancel at the neutral, leaving only the unbalanced current. Currents don't just collide and annihilate each other, but that is how some people mistakenly view it.

 

[Cold Fusion]

---The point of my first posting was to point out how both the current and the internal voltages change, not because there is a new conductor there, but because the center point of the wye is either locked down to a fixed voltage reference, or it floats. The current through the neutral conductor is actually the current which results from moving the floating neutral point to its grounded position. (Did that make any sense?) ---

Close enough - I got it.

cf