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Thread: wye connected loads

  1. #1
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    wye connected loads

    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?

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    Quote Originally Posted by PhaseShift View Post
    ---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.

    Quote Originally Posted by PhaseShift View Post
    ---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

  3. #3
    Quote Originally Posted by Cold Fusion View Post
    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.

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    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

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    Quote Originally Posted by Cold Fusion View Post
    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.

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    Quote Originally Posted by PhaseShift View Post
    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.

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    Quote Originally Posted by PhaseShift View Post
    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...

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    Quote Originally Posted by Smart $ View Post
    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?

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    Quote Originally Posted by PhaseShift View Post
    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.

    Quote Originally Posted by PhaseShift View Post
    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.

    Quote Originally Posted by PhaseShift View Post
    If we added a neutral CT to the configuration then the 3 CT's + N CT would all add to zero?
    I believe so.

  10. #10
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    Quote Originally Posted by Smart $ View Post
    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.

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