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Thread: Neutral connection to a wye wired motor

  1. #1
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    Neutral connection to a wye wired motor

    For a motor connected with a Wye connection when do you connect the neutral to the common point of the Wye and when do you not.

    Cannot find anything in the electric code or by googling.

    A collegue told me that if you connect the neutral you allow harmonic current to flow (using a VFD or soft start), and in case of a phase failure it still starts. It seems to me that you put the appropriate protection relay for phase failure detection such as an ANSI 46 or some similar protection relay. As for harmonic current flow, managing this with neutral connection seems wrong and filters should be used.

    It also comes to mind a comment someone once said to me that in the case of a minor ground fault on a motor it was much more easy to pickup in a Wye connected motor than a Delta connected motor.

    Came across an NFPA discussion paper about a sub-committee for the NEC 1996 edition that was looking into this matter of neutral connection, harmonics, etc. but there was no conclusion. In 2011 there must have been conclusions ?

    Can anyone direct me to some standard, or other document to clarify this point please?

    Best Regards,



    Christian

  2. #2
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    Quote Originally Posted by xian View Post
    For a motor connected with a Wye connection when do you connect the neutral to the common point of the Wye and when do you not.
    I can't give you an answer on which codes apply - not being from USA, I'm not familiar with NEC codes if those ae what you mean.

    However...
    My experience with 3-phase cage induction motors is that they are 3-wire with three stator windings. If all six ends of the three windings are brought to terminals, the windings can be connected in either a delta or star (wye) arrangement. When connected in star, the star point isn't normally connected to anything external to the motor.

    The different arrangements allow the motor to be operated on different supply voltages. As it happens, I have a nameplate for one on my desk at this moment.
    With the delta configuration it is rated at 220-240V and for star (which is actually how the motor in question is connected) it is 380-415V.

    The the two configurations are also required if you want/need wye/delta starting to reduce starting current.

    OK. That's some background. To answer your original question about when you would connect the neutral to the common point of the wye, the answer, in my experience, is never.
    Others here may have seen an odd-ball occasion/circumstance where it has been.

  3. #3
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    Quote Originally Posted by xian View Post
    Can anyone direct me to some standard, or other document to clarify this point please?

    A great place to find the answer would be the connection diagram on the motor. I have never seen one indicate connecting the neutral to the motor winding.

  4. #4
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    I agree with the above posts. The wye point is wired internally in the motor or the leads are available in the make up box so the motor can be wired wye or delta.

    In neither case do you connect to any supply neutral to the motor windings.
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

  5. #5
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    ...Cannot find anything in the electric code or by googling.
    Try, google, image, use the term "Diagram ______"
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  6. #6
    I think this was stated above, but I will reiterate it. The wye and Delta configurations of a motor are internal configurations, not how they are connected to the source. The motor is always connect to the source in a delta configuration regardless how it is internally configured.

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    Quote Originally Posted by Rick Christopherson View Post
    I think this was stated above, but I will reiterate it. The wye and Delta configurations of a motor are internal configurations, not how they are connected to the source. The motor is always connect to the source in a delta configuration regardless how it is internally configured.
    I don't understand what you mean when you say that the motor is connected to the source in a delta configuration.
    Don, Illinois
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    Quote Originally Posted by don_resqcapt19 View Post
    I don't understand what you mean when you say that the motor is connected to the source in a delta configuration.
    I don't either. The supply to a three-phase cage motor is three-phase three wire irrespective of the configuration of source or the motor.

  9. #9
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    Wye common connection to neutral

    Hello All,

    Thx for your efforts but the question remains unresolved as I can see. My experience is more in medium power ( 400Hp to say 5000 Hp ); a clue that came up in a discussion today, at my end: motor people may not know the answer (e.g. you have motor connection diagrams with no indication of how to connect the source), rather it would be the motor protection relay people that would as it's their product line that would have responsibility and technical know how.

    In very small motors (i.e. less than 100Hp) I would expect that the neutral not to be connected and for phase fault to ground or grandual leakage to ground with the deterioration of insulation on a winding one could pick this up with a Zero-sequence CT and a 51G relay. But if you connect the neutral, from what I gather, you can use a 51N, which uses a less expensive CT on the neutral than a ZS CT, but does not cover all failure modes.

    Still I would have liked to find some literature for guidance and explenation. I will continue to drill down this point, but if anyone has more information sooner I'll be real happy to read it.

    Cheers,


    Christian (xian)

  10. #10
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    Quote Originally Posted by don_resqcapt19 View Post
    I don't understand what you mean when you say that the motor is connected to the source in a delta configuration.
    I interpret what he said to mean you always just run 3 wires to the motor (Y-D and PW notwithstanding), but as stated elsewhere, in truth the source connection to the grid is irrelevant.

    All that matters is that the motor internal connections match the source voltage, phase-to-phase. Forget the issues of Wye or Delta inside the motor, that doesn't matter as far as you are concerned when connecting it, other than if it is dual voltage motor and the nameplate connection diagram happens to use those terms.

    I have never heard of using a 51N relay on a motor circuit, maybe that's why. 51N is typically used in feeder protection. Motor Protection Relays typically have a 51G function built-in and you have a choice on GF protection of just using the Residual Ground Current method, which uses existing CTs, or adding a ZSCT. If you are looking to save the cost of the ZSCT, you can just use the standard RC GF protection feature.
    Last edited by Jraef; 06-16-11 at 04:22 PM.
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