Page 1 of 4 123 ... LastLast
Results 1 to 10 of 36

Thread: non Linear loads

  1. #1
    Join Date
    Dec 2006
    Location
    Chapel Hill, NC
    Posts
    32,006

    non Linear loads

    A friend called and asked some questions about a job he may do.

    A residence with a remote building where the remote building has a 200 amp single phase service. The owner wants to install many, many computers in the building at 800 watts each. He is not sure how many but it may be 50 or so.

    The question is if you have a load on the neutral of 1600 watts for one circuit (2 machines), and 1600 watts on the neutral for the other phase, then is load on the neutral 1600 watts. If I think about the service as a multiwire branch circuit then the neutral load would be "zero" but I don't think it works that way.

    Secondly, once the neutral is calculated and the non-linear loads are more than 50% then how do we size that neutral ?

    Thirdly, do we need to even worry about this on single phase since the code mentions harmonics for a 3 phase wye and not single phase.

  2. #2
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    32,558
    Quote Originally Posted by Dennis Alwon View Post
    A friend called and asked some questions about a job he may do.

    A residence with a remote building where the remote building has a 200 amp single phase service. The owner wants to install many, many computers in the building at 800 watts each. He is not sure how many but it may be 50 or so.

    The question is if you have a load on the neutral of 1600 watts for one circuit (2 machines), and 1600 watts on the neutral for the other phase, then is load on the neutral 1600 watts. If I think about the service as a multiwire branch circuit then the neutral load would be "zero" but I don't think it works that way.

    Secondly, once the neutral is calculated and the non-linear loads are more than 50% then how do we size that neutral ?

    Thirdly, do we need to even worry about this on single phase since the code mentions harmonics for a 3 phase wye and not single phase.
    I won't claim to be an expert on harmonics, but we are always told they are additive in the neutral. So something to chew on a little before someone that knows harmonics better chimes in - for single phase multiwire applications the effects would not likely be as bad as they are for three phase multiwire applications as the additive effects of the harmonic currents is only coming from two ungrounded conductors instead of three. What I don't know is if the harmonics are still additive when there is 180 degree phase angle or if it is only additive because of the 120 degree angles of a wye system.

  3. #3
    Join Date
    Feb 2003
    Location
    New York, 40.7514,-73.9925
    Posts
    4,246
    It is just a multiwire circuit, so not considering the harmonics, the neutral conductor will have 0A.

    Harmonics are load based and will differ depending on the kind of power supplies are in the computers.

    3rd harmonic and all of its integer multiples (collectively called triplen harmonics) generated by 120ophase-shifted fundamental waveforms are actually in phase with each other, that is why on a 4 wire feeder, there is a concern of excessively high current that may be more than the individual phase conductors. This led to years of oversized and double neutrals.

    The triplens are not additive for 120/240V with the 180ophase-shifted fundamental waveforms

    Most will just run a single full sized neutral to accommodate either one circuit full on and the other off, or both on and significant harmonics on the neutral.
    Ron

  4. #4
    Join Date
    Dec 2006
    Location
    Chapel Hill, NC
    Posts
    32,006
    Ron, I don't think a calculation of a neutral conductor is calculated that way. The branch circuits are not multiwire branch circuit so if you calculated the load of the neutral as a service with just those 2 circuits I don't believe the load would be 0

    I guess you would have to size the neutral to the largest load on one phase as you could have one phase off

  5. #5
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    32,558
    Quote Originally Posted by Dennis Alwon View Post
    Ron, I don't think a calculation of a neutral conductor is calculated that way. The branch circuits are not multiwire branch circuit so if you calculated the load of the neutral as a service with just those 2 circuits I don't believe the load would be 0

    I guess you would have to size the neutral to the largest load on one phase as you could have one phase off
    In a two wire (120 volt) branch circuit the current (linear as well as non linear) is same in both circuit conductors. You are concerned about additive effects of harmonic currents in a neutral conductor of a multiwire circuit - which your feeder and service conductors would have this apply to them. But as Ron mentioned the linear load on the neutral of the single phase source is going to be zero (the linear balanced load) So you are starting out with less current on the neutral to begin with before any harmonics start to come into play on such a system, is kind of what I think he is saying. Then as I said earlier you don't have a third phase to add even more harmonics to the neutral.

  6. #6
    Join Date
    Jan 2016
    Location
    PA
    Posts
    3,473
    Grow lights



  7. #7
    Join Date
    Dec 2006
    Location
    Chapel Hill, NC
    Posts
    32,006
    Quote Originally Posted by Ingenieur View Post
    Grow lights

    No pc's-- lots of them. My friend didn't know why he had so many but he thought it had something to do with bitcoins

    So it sounds like we really don't have to worry about the neutral load at all... He will be happy to hear this.

  8. #8
    Join Date
    Dec 2012
    Location
    Placerville, CA, USA
    Posts
    17,659
    Quote Originally Posted by Ingenieur View Post
    Grow lights
    Many computers usually means Bitcoin mining, so minimizing power cost is critical.
    For a single phase MWBC, all even order harmonics would be additive.
    But to get even harmonics would require a current asymmetry between positive and negative half cycles, such as a single half wave rectifier input.

    Sent from my XT1585 using Tapatalk

  9. #9
    Join Date
    Jul 2006
    Location
    San Francisco, CA, USA
    Posts
    6,664
    "Bit coin mining" sounded like something fun and interesting... until I looked at the details. BOOOOORING! It's basically an incestuous process of setting up computers to help process and verify other bitcoin transactions from all over the world as part of a decentralized network based system, for which you are rewarded with... bitcoins.

    The thing about the Power Supplies will boil down to this: Are the Switch Mode Power Supplies "power factor corrected" or not? If they are, then the harmonics is insignificant. Everywhere else in the world EXCEPT the US require SMPS to have PFC, we do not. But indirectly we generally end up with the PFC versions anyway, because manufacturers don't really want to bother making different versions and our domestic marketplace is not big enough for a US mfr to stand alone and compete against suppliers selling all over the world.
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

  10. #10
    Join Date
    Jul 2013
    Location
    Renton, WA
    Posts
    625
    There is probably a delta-Y somewhere in the system.

    Circa 1992, company installed about 200 new computers in the offices. Not PFC corrected inputs, straight rectifiers in those older power supplies.

    About a week later the poco transformer on the roof smoked due to all the triplett harmonics.

    OP should check on the type power supplying the new computers, they likely have a PFC circuit in the front end, so no problem. Easy to check, slap on a CT and look at the current waveform.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •