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  1. #31
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    Aug 2011
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    I always thought that if the 3 phases are balanced, you end up with next to zero current flowing through the neutral.

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  2. #32
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    Sep 2011
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    Emerald Central Qld
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    I am still getting a diesel 3 phase gensetas the main standby or full supply as the electrical system is still pretty unreliable ,I use power on a 24/7 basis .
    Its ok for the 9-5 office dwellers and the like to have solar and batteries ,but many of the solar setups swicth off if the mains drops out during a storm , that is not what I specified when the fabulous roof decoration was purchased , I found out the hard way when a transformer died and let its smoke out .
    6-8 hours without any power .

    Solar may suit some but currently its not giving me any pleasure.

    Michael

  3. #33
    Join Date
    Sep 2012
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    York, North Yorkshire UK
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    6,439

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    Quote Originally Posted by simonl View Post
    I always thought that if the 3 phases are balanced, you end up with next to zero current flowing through the neutral.

    Simon
    That is correct for a balanced load such as a 3 phase "Y" wired motor !

    Though you might be surprised how many times you get a neutral burnout, at least here in the UK, where estates of houses have one side of the street wired to one phase and the other side wired to another and the third one wired somewhere else. Simply because all the return current has to flow through the neutral. All the major power failures where I live have been due to this cause ! Except for the one time the local sub station transformer went up in flames, and that was blamed on an overloaded neutral.

    These are "Y" connected systems. In a "Delta" connected system the current flows between the phases, the neutral may not even be present.
    Best Regards:
    Baron J.

  4. #34
    Join Date
    Jun 2008
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    The Whitsundays
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    229

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    Quote Originally Posted by simonl View Post
    I always thought that if the 3 phases are balanced, you end up with next to zero current flowing through the neutral.

    Simon

    Not sure if this is addressed to me, I'll answer it anyway.

    In a low voltage delta system the phase currents flow to and from each phase without reference to a neutral. A 3 phase delta wound motor would be a good example of this. Notice that at any given time in the A.C. cycle the curernts flowing are proportional to the applied voltage i.e. limited only by the impedance of each coil. This load is said to be linear

    In a star connected system the currents flow between the active to the neutral via the load. The current may not return to it's origin via the neutral. The easy example is a street of houses with three phase and neutral supply, with each house having only single phase power to it. House number one is on A phase, House number two is on B phase & house number 3 is on C phase. When A phase of the A.C. cycle is at it's positive peak, B and C phase will be below zero volts. The current that house one draws goes back to the neutral. However as the other two phases are below zero that same current will split and return to it's source via houses two and three. If all three houses are drawing the same current from the mains, none of the current will travel back in the neutral to it's origin. There will be current flow in the neutral between houses. If house two stops drawing curent then part of the current from house one goes through house three and back to it's source and the remainder of the current travels through the neutral to the source. Note that we are talking about linear loads.

    Imagine a street of houses where there are 7 houses on A phase, 9 houses on B phase and 5 houses on C phase. The load on the individual phases is 26 amps, 41 amps & 8 amps. The neutral carries the out of balance current, in this case 35 amps. We know that conductors have resistance and that it takes a certian voltage to cause a given curent to flow through a conductor. In this example the 35 amps in the neutral requires say 6 volts along it's length for this current to flow. Thus the end of the street furthest from the transformer will have the neutral 6 volts above earth potential, enough to cause shocks and tingles to those living in that area.

    Examine a modern computer switched mode power supply. These are not linear loads. During the A.C. cycle they only draw current if they need to and only for as long as they need to, could be milliseconds. They do not care what part of the A.C. cycle they are drawing load from. The load is not proportional to the A.C. waveform. They are non linear loads. Going back to our three houses, if a switched mode power supply turns on in house one the likelyhood that there is a switched mode power supply in houses two or three able to take the current from house one is pretty much nil. Thus all the current from non linear loads pretty much ends up in the neutral. Keep in mind that almost all electronics are switch mode power supplies.

    Does this answer your question?

    Cheers

    The Beryl Bloke
    Equipment er.... Projects I own

    Lathes - Sherline 4410 CNC
    Mills - Deckel FP2LB, Hardinge TM-UM, Sherline 2000 CNC.

  5. #35
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    Melbourne
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    Yes, thank you. A very good explanation.

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

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