3 phase neutral current

Discussion in 'Electricians' Talk' started by peter palmer, Apr 19, 2018.

  1. peter palmer

    peter palmer Screwfix Select

    Doing mainly domestic I don't really do a lot of 3 phase stuff but like to watch videos and what not on the more technical stuff, what I cant fathom at the minute though is balanced 3 phase loads. I get that if you had a 3 phase motor then the current on the neutral would be zero because basically the voltage/current is just going through the three live conductors at 400V.

    But if you had a house with a 30A cooker connected to one of the phases in the street 30A would flow back down the neutral conductor. What happens if there are 3 houses, each on a different phase with identical cookers connected, I would assume that there would be 3 lots of 30A out of phase with each other returning down the neutral, why would it be zero. For that to happen the return current would have to flow back through the other house's phase conductor instead.
     
  2. Lectrician

    Lectrician Screwfix Select

    Because it flows down your neutral and back through one of your neighbours neutral.

    AC is back and forth. There phase are all out of phase by 120deg. As one phase pushes from L to N, the others will be pulling N to L, by varying degrees.
     
  3. peter palmer

    peter palmer Screwfix Select

    So the path is through your cooker, down your neutral, through your neighbours neutral, through their cooker and back down their live/line conductor?

    So if thats the case you could do away with the neutral return and still operate 230V appliances in each house. I guess then that its effectively 2 x 230V appliances connected in series for a balanced 400V load.
     
  4. Lectrician

    Lectrician Screwfix Select

    There will never be a case where it flows just down one of your neighbours. It is three phase, so will return via two of them, the amount through each would be dependent on the phase position at the time.

    With SPLIT phase you push and pull through just two.
     
  5. MGW

    MGW Screwfix Select

    With just three or even six items you will never get the phases balanced. However with 30 items then yes, so three houses with 30 items each and the neutral current starts to drop, with 12 houses it drops even more as the average current becomes more steady. So remove the neutral to a supply to 24 houses and there will be points when one phase hits zero and another 400 volt, but there is often enough current through the earth to keep things running for a few hours before some one realises theft of copper from the transformer has resulted in no neutral.
     
  6. Lectrician

    Lectrician Screwfix Select

    If the phases were well balanced, and the neutral open circuited, if the neutral current was near zero then the voltages would all sit happily around 230v. It is only when the phases are poorly balanced that an open circuit neutral would see near 400v on the phase to N.
     
  7. Bob Rathbone

    Bob Rathbone Screwfix Select

    Lectrician is correct about the current flow, but to ease confusion, it is best to consider each house as a single phase load connected to a 3 phase system. The neutral currents will not all flow in the same direction at any given time (instant) so the neutral becomes a pathway for currents flowing in both directions and at different instants in time. At the main neutral connection to the transformer, the current will be small, just the imbalance current, as the neutral current from one load will have found it's way back to the supply through another load or two on different phases. You must consider current, voltage and instantaneous time, when dealing with ant AC supply and especially with 3 phase.
    The neutral is necessary when single phase loads are connected to a 3 phase supply as we cannot ensure a balanced load at all times, as someone will take a shower or switch off the cooker and unbalance the system. The neutral takes care of any imbalance.
     
  8. rogerk101

    rogerk101 Screwfix Select

  9. Bob Rathbone

    Bob Rathbone Screwfix Select

    Roger, show me a low voltage public supply that is derived from a delta connected transformer, it is mainly used as the primary in supply transformers as in Delta / Star, rarely on the secondary as their is no place to earth the system and their is a strong possibility of circulating currents in the delta winding that could cause overheating.
     
  10. Lectrician

    Lectrician Screwfix Select

    Delta secondaries - I have NEVER come across. As said, no star point, and so no facility to earth. The 33kv/11kv transformers are DELTA primary and star secondary, the star point grounded. The 11kv network is distributed as three phases plus earth. No neutral, but derived from a STAR. The 11kv supplies the DELTA primary of local subs, with 400v STAR secondaries.
     
  11. Lectrician

    Lectrician Screwfix Select

    At college we used to have a reduced three phase supply - I believe it was something like 110v P to N, and 173v P to P. We would use 240v lamps, one per phase, commoned to the neutral. All lamps roughly equal brightness. We would then pull the neutral out and the lamps would stay lit, although their brightness would alter slightly as lamps have a wide tolerance when manufactured. You could also drop a lamp out completely with the neutral gone and see the effects. As we used 240v lamps, and the maximum phase voltage was 173v, you couldn't blow the lamps, but could observe the effects and measure the voltage. This was before all the health and safety! We would use un-shielded banana plugs on the 240v supply when wiring up discharge lighting control gear!
     
  12. Risteard

    Risteard Screwfix Select

    It would need to be 100V phase to Earth to give 173V between phases. 110V phase to Earth would give 190V between phases.
     

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