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Balance an unbalanced 3 phase connection of appliances?

  1. Sep 18, 2014 #1
    Hi there, I want to know what is the best way to connect these devices to a 3 phase power supply outlet and minimise the unbalanced load as much as possible safely? I am only worried about the single phase appliances. What type of connection would you recommend?

    3x Power Supply units Single Phase 10A input
    2x Fans Single Phase 2A input each
    4x Variable Speed Drive Single Phase, 2 of them will take 6.7A rest two 12.5A
    2x VSD 3 phase 10A - 14A

    If you have any explanations that would be greatly appreciated as well. This is not a homework.
  2. jcsd
  3. Sep 18, 2014 #2


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    You have single phase devices with these requirements in amps;
    10, 10, 10
    2, 2
    6.7, 6.7, 12.5, 12.5
    Total is 72.4A. Divide by three to give ideal 24.1A per phase.

    Allocate loads to phases, all with neutral as common.
    Try different combinations such as the following;
    Ph1: 12.5, 10, 2 = 24.5A
    Ph2: 12.5, 10, 2 = 24.5A
    Ph3: 6.7, 6.7, 10 = 23.4A

    Phases will be matched within 1.1A. Neutral current will be about 1.1A.
  4. Sep 18, 2014 #3
    What should I do with the neutral connection? Does it have to be grounded?
  5. Sep 19, 2014 #4
    Does this mean each of those devices will be connected in series to each of the phases (across line to neutral that is)?

    Neutral as common as in connect the loads in Y formation?
    Last edited: Sep 19, 2014
  6. Sep 19, 2014 #5
    There has been a few changes

    Ph1: 12.5,2.3,1.3 = 16.1A
    Ph2: 12.5,2,2 = 16.5A
    Ph3: 6.7,6.7,2 = 15.4A

    Current in neutral = 0.96A
  7. Sep 19, 2014 #6


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    You seem to have got the idea.
    Each phase of a three phase system is a single phase supply when considered with the common neutral.
    To run single phase devices from a three phase circuit you must have a neutral or a transformer.
    Except at one point on the power distribution board, the neutral is not connected to earth.
  8. Sep 19, 2014 #7
    What is this point? Can you please elaborate?
  9. Sep 19, 2014 #8

    jim hardy

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    He's not said yet what is his phase to phase voltage or what voltage his appliances take.

    myster - are you conversant in the √3 difference in 3 phase line to line versus line to neutral voltage?
  10. Sep 21, 2014 #9
    The power supply provides 415V, all the individual devices take 240V.

    What I am having trouble understanding is when I connect the selected devices on the line to neutral(across each phase) in series, the current flowing through them is the same but voltage across each will be different. But I want equal voltage across the devices and different current according to the the ratings of them.
  11. Sep 21, 2014 #10

    jim hardy

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    that's √3 difference alright.
    Does that supply have four wires, one for each phase and one for neutral?
    If it's just three wires you have no neutral. You need the neutral.

    That's a little tricky to decipher:
    Let's take one device at a time for starters.
    Connect one end of it to any phase, other end of it to neutral and It'll see 240 volts. It'll accept current in proportion to its load.
    Now connect another device but to a different phase, and neutral.
    It too will see 240 volts and draw whatever current it needs.
    They are NOT in series provided the neutral wire goes back to your three phase source neutral..
    Series means they must have same current. These loads don't suffer that constraint, they are free to draw different amounts of current and the current difference will return through the neutral.
    So they're NOT in series IF you have a neutral.
    Voltage is fixed by the phase to neutral voltage of your supply, 240 volts.

    Does your source provide that necessary neutral?

    I think you've thought yourself up an imaginary problem . No problem, I do it all the time.
  12. Sep 21, 2014 #11
    Yes the source has a neutral. It is not an imaginary problem.

    I was thinking of connecting the devices in parallel across each phase so they each see the same voltage and draw whatever current they require.
    Here is a diagram, let me know what you think

  13. Sep 21, 2014 #12
    I have several devices in the same phase, hence the question about them being in series
  14. Sep 21, 2014 #13

    jim hardy

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    1, 2 and 3 are not in series with one another. Neither are 4, 5 and 6.
    As you said each can draw the current it needs.
    Devices in series must carry the exact same current by wiring not by chance.

    1, 2, and 3 are in parallel with one another. You could replace them with one box labelled 123....
    4, 5, and 6 are in parallel with one another. You could replace them with one box labelled 456....
    123 is NOT in parallel with 456 for they have different voltage sources.
    Devices in parallel must see same exact voltage by wiring not by chance..

    123 is NOT in series with 456 because the neutral can return their differing currents .
    Devices in series must carry the exact same current by wiring not by chance.

    I think you've got it, though..

    old jim
  15. Sep 23, 2014 #14

    jim hardy

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    Okay, now we have a schematic.
    From your first post:
    Physically what do you have? Just one single outlet?
    You need a hand from somebody who's familiar with electrical work and codes where you are. I''d guess you are in UK?
    Inquire at your local electrical supply house. They might have an industrial drop box with a cord and several outlets already assembled. We used them at work for our 480/277(US).

    old jim
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