3-phase questions...need some help

Discussion in 'General Electronics Chat' started by lespaul, Jan 30, 2008.

  1. lespaul

    Thread Starter Active Member

    Jan 30, 2008
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    Hello and glad to be a new member. I work in the live entertainment industry and we commonly use 3-phase power distros/sources to distribute power to our equipment. This equipment is usually 110VAC powered gear such as audio amplifiers, video switchers, cameras, etc. However, we do also use large scale projection systems that require 208VAC to power them. The case I need help on is that I have 4 projectors at 208VAC that draw 17.3amps at that voltage. The 3-phase power system ordered is 120VAC @ 60amps per phase. Does this mean that I have 120amps available if I connect the projectors across two phases (208VAC)?? The total current of the 4 projectors is 69amps and would be bad news if i only had 60amps of available current. Thanks for your help...Les
     
  2. subtech

    Senior Member

    Nov 21, 2006
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  3. rwmoekoe

    Active Member

    Mar 1, 2007
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    do you agree if the projectors aren't sourced from one single phase?
    if so, my suggestion is that you should step up the 3 phase power source to 208 vac each. subtech has provided us with the technical info on that subject.
    then put 2 of your projectors on one phase, and the other 2 to the rest of the phases. the idea is to spread the loads as evenly as possible across the 3 phases.
    we have to put precaution on the phase that handles the 2 projectors though, as it would have been occupied at its full capacity (1 projector requires 30amps at 120vac, times 2)
     
  4. lespaul

    Thread Starter Active Member

    Jan 30, 2008
    49
    2
    Thanks for the help. I probably should have been more clear about the application though. The power distro. system is 208VAC @60amps. It's just 120VAC between each phase and neutral/ground. We typically connect the projectors to the power distro using a hubble - L630 connector. That connector is wired to two phases and the neutral, giving you 208vac which will power the projector. I am just wondering if I am wired between two phases do I have 120amps capacity (A -phase to neutral = 120VAC @60amps & B-phase to neutral= 120VAC @60amps) together they give me 208VAC @ 120AMPS
     
  5. rwmoekoe

    Active Member

    Mar 1, 2007
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    i see.
    the voltage between any two different phases is 208vac. but the current is less by the same factor as with the phase to ground (neutral) configuration, so that the maximum power is the same in both cases.
    i suggest you put each projectors to each phase-to-phase, and one projector that remains at either of the 3.
    only this time there's no need for the stepup transformer.
    phase a-b to 1 projector, phase b-c to 1 projector, phase c-a to 2 projectors.
    what do you think?
     
  6. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    the problem is u will only get 60 A as the max from the system since it seems to be a star configuration.had it been delta the rating wud have been 60*1.73(which isnt the case here)
    wud it be possible for u to connect two other projectors between another two phases?
    this way u can actually connect 9 projectors ,3 in each phase-phase combination.
     
  7. lespaul

    Thread Starter Active Member

    Jan 30, 2008
    49
    2
    Recca,

    Yes, the power we normally deal with is WYE configuration (star). So what you are saying is that the current (power) across the phases does not double. That makes sense having read your theory on 3-phase power. I will just order a distro for 100amps per phase and we should be good. Great bunch of folks on this site...thank you for the help.
     
  8. rwmoekoe

    Active Member

    Mar 1, 2007
    172
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    les,
    recca asked you about this:
    you see? if you can use all three phases phase-phase, you can connect 9 projectors at a time. isn't that enough? besides, it's better because you'll spread the load evenly instead of loading just one phase.
    using the power source in a star or delta config. is the same as long as the power is spread evenly.
     
  9. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    wait a min!
    that wud have been the case with phase to neutral loading....
    i seemingly went out of phase with the situation.;)
    but anyways..
    this seems the best alternative other than opting for a higher capacity system.

    note that max power from this system is going to be 1.73*60*208...(under balanced loading that wud be 5.9 projectors in all :p)
    and the current thru any of the phase shud not exceed 60 amps..keeping this in mind the
    above suggestion still holds good...(i think it will be a good practice to connect the neutral of the system to the neutral of the supply(or ground) so that unbalanced current flows thru the neutral)
     
  10. rwmoekoe

    Active Member

    Mar 1, 2007
    172
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    :)
    it's a good thinking about the unbalanced current flow thru the neutral.
    the current thru any of the phase-to-neutral loading shouldn't exceed 60amps,
    or
    the current thru any of the phase-to-phase loading shouldn't exceed 60/1.73amps or 34.7amps. in this case, a max of two projectors are connected at a phase-to-phase loading, which consumes max 17.3*2 or 34.6! (that was close!)
    i wonder if the projectors and the power system are build for each other, because 6 unit will fit just in the power system :)
     
  11. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    actually since it is a star the current in phase and line remain same.
    thus 60amps is still the limit.
    the reason i think the the limit comes down to 2 projectors is that a phase is common to two other phases.thus there will be a vector addition of current due to loads connected in each phase to phase loads...thus a vector addition of currents due to loads between R-B and R-Y resulting in a different current in R phase.
     
  12. lespaul

    Thread Starter Active Member

    Jan 30, 2008
    49
    2
    Thank you onece again for all your help. We decided to increase our power to 100amps per phase due to other pieces of video equipment that needed to be powered. The interesting concept here is that the distro has 6 Edison connectors on the C phase and 4 L6-30 hubble connectors for the A/B phases. this should all work out fine.
     
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