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Thread: May I ask a question about the single vs two phase stuff

  1. #11
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    Quote Originally Posted by Russs57 View Post
    But from a pratical point....I could have two motors, both called single phase, feed from a 3 phase 480/277 supply. Both would have run caps, but one would get 277 and one would get 480. Seems the one that is 480 is a true two phase motor (albeit 120 degrees apart) and therefore should be able to run without the cap.....but it doesn't seem to.

    I feel I'm failing to understand something here.


    Its only 2 wires- you would need to make use of the neutral. 3 wires to the motor and 2 windings.
    I'm in over my head...

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    Quote Originally Posted by Russs57 View Post
    But from a pratical point....I could have two motors, both called single phase, feed from a 3 phase 480/277 supply. Both would have run caps, but one would get 277 and one would get 480. Seems the one that is 480 is a true two phase motor (albeit 120 degrees apart) and therefore should be able to run without the cap.....but it doesn't seem to.

    I feel I'm failing to understand something here.
    the 480 is not 2 ph (although comprised of 2 phase lines)
    Vab is a ph
    Vbc
    Vca

    each is a single ph (the sum of 2 phase coils in a wye)

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    Quote Originally Posted by Russs57 View Post
    But from a pratical point....I could have two motors, both called single phase, feed from a 3 phase 480/277 supply. Both would have run caps, but one would get 277 and one would get 480. Seems the one that is 480 is a true two phase motor (albeit 120 degrees apart) and therefore should be able to run without the cap.....but it doesn't seem to.
    The two legs for the 480 volt motor are only 120 degrees apart relative to the neutral. For a circuit with only two wires, there is no notion of phase. When discussing phases, there are necessarily at least 3 wires involved: one defines a common reference, so that the phase difference between the other two wires can be described.

    Cheers, Wayne

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    Thank you gentlemen.

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    180722-1321 EDT

    I have two single phase generators driven by one motor with a 1 to 1 ratio between the motor and each generator. Additionally there is an adjustable mechanical coupling between the two generators such the relationship mechanically can be anywhere between and including 0 and 360 degrees. Is this system a two phase source?

    By design this system has two generators that are synchronized.

    .

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    Quote Originally Posted by kwired View Post

    They are "out of phase" from one another from their relationship to a particular reference point at any given moment in time.
    Isn't that all moments in time? They are out of phase all the time. That's not nit picking - I think it's an important concept to understand.
    Quote Originally Posted by kwired View Post
    There is no "phase" relationship that creates a rotating field for an ac induction type motor without additional methods being used to create such rotating field on such "split phase" source.
    True. But does that doesn't mean that you don't have two phases displaced by 180° from each other.
    Si hoc legere scis nimium eruditionis habes.

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    Quote Originally Posted by Russs57 View Post
    But from a pratical point....I could have two motors, both called single phase, feed from a 3 phase 480/277 supply. Both would have run caps, but one would get 277 and one would get 480. Seems the one that is 480 is a true two phase motor (albeit 120 degrees apart) and therefore should be able to run without the cap.....but it doesn't seem to.

    I feel I'm failing to understand something here.
    It is two phases in relation to the entire three phase supply, but to the motor being supplied you only have a single voltage and a single current wave form in the supply conductors, and the "phase angle" between two points is always 180 degrees. As already mentioned that 120 degree angle is when referenced to the neutral point of the overall supply system.

    Comes down to what I earlier mentioned exact meaning of "phase" depends on what is being referenced to what and that the word phase alone is too generic to mean the same thing for all conditions we use this word for.
    I live for today, I'm just a day behind.

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    Quote Originally Posted by gar View Post
    180722-1321 EDT

    I have two single phase generators driven by one motor with a 1 to 1 ratio between the motor and each generator. Additionally there is an adjustable mechanical coupling between the two generators such the relationship mechanically can be anywhere between and including 0 and 360 degrees. Is this system a two phase source?

    By design this system has two generators that are synchronized.

    .
    yes
    and if set at 180 deg and 1 leg from each commoned indistinguishable from 120/240/1 split phase

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    Quote Originally Posted by jaggedben View Post
    An important reason not to call split-phase 'two-phase' is that two-phase actually means something else.

    Otherwise, mathematically sin(n)=-sin(n+180), and thus it's a bit pointless to argue over which is a 'better' mathematical representation of what you read with your meter. Just specify which probe on your scope is being used as reference.

    From a conceptual point of view, if the two sources are actually derived from the same transformer primary, it makes more sense to think of it as 'single phase with a center tap'. If they are something else (say two inverters that can use software to have various relationships) then perhaps speaking about mutliple phases is more justified. Ultimately, however, one's obligation is only to employ words to accurately represent what is being done physically, which can be done multiple ways as long as one defines terms clearly. Trying to settle what it 'is' is relatively pointless.
    "Electricity is really just organized lightning." George Carlin


    Derek

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    Last edited by Ingenieur; 07-22-18 at 03:00 PM.

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