1. Subscribersonhouse
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    31 Aug '16 12:03
    Ok, so I have an electric motor powered by 3 phase AC. The rule is, the motor rotates one way say with phase A, B, C to terminals X, Y, Z

    If any 2 are reversed, B, A, C or C, B, A or A, C, B, the rotation of the motor reverses.

    Only one motor direction is preferred.

    So are the odds just 50/50 or is it more complicated than that, trying to figure out beforehand which rotation the motor will take?
  2. Standard memberapathist
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    02 Sep '16 09:421 edit
    There's only one combination that produces the correct rotation, so assuming the wires and/or terminals are unmarked then there is a 1 out of 3 chance of placing the first wire correctly, and then 1 out of 2 chance of placing the second wire correctly and then the third wire is easy. So 1/3 * 1/2 * 1/1 which is 0.166... or about a 17% chance of wiring the machine correctly. I think.

    But this has nothing to do with the Monty Hall problem which relies on hidden information that affects the probabilities.
  3. Standard memberapathist
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    02 Sep '16 23:56
    Now I think I made a mistake. There are six permutations, three of which involve switching two of the wires (and so reversing the rotation). So presumably the other three would not reverse the rotation. 50/50 chance.

    ABC forward
    ACB reverse
    BAC reverse
    BCA forward
    CAB forward
    CBA reverse
  4. Subscriberjoe shmo
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    03 Sep '16 00:57
    Originally posted by apathist
    Now I think I made a mistake. There are six permutations, three of which involve switching two of the wires (and so reversing the rotation). So presumably the other three would not reverse the rotation. 50/50 chance.

    ABC forward
    ACB reverse
    BAC reverse
    BCA forward
    CAB forward
    CBA reverse
    You can always get the propper rotation on the second try. 50/50
  5. Subscribersonhouse
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    04 Sep '16 04:42
    Originally posted by joe shmo
    You can always get the propper rotation on the second try. 50/50
    That was my intuition, 50/50 but wasn't sure.
  6. Standard memberHandyAndy
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    04 Sep '16 18:45
    Originally posted by sonhouse
    That was my intuition, 50/50 but wasn't sure.
    Monty celebrated his 95th birthday Aug.25 behind closed doors.
  7. Standard memberapathist
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    05 Sep '16 08:27
    Originally posted by joe shmo
    You can always get the propper rotation on the second try. 50/50
    Explain your reasoning please.
  8. Subscriberjoe shmo
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    05 Sep '16 15:131 edit
    Originally posted by apathist
    Explain your reasoning please.
    "By flipping just two of the three wires". That was the original method described by sonhouse. Using this method you can always get the propper rotation on the second try. I think because I ommited that condition in my response to your statement you thought I was mistaken.
  9. Subscribersonhouse
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    06 Sep '16 14:281 edit
    Originally posted by joe shmo
    "By flipping just two of the three wires". That was the original method described by sonhouse. Using this method you can always get the propper rotation on the second try. I think because I ommited that condition in my response to your statement you thought I was mistaken.
    That is in fact the case, flip any two wires of a three phase power supply to a 3 phase motor and the spin reverses. Not sure what happens to flipped wires in higher phase numbers though. There are 4,5,6 phase equipment also. My guess is if you flip one phase of a quad, it will either stop running or have seriously reduced torque. I don't think flipping 2 phases out of 4 will reverse spin though. 3 phase is power to each phase at 120 electrical degrees apart. 4 phase is each phase is 90 degress apart, 5 phase, 72 degrees apart, 6 phase, 60 degrees apart.

    Higher phase numbers gives smoother rotation like internal combustion engines, 1 cylinder is pretty shaky, 2 better, 3 better, 4 better (on balancing and left over jerkiness of the engine rotation)
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