- 31 Aug '16 12:03Ok, 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? - 02 Sep '16 09:42 / 1 editThere'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. - 02 Sep '16 23:56Now 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 - 03 Sep '16 00:57

You can always get the propper rotation on the second try. 50/50*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 - 05 Sep '16 15:13 / 1 edit

"By flipping just*Originally posted by apathist***Explain your reasoning please.***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. - 06 Sep '16 14:28 / 1 edit

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.*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.

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)