# Variation on the Monty Hall problem?

sonhouse
Posers and Puzzles 31 Aug '16 12:03
1. sonhouse
Fast and Curious
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. apathist
looking for loot
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. apathist
looking for loot
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. joe shmo
Strange Egg
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. sonhouse
Fast and Curious
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. HandyAndy
Non sum qualis eram
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. apathist
looking for loot
05 Sep '16 08:27
Originally posted by joe shmo
You can always get the propper rotation on the second try. 50/50
8. joe shmo
Strange Egg
05 Sep '16 15:131 edit
Originally posted by apathist