Stat guys, probability of getting rotation right?

[chris kennedy]

Does anyone know the math behind arbitrarily hooking up 3? motors and getting the rotation right. Experience tells me its better than 1in3 but I was asked today and I had no answer.

Thanks

 

[sii]

My experinece has been about one in twenty. Seems like I never get it the first time.

Seriously though, it's only cw or ccw, I would think 50/50.

 

[iwire]

Seriously though, it's only cw or ccw, I would think 50/50.

I agree.

 

[Cow]

I've been meaning to get one of these, it's non-contact too:

http://www.extech.com/instruments/resources/manuals/480403_UM.pdf

 

[cadpoint]

There's 3 positions on one side 3 on the other, odds wise U have a 1 in 4 chance to get it right out of 12 possible combinations.

A A
B B
C C

A B
B C
C A

A C
B A
C B
----
A B
B A
C C to reverse it

Negating the Bond!

 

[jeremysterling]

I've never been to Vegas, but I'd make book on 1 out of 2.

 

[chris kennedy]

I've never been to Vegas, but I'd make book on 1 out of 2.

50-50 or 1 out of 2, something tells me there are more possibilities with three wires.

 

[Mgraw]

A A
B B Same
C C

A B
B C Same
C A

A C
B A Same
C B

A B
B A Reversed
C C

A A
B C Reversed
C B

A C
B B Reversed
C A
50/50

 

[jeremysterling]

I've been meaning to get one of these, it's non-contact too:

http://www.extech.com/instruments/resources/manuals/480403_UM.pdf

I use this:

http://www.fluke.com/fluke/usen/Electrical-Test-Tools/Electrical-Testers/Fluke-TPRO-T.htm?PID=56197

The boss still doesn't like it because it detects a rotary field with only 2 leads. What are the odds?

 

[iwire]

50-50 or 1 out of 2, something tells me there are more possibilities with three wires.

Not if your only wish is correct rotation.

There are only two directions.

Stopped or smoking are not directions. :grin:

 

[topgone]

I've never been to Vegas, but I'd make book on 1 out of 2.

Yep. If the rotation is wrong, you just reverse any two leads, so that makes it 1 in 2!

 

[cadpoint]

Post 8 is correct for the permutations, but his probablity is wrong!

Only combinations that have at least one set of like letters will run. Therefore one has a two out of three chance of arbitraly hooking up a motor that will run. But there is only a one in six chance of hooking the motor up correctly(forward), because of the six combinations only one has three matching letters.

One has a 16% chance of hooking it up correctly.
One has a 67% chance of hooking it up and it running.
One has a 50% chance of it running backward.
One has a 33% chance of it not running at all.

I agree with Bob, the correct way is "1" in respects to the desired application.

I think the Smoke, Chatter will be released with my second and thrird choices.

Thanks Cathy! :roll:

 

[eric9822]

I know it's 50/50 but in over 20 years I have only gotten 1 right on the first attempt. That explains why I don't buy Lotto tickets.

 

[hillbilly1]

I wired one plant where the customers engineer would run a pool on how many motors would run the correct direction versus backwards. You would figure with all of them wired the same, either they all ran backwards, or all would run correctly, but you have to take into consideration that some equipment required clockwise rotation, while others required counter clockwise rotation.

 

[hurk27]

A-A
B-B = CW
C-C

A-A
B-C = CCW
C-B

A-B
B-A = CCW
C-C

A-B
B-C = CW
C-A

A-C
B-A = CW
C-B

A-C
B-B =CCW
C-A

So there you have it, 6 possibilities that result with 3 CW and 3 CCW = 50/50


There are only two rotations, ABC (CW), and ACB (CCW) if you look at the order of your wires, it will be one or the other.

 

[hurk27]

If you also go through the combos of both supply and load connections, there will be 192 possibilites, with an outcome of 96 CW and 96 CCW still 50/50

 

[Smart $]

If you also go through the combos of both supply and load connections, there will be 192 possibilites, with an outcome of 96 CW and 96 CCW still 50/50

192?

How about 3!? = 216 if considering the labeling the wire ends (1 correctly labeled and 5 mislabeled possibilities). Otherwise only 3!? = 36... 3 supply terminals, 3 wires, and 3 motor leads.

 

[Mgraw]

Post 8 is correct for the permutations, but his probablity is wrong!

Only combinations that have at least one set of like letters will run. Therefore one has a two out of three chance of arbitraly hooking up a motor that will run. But there is only a one in six chance of hooking the motor up correctly(forward), because of the six combinations only one has three matching letters.

One has a 16% chance of hooking it up correctly.
One has a 67% chance of hooking it up and it running.
One has a 50% chance of it running backward.
One has a 33% chance of it not running at all.

I agree with Bob, the correct way is "1" in respects to the desired application.

I think the Smoke, Chatter will be released with my second and thrird choices.

Thanks Cathy! :roll:

Not sure what you are saying. In all six cases the motor will run. Three will run CW and three will run CCW.

 

[millelec]

I was taught the 50/50/90 rule. There's a 50% chance of getting it right or wrong, but you'll still get it wrong 90% of the time due to bad luck

 

[jeremysterling]

I was taught the 50/50/90 rule. There's a 50% chance of getting it right or wrong, but you'll still get it wrong 90% of the time due to bad luck

That is why I've never been to Vegas!

I try to not "arbitrarily" (as stated in the OP) hook up 3? motors. I know sometimes we have no choice.

I think most motors turn CCW when looking at the shaft end with ABC supply. Someone correct me if I'm wrong.

Is it possible to manually spin the motor and then read the output to determine rotation?