Stat guys, probability of getting rotation right?

[Dennis Alwon]

The probability of getting the right rotation is 50/50 however if it is wrong then the probability of getting it right the first time is 100% unless you factor in stupidity into the equation. :grin:

 

[jumper]

The probability of getting the right rotation is 50/50 however if it is wrong then the probability of getting it right the second time is 100% unless you factor in stupidity into the equation. :grin:

Dennis, can I assume this is what you meant?

 

[mivey]

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

Yes .

 

[mivey]

Only combinations that have at least one set of like letters will run.

Please elaborate.

 

[Mgraw]

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

There are meters for that also.
Example:
http://www.time-mark.com/Documents/Datasheets/103-108C.pdf

 

[iwire]

If I can manual spin a motor I can just as easily temp it and jog it, no tester needed.

 

[charlie b]

Current flowing through a wire creates around itself a magnetic field. With the manner in which a three phase motor's stator is wound, and the manner in which a three phase generator provides power to the distribution system, the net magnetic North created within the windings will rotate around the stator. The direction of rotation is a function only of the phase sequence of the currents flowing through the stator. Once the magnetic field is rotatating in one direction or the other (i.e., CW or CCW), the rotor will be driven in the same direction.

There are only two possible sequences (ABC or ACB), regardless of how you made the connections. All other sets of letters are identical on one of these two sequences. Thus, there are indeed only two possibilities, and the odds are 50/50.

That said, is the wiring choice really arbitrary? Are there no clues from the color codes sometimes used in wiring systems, or in the information provided by the manufacturer? Never having connected a motor myself, I would not know whether the terminals are labeled in any useful way.

 

[Mgraw]

In most situations I would agree a meter would not be needed.

 

[mivey]

Are there no clues from the color codes sometimes used in wiring systems, or in the information provided by the manufacturer? Never having connected a motor myself, I would not know whether the terminals are labeled in any useful way.

The terminals are labeled. Sometime the source wiring is labeled. But I would never trust somebody else's labels unless there was no chance of damage if the motor turned backwards.

 

[cadpoint]

Based on the original question if one wants to connect L1, L2, L3 (ABC) to the respective poles of T1, T2, T3(ABC); there is only a one is six chance to do it right!

But if the original question is that it will run clockwise then it?s only 50-50 it will happen!

Having reviewed simple 480 3 phase constant speed simple wye (star) and delta motors per the diagrams and the wordage.
One is only allowed ABC to ABC and ABC to BAC per standard practice, IE reversing only the first two poles.

http://www.industrial-electronics.com/AC-DC-motors/14_Reversing-Rotation-Three-Phase-Induction-Motor.html

Will any of the other combinations work, I frankly don?t know, I do know from reading forward and backwards in the link
given all these other odd combinations might well work for a simple 480 3 phase constant speed but are not NEMA SOP!

He?s the kicker, my bride reminded me that permutations was taught in HS (for us older readers) and she now teaches it in the fifth grade!

Thanks again Baby!

 

[Barndog]

The probability of getting the right rotation is 50/50 however if it is wrong then the probability of getting it right the first time is 100% unless you factor in stupidity into the equation. :grin:

they are both 50/50. its either CW or CCW/ Right or Wrong. if you only have to choices its always 50/50.

 

[mivey]

Based on the original question if one wants to connect L1, L2, L3 (ABC) to the respective poles of T1, T2, T3(ABC); there is only a one is six chance to do it right!

Without a pre-existing reference, the designation "A" is completely arbitrary and we have a 3 in 6 six chance of doing it right.

But if the original question is that it will run clockwise then it?s only 50-50 it will happen!

There you go.

Having reviewed simple 480 3 phase constant speed simple wye (star) and delta motors per the diagrams and the wordage.
One is only allowed ABC to ABC and ABC to BAC per standard practice, IE reversing only the first two poles.

http://www.industrial-electronics.c...ing-Rotation-Three-Phase-Induction-Motor.html

That is not what they are saying

Will any of the other combinations work, I frankly don?t know, I do know from reading forward and backwards in the link given all these other odd combinations might well work for a simple 480 3 phase constant speed but are not NEMA SOP!

Your previous 50/50 statement is still true. Not sure what NEMA SOP you are referring to.

He?s the kicker, my bride reminded me that permutations was taught in HS (for us older readers) and she now teaches it in the fifth grade!

Thanks again Baby!

Kids are just smarter than us old fogies. If you don't believe it, just ask them!

 

[Dennis Alwon]

Dennis, can I assume this is what you meant?

I am glad you are here interpreting for me. I am also glad you understand my language.

Yes that is what I meant.

 

[Dennis Alwon]

they are both 50/50. its either CW or CCW/ Right or Wrong. if you only have to choices its always 50/50.

If you get it wrong the first time then the second time is not a 50/50 chance- change any 2 and you get it right however , as I stated , that does not take into consideration stupidity.

 

[Besoeker]

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

Yes. Six.
Three possibilities for the first wire, two for the next, and one for last. That gives the six. Of the six, three give one direction of rotation and the other three give the opposite. Without any other information you'd have an equal chance between correct and incorrect rotation.

If you have data on the motor you should have the required information to get the direction right.
From the ABB motor manual on a recent installation:

"The shaft rotates clockwise when viewing from the shaft face at the motor drive end and the phase sequence - L1, L2, L3 - is connected to the terminals as shown in Figure 1."

I should add that the manual is applicable to all ABB motors from frame size 56 to 450. That covers motors ranging in weight from 4.5kg to 4500kg.
Or around 10lb to 4.5 tons.

 

[zog]

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

Navy saying was 50-50-90

50% chance you get it right, all you need to do is swap any 2 legs but 10% chance you screw that up too

 

[stevebea]

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

The law of averages says you have a 50% chance of getting the correct rotation.

 

[chris kennedy]

Navy saying was 50-50-90

50% chance you get it right, all you need to do is swap any 2 legs but 10% chance you screw that up too

Thats a good chuckle right there.:grin:

 

[topgone]

If you get it wrong the first time then the second time is not a 50/50 chance- change any 2 and you get it right however , as I stated , that does not take into consideration stupidity.

As posted above, including what others have said earlier, the answers all say 50/50! Because I don't think there is anybody in this forum stupid enough to get those motor connections wrong the second time around. Say it ain't so, buddies?

 

[__dan]

large sample

large sample

Flip a coin ten times and the hit rate for heads or tails could easily range from 3/10 to 7/10. The hit rate only converges to 1/2 with a very large number of samples, say 50/100 or 500/1000.

Probability is not deterministic with a single sample or small number of runs.