Different Directions of "False" Rotors.

[YorkMech1]

Many times "Only" the stator of certain motors, up to 2500HP, for rewinding.
Since the proper rotation of the rotor is critical and must be determined before re-assembly, I test the rotating field with a false rotor I carry from a very small fractional HP motor, mounted inside a short piece of pipe with caps on each end and holes drilled for the shaft.
Upon energizing the stator @ reduced (half) voltage & known Ph.Rotation, Inserting the false rotor gives me the direction of rotation.
Many times I just Energize the stator (Half-Voltage) and flip a "Ball-Bearing" inside the stator and observe direction the lil' ball takes off inside the stator..
The Ball-Bearing "Always" goes in the "Opposite" direction of either the "False" Rotor or the actual rotor....
My question is simple......Why "Opposite"?????

 

[electricpete]

Interesting. Do you put a whole bearing in or just a single ball? If you put a whole bearing in is it "supported" by the inner or outer race?

 

[llangford]

Yes, electricpete, this IS interesting. The rotation should be in the same direction, either CW or CCW as viewed from one end of the bore of the stator. The ball or bearing will rotate on its own axis in the opposite direction but only because it has to 'roll' around the stator. The hand-held 'rotor' is in the air and rotates freely. I'll check this out tomorrow.

Oh by the way, in an emergency your rotor can be replaced by an aluminum can with a rod punched through the center of the top and bottom so the can is allowed to spin. A round metal capacitor hung from a string also works. Old Fogie Stuff.

Just thinking out loud.

 

[YorkMech1]

electricpete;
Sorry for not being specific on the ball bearing.
I was referring to a "Ball" from an old bearing of 1/2" to 3/4" in diameter...larger balls tend to become "Mindless-Missiles" when you turn off the power..Fairly safe as long as power is maintained and reduced to a minimum before disconnecting...Quite a little show should someone put 30 or 40 of them in at one time...
So I hear....I would Never do that...Noooooo...Not me..
This little known phenomena has bothered me for years.

 

[motorman]

Motor users do not know much about the "can test" because there is no need to. After all, if the motor is rotating wrong, just switch any two leads. But this test is very familiar in motor repair and manufacturing.

The “can test” is how a stator is checked to insure that it is connected right before doing a running test. It is especially handy when checking very slow speed (i.e. lots of stator slots) windings with multiple parallels. The first time I witnessed this test was when it was being used to settled an argument between the first and second shift winding supervisors about how to connect a stator (the day shift took the winding data before the stripping so he had to be right, right?) With power on the stator, a can that is free to rotate on a shaft (nothing fancy, just poke a wooden dowel through the top and out the bottom of a coffee can) is moved round the I.D. of the stator close to the bore. If you get to an area where the can changes directions; trouble. If you get in an area where the can just stops, trouble. These groups are connected wrong.

By the way, the steel ball will be whipping around the bore at sync speed so you might want to be cautious… (I recall the story from an old timer where a winder had left a bucket underneath some shop towels in the bore of a 4pole, 12,000hp stator. Someone energized the stator and that bucket went flying around inside at 1800rpm. You can imagine the racket, much less the damage to the soft core iron, all under the watch full eyes of the customer witness!… bucket did not fair very well either! I can just read it now in the papers: rash of stators all over the world are being damaged by ball bearings being thrown into them because of internet post!)

 

[YorkMech1]

motorman;
Interesting about the bucket getting picked up by a stator...Thats sounds a little scary...I also noticed that the lil' "Ball-Bearing" Test I mentioned, never gets the Steel Ball "Anywhere" near the Syncronous-speed. Also the ball slows to a crawl as the voltage is slo

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