Motor Magnetic Center 1

[vanskat]

I have been thrown a concept on Motor magnetic center.

After a recent failure it was argued that over tightening a coupling restricted axial movement that is normally expected on larger induction motors once they have been energized or return to rest!

Personally, I have never heard of this theory nor have I seen or can find any motor spec containing such information

How real is this?

 

[aolalde]

NEMA MG1-20.81 END PLAY AND ROTOR FLOAT FOR COUPLED SLEEVE BEARING HORIZONTAL INDUCTION MACHINES.
Operating experience on horizontal sleeve bearing induction machines has shown that sufficient thrust to damage bearings may be transmitted to the induction machine through a flexible coupling. Damage will be avoided if the following limits are observed by the machine manufacturer and the assembler of the motor to driven equipment:

Motor Speed min. Rotor Max coupling
HP rpm end float end float

500> 1800 > 0.25” 0.09”
300-500 3000-3600 0.50" 0.19”
600 < All 0.50&quot; 0.19&quot;

 

[electricpete]

It is very well established that there should be careful attention given to positioning the motor properly within the end-float mentioned by aolalde.

Ideally magnetic center and mechanical center are not too far apart and you can set on either one. Most important requirement is to set so the motor won't run on a thrust shoulder. 2nd most important is to allow it to run on magnetic center.

There is a very lot that can be said on this subject. Can you give more details of your situation. Is it sleeve bearing machine? What type of coupling? Details of failure.

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[aolalde]

To find the magnetic center energize the motor uncoupled and under full voltage. Use a static reference on the bearing pedestal and scribe a mark around the shaft. Stop de motor. Rotate the shaft by hand with a wrench help, push the shaft until it touches the bearing shoulder, then pull the shaft while rotating it until it touches the other bearing shoulder. The motor en float is the space found between the shoulders. The magnetic center should be close to half the way.

Locate the shaft mark for the magnetic center against the reference in the pedestal and align the coupling. The max allowed displacement coupled should be into the limits and never should touch the bearing shoulders while reaching the natural magnetic axial center

 

[macmil]

There was a recent thread on a similar topic; the coupling was an internal gear coupling, and wear caused an axial thrust on the motor shaft out of magnetic centre and against the sleeve bearing shoulder. I had this experience 25 years ago with several sleepless nights. If indeed you have a gear coupling you may want to review the thread.

 

[gsimson]

Dear alocade,
For measuring the physical center it is easier to push & pull the shaft when the motor is running instead of stopping.
Simple pencil can push the shaft as the shaft is floating on the oil film. Rotating the shaft in stationary condition may score the bearing surface.

 

[edison123]

I normally scribe the magnetic centre on the shaft on both sides of the ODE (outboard) bearing pedestal, when the uncoupled motor is run with full rated voltage. This way, by turning the rotor by hand and pushing it along the axis on both directions, I can measure the rotor play with respect to the scribed centres. If you mark only on one side of the shaft, the mark may disappear into the bearing housing in one direction thereby making the measurement of the end play in that direction difficult.