Balancing of Electric Motors 2

[CollinM]

HI. We are currently commencing the overall program on out compressor motors on our locomotives. As part of the scope of work, we are replacing the bearings on the motor side. My question is, should the motor now be balanced or can the motor be assembled and placed in service without being balanced? Is their a specific RPM rating which states that: motors exceeding a certain RPM should be balanced or is it the norm that all motors, irrespective of their RPM rating, should be balanced? What would the long term effects of an unbalnced motor be?

 

[electricpete]

the last part - immediate effect of unbalance will be 1x vibration which should be detectable by vib monitoring. This puts additional load on bearings etc.



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[jbartos]

Suggestion: Motors up to tens of HP size are normally not balanced. Larger motor sizes are prudent to balance. Balancing is always beneficial since it reduces centrifugal forces on bearings, reduces vibrations, etc.

 

[jbartos]

Suggestion: Visit

http://www.skfcm.com/service/support2/New Library/CM3047 Trim Balancing.pdf

for: Trim Balancing

 

[bklauba]

Normally just replacing the bearings will have little or no effect on the balance of the motor.

As both the intertia of the armature and the speed are greater, so the need for balancing becomes greater.

Another factor to consider is with what type of equipment the motor will used. This is expressed in what is referred to as "Balancing Quality Grades" expressed as "G" numbers, running from G0.4 (very well balanced, e.g. gyroscopes) to G4000 (somewhat balanced, e.g. crankshaft drives of slow, rigidly mounted marine diesel engines, with uneven # of cylinders).

For reference, take a look at Fundmentals of Balancing, published by Schenck Trevel Corp., Deer Park, L.I., NY, 2nd edition, March 1983. ppgs, 46 - 47. Here you will find the "G" table, and chart showing what levels of unbalance can be tolerated for various speeds, and various "G" grades. These are also commonly found in both instruction manuals for balancing equipment, and included in vibration analysis course textbooks.

Hope this helps!!

 

[jclough]

Motors of all types and sizes are balanced regardless of size. Most are balanced ot a G2.5 level or better. If you follow the Uper chart from ISO 1940 you'll fing that slower motors will be well balanced as the faster ones.

Don't rely on vibration monitoring to check unbalance. Vibration level is related to the stiffness of the mounting.

 

[electricpete]

I think it is fairly typical that if work is done in an outside shop, they will be required to balance the machine.

I think for in-house work, it is not as common to require a balance check. For example we replace bearings on NEMA frame motors without balancing and we do in-place split sleeve bearing replacement without removing rotor or balancing. We rely on solo-run installed in-place before coupling to identify problems. I agree with jc this is not a perfect way to find unbalance. If we find a problem at that point we may or may not be able to balance in-place.

I think it is a matter of effort vs payback. Depends in part on where the motor will be worked, will rotor be pulled... how easy to get it to the shop, do you have your own shop or have to send it out for balance, etc.

So far we have taken the shortcut and no balance and never gotten in trouble.

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[jbartos]

Suggestion: If a motor is disassembled and the balancing equipment available, it would be a shame not to balance it. By balancing the motor, it is made sure that the repaired motor is in a very good (A1 or so) condition. I think that the customer will be willing to pay a little extra for it.

 

[gsimson]

All motors are balanced when manufactured , then what is the factor which causes unbalance ?
Wear & tear can be only at the bearing support point.
Whether the rotor has bulged due to hot spots?
Whether any stotor - rotor rubbing has taken place ?
Whether rotor bar has broken?
If any unbalance is detected in a rotor , the root cause is to be analysed and rectified before balancing is done. Simply balancing does not help as the rotor developes unbalance only when there is an unwanted stress in the rotor.

 

[Iwe007]

If your program calls for replacing bearings as part of your maintenance procedure I would be led to think that your motors have served for quite a few hours (10K , 20K hours or more !) which is typical bearing life.
You should also call for balancing of the rotors at the same time. This is helpful because you get a chance to visually see the the condition of both the rotor and the stator, at which point you can determine if any preventive repairs are necessary or not.
Yes, a balnced rotor will help your motor live longer and work smoother, quieter and be more efficient.
I design, manufacture, test and maintain motors, hopefully that adds some weight to my point of view.

 

[jbartos]

Comment: It is good to see in the above postings that the electrical expertise is very considerate to the mechanical aspects of the motor functioning.