Rotating electric machines vibrations measurements

[JPetersen]

Hi,

have anyone worked with IEC 34-14, dealing with subj.??

If so you may be able to answer me: why do it state that vibration severity should be measure at NO LOAD, when I just now realize that f.x. rotor winding interturn shorts can easily be detected as high vibrations at loaded condition!
In any case vibration measurements should of course be talken in normal operation mode, or at the load condition which give highest vibration readings - right???

Comments and experiences are highly appreciated

/Petersen

 

[weh3]

Isn't the purpose of the standard to give a standardized way of measuring the vibration? Can the standard take into account all possible loading conditions? Is the measurement intended to determine the stability of your mounting or the integrity of the motor?

Wouldn't the standard logically assume that your equipment is in working order? This is not the case if there are shorted windings. Then you have an electrical problem, not a mechanical one.

Wouldn't any load at all stabilize the motor and mountings, so that mechanical vibrations would be greatest at no load? How would the standard account for all dynamic conditions that might cause resonances, etc.?

Regards,
William

 

[JPetersen]

Thanks for a prompt reply,

agree with you to some extend. But what if thermal heating of the rotor, during load, for some reason causes bending of the rotor, resulting in high vibrations. Rotors are usually balanced dynamically in cold condition.
What if vibrations in load condition is severe due to above? - what is the limit for vibrations in loaded condition.

/Petersen

 

[weh3]

I was only providing food for thought in the form of questions, and you are rapidly getting beyond my expertise. Have you read the standard?

Regards,
William

 

[electricpete]

I'm not familiar with the standard but just wanted to throw in my 2 cents comments fwiw.

Loaded and unloaded run obviously provide the best identification of possible problems.

In many cases particularly rewind of a large motor, loaded run is expensive. When we rewind a large motor we do unloaded run with vibration check. We don't get to check the loaded performance until it gets installed in the plant. For new large motor we request loaded run.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

NEMA MG-1 2003 section 7.8.1 specifies that the limits listed apply to no-load conditions.

Once again I think it is a matter of convenience and cost. Purchaser can certainly request loaded test if he desires.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[JPetersen]

Electricpete, if you intend to request a vibration check for your motors in loaded condition, would you specify/use the same limits as for unloaded test?

Weh3 is speaking the motor manufacturers point of view -
"Can the standard take into account all possible loading conditions?"

and further

"How would the standard account for all dynamic conditions that might cause resonances, etc.?"

If possible to do a full load test - should we stick to the unloaded limits? Otherwise what do you do?

/Petersen

 

[electricpete]

"if you intend to request a vibration check for your motors in loaded condition, would you specify/use the same limits as for unloaded test?"
Yes, we apply the NEMA MG-1 unloaded vibration limits for both unloaded and loaded vibration data. I'm not sure if the remaining questions are directed to me but I'll give my two cents.

"Can the standard take into account all possible loading conditions?"
No. I believe no-load and full-load are good enough (excluding VFD applications).

"How would the standard account for all dynamic conditions that might cause resonances, etc.?"
NEMA gives instructions for testing either in rigid or resilient mounted configuration. Either configuration assures that mounted resonant frequency during test is well separated from running speed. However there is no easy way to assure that resonance amplification will not occur once installed. Likewise installation problems like base looseness or coupling misalignment cannot be anticipated or prevented by repair shop testing.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

I said no-load and full load are good enough... I meant for vibration considerations. Often there are more points specified for efficiency determination.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[powerjunx]

Jpetersen ,search this thread # : 254-33520, this would help!