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Vibration

Vibration

Vibration

(OP)
Hi,

We are experiencing severe vibration problem with a 500 k W 2 pole 400 V 50 Hz squirrel cage electric motor. The motor is driven by a VSD. The motor was uncoupled from the drive and run at nominal speed, the vibration was 2-3 mm/s in the X and Y axis and 1.0 mm/s in the axial position. Above 3050 rpm, the vibration reached 30 mm/s in the X and Y positions and 2,5 mm/s axially. It is important to note that new bearings were fitted to the motor.
According to the vibration expert, the rotor might need balancing. My opinion is that if the rotor was not balanced, the vibration would have been observed even below nominal speed.
Any opinion?
Thanks.

Guardiano

RE: Vibration

Certainly, if you have a resonance near 3050, then you can see a dramatic increase in vib as you increase speed to that range.

Let's say your first resonant freq was far above that.  Then you would see displacement approx proportional to speed^2 and velocity proportional to speed^3 as you vary speed far below resonance if the excitation was due to unbalance (unbalance generates speed^2 force)

Is this a vertical?  I would be more inclined to believe both directions reach resonance at about the same speed on a vertical machine than on a horizontal machine.

The vib analyst may have other info at his disposal.  Is the vib entirely 1x.  And if so, phase change can also give a clue whether resonant behavior is occuring or not
 

=====================================
(2B)+(2B)'  ?

RE: Vibration

Also, sometimes you can fairly easily test whether a motor is resonant by temporarily bracing the machine, with speed below what you suspect is resonance speed.  If it is a resonance, then the increase in stiffness should drive vibration down.  Sometimes we use a timber wedged between the motor (preferably near top of vertical) and the nearest convenient support. Hammer tighter into  place to tighten.   Turnbuckle is another approach for temporary bracing. Use common sense to avoid killing the patient during diagnosis.

=====================================
(2B)+(2B)'  ?

RE: Vibration

The vibration is not always visible at the slower speeds if unbalanced, it is dependant on how much imbalance and if you are approaching a critical frequency.  We have a policy that if a rotor comes out of the motor, it gets balanced.  This also gives a great change to check the runouts on the rotor to ensure nothing is slightly out.  Sometimes the smallest thing causes big problems.

You might want to double check the bearings.  Make sure the right bearings were put in and installed correctly.

RE: Vibration

May you feed motor to grid directly? Remove VSD and check vibration at nominal speed even can't reach 3050rpm. Some VSD have high harmonics. What is cable lenght between VSD and motor?
Another solution, much complicate is to drive motor by other motor (beter with variable speed capability) and check vibrations.

RE: Vibration

To back up the idea that just because you don't see as much vibration at low speed as you do at higher speed, that is why VFDs have what is called "critical frequency lockout" or "skip frequency" features. Mechanical resonance is very real and dangerous.  

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