Interesting case

[dccap]

A 660kw induction motor when stopped after large vibration during service, has shown damage to bearing .When the shaft was attended (welding was carried out at damaged area of bearing seat) and bearing were replaced, the motor was re-assembled after rotor balancing was carried out.
Now there is seen a very special problem of large scale vibration when motor crosses the 2200 rpm speed. This vibration goes on increaing as the speed gets up. All these tests are carried out at shop floor at no load.
What could be the reason and remedy of such probelm

 

[irdrop]

Dear Dccap,
Please give the rated speed of the motor and supply frequency. Have u done the balancing at rated speed or at 2200 rpm. Have you checked the rotor bars. Loose rotor bars (I assume it is a squirrel cage motor)can give u such vibrations at subsynchronous speeds. Please also give the bearing details.
--irdrop

 

[jbartos]

Suggestion: The additional balancing is needed. It appears that the rotor balancing was done when the motor was disassembled.

 

[electricpete]

Am I correct to assume from your description these are rolling element bearings?

What is the frequency of the vibration observed in the shop? How about the frequency of the vibration observed in the field?

Horizontal or vertical motor? Vibration equal at both ends? Vibration equal in both radial directions? Any axial vibration?

Does the vibration change with voltage (constant speed)? o power-down?

The speed dependence at first suggested a resonance but maybe not since you say vib continued to increase.

There seems two obvious broad scenario's:

1 - some original problem created high vibration which damaged the bearing... bearing replaced and original problem remains.
2 - the shaft repair caused the problem. Is it possible there was residual stress created which produced a shaft bend. Runout checked? Perhaps there is some problem with bearing seating on the repaired area. Just thinking out loud.

 

[jbartos]

Suggestion: Have you checked the motor stator for turn-to-turn shorts?

 

[Mendit]

This could well be a welding defect. You may have subsurface cracks beneath the welding. The best way to detect these is UT, use an angle beam transducer and scan across the welded areas. Welding on motor rotor shafts can be difficult, its not just a matter of laying down a few beads with the stinger.
1 undercut and NDT
2 Ample preheat
3 Proper welding with correct filler materials
4 Post heat to stress relieve
5 Machine to proof welding
6 NDT welded sections
7 Final machine, ensure rotor iron is concentric to journals and to coupling diameters

 

[jbartos]

Suggestion: The welding can hardly be perfect, i.e. fully homogeneous. Therefore, some vibration origin potentially exists there.