A 6.6 KV, 1500 HP, 3000 RPM cage horizontal sleeve bearing motor is driving a compressor through an intermediate shaft having 4 flexible plates of 3 mm thick at each end.
The vibration peak velocity (mm/sec) with the motor decoupled was as below
DE - H/V/A - 16/9/7
ODE - 15/8/8
Trim balancing on the decoupled motor was done in situ at 3000 RPM with the following final peak velocity levels
DE - H/V/A - 2.6/0.5/0.5
ODE - 1.9/0.5/0.4
The motor was also found to have an axial play of about 5 to 6 mm in the decoupled state at 3000 RPM. The motor does not have a thrust bearing.
Motor was then coupled to the compressor through the intermediate shaft and the vibration readings of the motor did not change much in the coupled state. The axial float was also reduced to 2 to 3 mm.
After about 15 hours of running on load, the motor vibrations suddenly jumped to
DE - H/V/A - 10/6/8
ODE - 11/6/8
The bearing oil temperature was around 80 deg C, which is normal as per the client.
The motor vibrations dropped to the after-balancing levels the moment the motor supply was cut off.
The motor is found to be electrically ok. I am suspecting that the flexible plates could be the problem for this sudden increase in the motor vibration level (like cracked plates). I also attribute the drop in vibration when the supply is cut off to the fact that the torque transmitted through this flexible plates drops the moment the supply is cut off.
Are my surmises correct ? What else could cause this sudden jump in the motor vibrations and its vanishing when the supply is cut off ? Could the axial float be the cause ? If yes, then how the motor ran for 15 hours with the same axial float without any issues ?
(The rotor run-out, eccentricity, uneven air-gap etc. can be safely ruled out since this was a running motor and the motor sleeve bearings are supported from the end brackets fitted to the stator frame.)
Thanks in advance for your tips.
Muthu
The vibration peak velocity (mm/sec) with the motor decoupled was as below
DE - H/V/A - 16/9/7
ODE - 15/8/8
Trim balancing on the decoupled motor was done in situ at 3000 RPM with the following final peak velocity levels
DE - H/V/A - 2.6/0.5/0.5
ODE - 1.9/0.5/0.4
The motor was also found to have an axial play of about 5 to 6 mm in the decoupled state at 3000 RPM. The motor does not have a thrust bearing.
Motor was then coupled to the compressor through the intermediate shaft and the vibration readings of the motor did not change much in the coupled state. The axial float was also reduced to 2 to 3 mm.
After about 15 hours of running on load, the motor vibrations suddenly jumped to
DE - H/V/A - 10/6/8
ODE - 11/6/8
The bearing oil temperature was around 80 deg C, which is normal as per the client.
The motor vibrations dropped to the after-balancing levels the moment the motor supply was cut off.
The motor is found to be electrically ok. I am suspecting that the flexible plates could be the problem for this sudden increase in the motor vibration level (like cracked plates). I also attribute the drop in vibration when the supply is cut off to the fact that the torque transmitted through this flexible plates drops the moment the supply is cut off.
Are my surmises correct ? What else could cause this sudden jump in the motor vibrations and its vanishing when the supply is cut off ? Could the axial float be the cause ? If yes, then how the motor ran for 15 hours with the same axial float without any issues ?
(The rotor run-out, eccentricity, uneven air-gap etc. can be safely ruled out since this was a running motor and the motor sleeve bearings are supported from the end brackets fitted to the stator frame.)
Thanks in advance for your tips.
Muthu
