Motor 2X vibration problem

[Arnieg]

I have three 1500 HP, 4160 volt, 60 Hz, 3600 RPM, horizontal Siemens induction motors that drive centrifugal air compressor. One of the three units has had a higher vibration level than the other two from the start. At initial start up the motor had a high 2X running vibration level. The vibration levels measured by our vibration crew were approximately 0.16 in/sec horizontal, vertical and axial DE and approximately 0.10 in/sec NDE, (the motor vibration levels were approximately the same coupled and uncoupled at system startup). For lack of a better description the problem motor seemed to have more of a roar than the other two motors. The motor manual states that vibration alarm should be at 0.20 in/sec and trip should be set at 0.25 in/sec, (the motors do not have vibration monitoring). The air compressors have proximity probe vibration monitoring. For the first 1300 hours of operation the the vibration levels on all three stages of the problem air compressor remained constant. After approximately 1300 hours the vibration levels on all three stages of the problem unit started to increase. Over a period of 5 days the vibration levels on the air compressor increased by approximately 30%, (the first stage went from 0.28 to 0.42 mils). The motor vibration level of the problem unit was noticably higher to the touch than the other two motors. When standing on the steel motor base of the compressors it was obvious that the vibration level of the problem motor base was noticably higher than the other two compressors. The vibration crew was called back in and they measured vibration 2X running levels on the order of 0.9 in/sec, horizontal, vertical and axial on both ends of the motor. The unit was shut down and Siemens was contacted. When the Siemens technician arrived he reviewed our vibration data and informed us that our vibration crew was taking readings in the wrong places on the motor, not taking readings for long enough to properly detect modulation and not at sufficiently high resolution to accuratly distinguish between 2X running and 2X frequency vibration. Basically he stated that the vibration data that our vibration crew had taken was unusable. The Siemens technician than proceed to take vibration data and could not find anything wrong with the unit, (all vibration levels were less than 0.10 in/sec. The motor and motor base was no longer vibrating nearly as much to the touch and the compressor proximity probe measurements had returned to normal. (We continuously record compressor vibration data.) Based on the above information the Siemens technician concluded that we either had intermitant electrical problems, (phase inbalance), or that the motor base had entered into resonance but was no longer resonating when he colected data. Can anyone think of anything else that could cause the vibration problems that we experienced?

 

[Tmoose]

Possible Something else -

Some hold down bolts were tightened.

 

[petronila]

Helo Arnieg,

Taking account you sayd,2X Problems could be caused by: 1. missalignment 2. Looseness 3. Bent shaft or 4. Eccentric Gap. Is very strange your people find differents vibration readings than SIEMENS.

To find the problem´s sources is better to take a Phase Angle and to take vibration readings cutting the power this coul help to clarify whats happenning.

Due to Is very strange your people find differents vibration readings than SIEMENS call to one vibration expert looking what they sayd.

Regards

Petronila

 

[edison123]

Did the supply frequency change when these two measurements were made ?

Two pole motors are notorious for sudden vibration problems.

*Why make the same mistake twice when there so many new ones available*

 

[powerjunx]

Some service personnel would establish a fact that there equipment installed has no problem at all, though apparently it does. In a silent manner, a sly and witty manevuer on the machine peripherals was made by that technician. Thus, problem never persist. I myself do that before.

 

[arcflash99]

If you want to separate the electrical possibility from mechanical, perform a "coast down" test. Monitor vib and shutdown the motor. At this point electrically induced problems should go away. Any mech problems should remain and degrade as the motor coasts down.

 

[electricpete]

If 0.9 ips was measured on the bearing housings of a sleeve bearing motor, I would wonder whether the bearings had been damaged as a consequence of that high vibration.

I notice the machine was shut down in between when the vibration was observed by your guys and not observed by the oem rep.

How long was the motor run for the OEM.... long enough to stabilize thermally? Sometimes it takes awhile for problems to show.

Is it a gear coupling? I have heard of gear lockup relieved by shutdown/startup, although it doesn't seem relevant if the motor has only sleeve bearings operating away from their mechanical stops.

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