high vibrations synchronous motor under reciprocating loads 2

[RRUUEE]

During operation we experienced high vibration on the pedestal at nde side on new installed 6MW Motor, single bearing type, running speed 450RPM.
Harmonic spectrum showed peeks at 7.5 and 22.5Hz that can be explained by bending exciatation 1st and 3rd Order.
The calculated bending frequency of the train is near to 3x order running speed
other peaks are around 120 and 240 Hz with sidebands
We're searching for explanations for the higher frequencies
These frequencies can be explaind by loose stator coils and other facts, but the motor is new
we found a previouss thread thread237-297373 that sounds similar, but outcome was not mentioned.
Could it be that synchronous motor reacts on reciprocating loads by high vibration frequency (120,240Hz) specially when strong bending results in assymetric air gap?
How to verify this?
Help appreciated
see also thread237-297373

http://files.engineering.com/getfil...67-4a65-99de-2ab2daeb05f4&file=resonancee.png

http://files.engineering.com/getfil...00496f9&file=hammer_test_after_stiffening.png

 

[Strong]

"Problem identified as a resonating bearing housing
a fix will be tried out: implementation of an additional dampening element around the bearing housing"

Very hard to believe this "diagnosis" and "proposed fix". Perhaps OP can provide additional feedback with vibration data and photo/sketch/drawing so we can learn something from this discussion.

Walt

 

[RRUUEE]

Maybe final update after some work on site and evaluation

After presentation of results to involved parties it seems to be mutually agreed that
existing vibration levels are acceptable without further hardware changes

I share hereby some excerpts of the report and some facts about the approach
But without detailed figures. If there is some more interest, I can share

Main concerns were stresses on shaft, bearing housing and bearing

-Analysis showed high vibrations and are mainly due to mechanical resonance of the bearing pedestal.

-The excitation force most likely comes from typical and inherent forces in the reciprocating compressor.
-These forces (coming from Compressor) are low and do not produce significant vibration on the compressor frame, cylinders, and piping.
-There is only a raised noise floor

-Reducing high frequency forces in the compressor may not be practical,
-The mechanical resonance of the bearing housing and pedestal is the root cause of high vibrations on the bearing housing on the non-drive end of the motor.

4.1.A stresses on the shaft,

-reconstructed overall orbit by combining 1X to 5X proximity probe readings

Figure 4.1.1 Overall shaft displacement (not included)

-vibration-induced stresses are not in the danger zone.

4.1.A.III Impact test interpretation
Figure 4.1.6 (not included)
-it is perceived that 25 Hz is a MNF
-There is a peak for all three planes at about 23.5 Hz
-160 Hz could be another MNF of the shaft for a higher mode.
-The mode at 220 Hz seems to be more dominant by the response from the pedestal, so 220 Hz is believed to be a MNF for the pedestal.
-very small response are observed at 125 Hz and 260 to 285 Hz on the shaft.
Figure 4.1.7 (rotating shaft barring device, with running lift oil pump, not included)
-peak observed at about 70 Hz bracket related to the bracket that these proximity probes are installed on.
-270 Hz related to the natural frequency of the pedestal,
4.1.b Stresses on pedestal and grout,
-field measurements show higher than guideline vibration for a bearing housing on a motor (as per Motor Vendor standards)
-unlikely that this level of displacement will lead to structural problems
-Figure 4.1.8 (not included)
4.1.C Stresses on the bearing (Babbitt)
-displacements are less than 1.5% of diametrical bearing clearance for frequencies higher than 3X run-speed).
-The Overall vibration is a reconstruction of the waveform by combining 1X to 5X vibrations.
displacement of the bearing is less than 30% of the diametrical clearance of the bearing. And based on the Hugh article found acceptable

Reference:
Estimating Allowable Shaft Vibration Limits for Fluid Film Journal Bearings, James D. McHugh,

Figure 4.1.9 –1X to 5X vibration of the shaft relative to the bearing housing (not included)
4.2.e Torsional Measurement

-The first TNF is measured to be at about 72 Hz. For the second mode, the measurements showed a wide peak between 180 Hz and 195 Hz.
-The torsional natural frequency (TNF) is predicated to be at 74.4 Hz and 192.6 Hz, as per Burckhardt

The Outcome:
Good: Measurements confirmed the predicted bending and torsional natural frequencies
Bad: Vibration is a complex thing
I collect here some general points
Lessons learned (technical)
-Pay more attention to design of motor bearing design (found resonance in bearing housing)
-Consider also higher order (3x, 5x) as possible relevant excitation of natural frequency of bending
-Consider the lowering effects on natural frequencies by lubrication
Lessons learned (contracting)
-Ask upfront or specify motor vendor vibration acceptance limits
-quantify and try to validate alarming messages coming form site as early as possible by qualified personnel
-consider limited understanding of vibration issues and possible misunderstandings of vibrations limits found in international standards like e.g. ISO 10816-1 on all involved parties
-stay focused also under pressure from management side
-try to reduce involved parties and personnel to the necessary technical experienced ones

 

[electricpete]

Thanks for providing feedback (many don't do that). I vote you a LPS for that.

fwiw, I'll repeat an opinion that resonance is not the whole story because you don't have one (or even two) peaks dominating the spectrum. You have many peaks. If you're blaming it on many different resonances..that seems quite unlucky. Although maybe it's semantics. My thinking is that if the motor support is relatively flimisy (low natural frequency relative to the range of exicting frequencies originating from the compressor) then there are many higher order resonances expected in the excitation range. But I would characterize it more as low stiffness for the application rather than unlucky resonances.

Another pattern I just noticed is that the odd harmonics are higher than the even harmonics throughout the spectrum. We obviously cannot attribute this pattern to resonance, it must be attributable to the excitation pattern which presumably has half-wave symmetry.

I'm not drawing any conclusions, but I would pose an open question:
What mechanism do you think the vibration originating from compressor is being seen at the motor?...
A - Dynamic variation of torque transmitted through the shaft (resulting in radial vibration due to expected motor asymmetric stiffness).
B - Vibration transmitted from compressor to common foundation to motor frame. (could be investigated by measurements on the foundation)
C - Radial motion of shaft creates radial reaction on motor frame.
something else?

I tend to view them in the order listed (A most likely, C least likely).

For A, a follow-on question might be: why doesn't the compressor show the same radial vibration. I would suggest the compressor is designed from the ground up for these types of forces and torque pattern, the motor is not.


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

 

[electricpete]

reconstructed overall orbit by combining 1X to 5X proximity probe readings

Filtering out 1x / 5x components and then analytically combining them seems a little bizarre to me.
Most software would allow you to view the unfiltered orbit directly.

It also raises my curiosity about the shaft spectrum. You mentioned 1x and 5x but 1x and 3x were highest in the housing spectrum. Is shaft spectrum significantly different than housing spectrum? I'd be interested to see shaft spectrum or other representations of prox probe output.

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

 

[RRUUEE]

Electricpete

I share hereby some figures that can illustrate the difference in stiffness or rigidity between Motor and compressor bearing
On the left side the NDE end which vibrates. string reduction by a tapered section, followed by a long distance to the bearing
not shown: details of bearing housing

we assume relatively low stiffness as a contributing factor for the Vibration issues



we identified the source for the high amplitudes for low frequencies as follows
7.5Hz 1x recip load
22.5Hz 3x order typical for 4crank recip and relatively near to bending natural frequency

Root cause for the high amplitudes for the higher frequencies are partially unknown

72 Hz first TNF
180-195 Hz second mode TNF
220 Hz Resonance at bearing housing
280 Hz maybe another resonance transferred by foundation

http://files.engineering.com/getfil...0933d9b50ab&file=1320_TA_stiffeness_model.png

http://files.engineering.com/getfil...c1bf056c6&file=1320_FEM_ansys_deformation.png

I can share also some impact tests (also running) and spectra that show some differences.

http://files.engineering.com/getfil...Impact_test_results_on_the_shaft_pedestal.png

Plane 1: Compressor DE
Plane 3: Motor NDE Inboard
Plane 4: Motor NDE Outboard

http://files.engineering.com/getfil...mpact_test_results_on_the_shaft__rotating.png

http://files.engineering.com/getfil...tion_on_pedestal_in_horizontal__peak_hold.png

http://files.engineering.com/getfil...on_vibr_of_compressor_and_bearing_housing.png

http://files.engineering.com/getfil...=1320_Figure_4.2.13_–_frame_to_foundation.png

Torsion:

http://files.engineering.com/getfil...=1320_Figure_4.2.14_–_Torsional_Vibration.png

http://files.engineering.com/getfil....15_–_Torsional_Vibration-_Peak_Hold_Plot.png

Stator:

http://files.engineering.com/getfil...dy_state_vib_NDE_right_side_of_the_stator.png