Critical Speed

I'm guessing this is a multiple choice question off some examination so it probably is best to share the possible answers.

This isn't the best data for determination of critical speeds.  Since the plot lacks header information, we don't know much about the data, even the transducer type or mounting angle.  We also know very little information about the machine.  We see three peaks, and well into perhaps a 4th.  Except for some large nuclear 50 cycle generators, it might be a bit unusual to see passage through three criticals.  

Although we have well defined peaks in amplitude, we don't have the anticipated 180° phase shift that is expected in passage through a critical.  The 2nd peak, about 1200 cpm goes through about 165° phase shift so that might be a candidate.

Personally, if I was actually doing diagnostics on this machine, I would be carefully examining the polar plots, preferably of two orthogonal transducers, before making an assessment of how many criticals might be present.

 

I agree with John. To answer your question with confidence,  I would look for information, other locations and plot types such as polar.

 

Furthermore, looking at the vibration alone could be misleading if you want to know the system characteristics without knowing the forces. Many interpretations of Bode plot are based on the assumption that the exciting force is at 1X which is not always the case.

Regards- Ali M. Al-Shurafa 

The OP states "Critical Speed" which is sometimes referred to rotor critical speed or balance resonance when the rotor has distinctive deflection. I have only seen one rotor balance resonance below operating speed for large 2-pole motors. Therefore, I would expect that one or more (at least 2) of the 3 peaks to be natural frequencies of the structure and not the motor rotor. Structural vibration test by impulse-response or operating deflection shape (ODS) could confirm this hypothesis. The driven machine rotor should also be considered.

Walt

Walt Strong posted:

The OP states "Critical Speed" which is sometimes referred to rotor critical speed or balance resonance when the rotor has distinctive deflection. I have only seen one rotor balance resonance below operating speed for large 2-pole motors.

Walt, point well taken on motors but the OP does not refer to a motor, he refers simply to a rotor.  I have frequently worked on large nuclear TG sets, both 60 Hz and 50 Hz and 1000 Mw, where the generator, which is physically massive may pass through multiple flexural modes.

ALI HAIDERR posted:

Thanks all

It is Generator rotor directly coupled with frame 9 Gas turbine.

 

As best I recall, the frame 9 generator probes are 45° left and 45° right.  It is entirely possible you have a split critical.  You might try virtual probe rotation as a tool to help identify the dynamic behavior.

John from PA posted:
ALI HAIDERR posted:

Thanks all

It is Generator rotor directly coupled with frame 9 Gas turbine.

 

As best I recall, the frame 9 generator probes are 45° left and 45° right.  It is entirely possible you have a split critical.  You might try virtual probe rotation as a tool to help identify the dynamic behavior.

Dear john

Can you please explain more about virtual probe and split critical.may i face this state at frame 5 and 6 

Thanks

aziz58 posted:
Can you please explain more about virtual probe and split critical.may i face this state at frame 5 and 6 

 

Quoting from the book Rotordynamics by Agnes Muszynska

"Virtual Probe Rotation - A mathematical transform of vector data from an existing XY pair of vibration transducers to produce vectors that would be measured by XY probes mounted at some other, arbitrary, orientation."

The technique requires special software to be accomplished.  GE/BN diagnostic software has the ability to show data from the virtual probes but I do not know what else in the marketplace may have the capability.

As far as a split critical, see https://books.google.com/books...tical%22&f=false

 

A couple of points (leave out the o and that would be a couple of pints), there is more than one type of frame 9; this will make a difference.

Rigidly coupled machinery trains often have noticeable train modes, not simple rotor modes.  Gas turbines often have the casing and support system involved in the modes.  Sometimes it helps to use both shaft relative and casing measurements if you want to better understand the dynamics.

Generators may have pedestal resonances.  It can help to look x, y, and z on the bearing pedestal. 

Dear Ali,

With regards to your given plot and in the absence of other plot like polar, orientation and etc of probe, i assume that the critical speed is around 2000 rpm, the hump around 700 & 1300 rpm is like more on damping due to horizontal stiffness.

but then, you can contact your OEM or in manual for the critical speed, even in nameplate this will be seen.

regards,

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