Terminal Box Resonance : Examples of "Bump" Tests and Bode Plots

WHAT WAS THE PROBLEM?

Whilst testing a motor on a test bed, the client had problems in identifying the cause of the high vibration levels recorded.

Although the client possessed and used modern and sophisticated vibration monitoring equipment, they lacked the in-depth knowledge and experience to pinpoint the problem. Especially in this case as the problems were two, one masking the other.

WHAT DEI DID

• Confirmed that the clients fixed vibration monitoring equipment was set-up correctly and displayed the correct magnitudes.
• Used the clients fixed vibration monitoring equipment to record the run-ups and run-downs, to extract vibration magnitude/time trends, magnitude/speed trends and 1x component & phase bode plots.
• Used DEI equipment to collect vibration data from test bed, supporting blocks, motor casing and terminal box.
• Used DEI equipment to carry out "Bump" tests on the motor and terminal box, to identify a suspected natural response that occurred at the running speed frequency.

THE INITIAL BODE PLOT

• Note the sharp increase in amplitude as the running speed approaches 1200rpm (20Hz).
• Note the sharp phase change as the running speed approaches 1200rpm (20Hz).

TERMINAL BOX INITIAL RESPONSE

• Note the significant response at 21.25Hz. Running speed frequency is 20Hz.
• Note the Coherence value of 0.99692 at this response.
• Note that the interaction of the whole structure is likely to have a slightly lower response frequency than that of just the terminal box.
  

WHAT DEI DID THEN

• DEI observed that the terminal box end-plate and cable glands were not installed on the terminal box.
• Estimated the mass of the terminal box end-plate and cable glands and fixed this mass to the terminal box.
• Used DEI equipment to repeat the “Bump” tests on the motor and terminal box, to identify the new natural response frequency.
• Used the clients fixed vibration monitoring equipment to record another run-up and run-down, to extract vibration magnitude/time trends, magnitude/speed trends and 1x component & phase bode plots.
  

TERMINAL BOX FINAL RESPONSE

• Note that the significant response is now at 17.5Hz (running speed frequency is 20Hz).
• Note the Coherence value of 0.99111 at this response.
• Note that the interaction of the whole structure is likely to have a slightly lower response frequency than that of just the terminal box.
  

THE FINAL BODE PLOT

• Note the sharp increase in amplitude and sharp change in phase as the running speed runs through 825rpm (13.75Hz).
• Note the sharp increase in amplitude and phase change close to 1200rpm is no longer evident.
  

OUTCOME FOR THE CLIENT

Having adjusted the terminal box natural response frequency away from the running speed frequency:-

• The influence of the terminal box response now had no effect on the vibration characteristics of the motor at running speed (amplitude and phase).
• Allowed the root cause of the high vibration levels to be identified (electrical).
• Removed the concerns the client had about the condition and suitability of their new test bed.