electricpete,

Nice job analyzing the problem from one ODS movie. I wasn't holding information back on purpose. I attached the movie in the earlier post to show the power of ODS testing. You proved my point by seeing the issues and asking the right questions.

Run-up and coast-down testing on the machine identified resonance within the operating speed range (667-893 rpm or 75%-100% of maximum speed). The natural frequencies were 776 and 810 cpm and are the orthogonal first bending modes of the pump/motor structure above the building floor. Resonant amplification resulted in vibration above .4 IPS when the pump was operated between 83-91% of full speed in the discharge direction and between 83-87% of full speed in the cross-discharge direction.

A bump test confirmed natural frequencies of 810 cpm (discharge direction) and 765 cpm in the cross discharge direction. The pump structure looked very rigid and these (low) natural frequencies were unexpected. I didn't think the first bending mode would be in the operating speed range. The bump test did not excite the torsional mode at 51.4 Hertz.

When the pump was operated at 775 rpm (86% of speed), electrical line frequency of the VFD (3088 cpm or 51.4 Hz.) excited the torsional mode at 51 Hertz and turning speed excited a bending mode. The 51 Hertz torsional movie was included in an earlier post. The attachment below shows the bending mode at 774 cpm with the motor hidden in the picture. The movie shows the bottom of the motor (green) and the motor base plate (red) moving in phase with each other but out of phase with the top of the pump tube (blue). The C-face mount was soft and lowered the first bending mode of the pump structure into the operating speed range. The soft joint was about .004" and extended about 12" around the circumference of the motor mounting flange. Shims were used to fill the soft joint and the increased stiffness pushed the first bending mode from 86% of speed to slightly above full speed. Vibration was significantly reduced.

I used a triaxial accelerometer for the ODS test. To measure circular motor and base plates, I mounted the triax tangentially around the components at eight locations.

Pump_ODS_775_cpm_(turning_speed)-no_motor.avi (562 Kb, 18 downloads)

Hello,
How do you get the graph to move in Excel? I thought you use a macro but it is not? It works very well for modal but how do you use it on ODS? I am looking for buying expert software for ODS and Modal and given the price of Me’ scope I think I’ll go with Excel and maybe MDS 5.0 from adash at 1990 euro it’s a bargain! What do you think?
Best regard,