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Forced Vibration

Vibration on machinery can be evaluated using general vibration response equations near resonance using general details either identified by testing are known about the system. Changes in vibration amplitude resulting from speed changes can be evaluated using these relationships. This can be a very useful prediction and diagnostic tool. One particularly problem includes the use of variable frequency drives (VFDs), that can create lots of opportunities for using basic single degree of freedom (SDOF) math to evaluate the impact of speed changes.

As an example, a vertical motor was mounted on a rigid foundation with a rather flexible support pedestal with minimal mass of the support. The weight of the motor is 960 lb, which can be considered to be the moving mass (modal mass). The motor operates at a normal speed of 1195 RPM ...across the line... (without the VFD). A VFD is proposed that will allow operation up to 80 Hz on the drive (60 Hz is line frequency). Evaluation of the vibration response through the entire speed range is desired.

Using the impact test data above, what is the expected 1xRPM vibration amplitude of the motor at 80 Hz VFD operating frequency provided that the motor has the following features:

A. 0.063 in/sec pk

B. 0.00038 mils pk-pk

C. 0.00085 in/sec/lb

D. 2.6 mils pk-pk

The correct answer is A.


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Sample Question Answers