Balancing in a Critical
Working at a water treatment plant in Norfolk VA, there was a Variable Frequency Drive motor that had a vibration problem. The engineer for the Water Treatment Plant demonstrated the condition on the vertical VFD Motor. The pump worked best at a certain speed. That speed happened to be the critical speed of the motor. The motor vibrated horribly at that speed. One of the options is to balance in the critical speed. The offset for this is operating outside of the speed will induce vibration due to unbalance. If the motor is going to never operate outside of the critical then this is a viable option. I was recommended for this job by someone from the Vibration Institute. The motor was tested in place with vibration sensors and an rpm sensor installed. The critical speed was verified and the motor was balanced at that speed. Again the trade off is operating outside of that window the pump would vibrate. In this instance the motor ran smooth in its critical.
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