Using a VFD to vary the speed causes components in pumps and motors to vibrate. At standard RPM these mechanical frequencies have been balanced, and there is no vibration. At a lower RPM the motor shaft will vibrate. At a different RPM the laminations in the motor will vibrate. Another RPM will cause the impeller to vibrate and so on. Every component in a pump or motor has a mechanical or resonance frequency. When a VFD varies the speed from 0 RPM to full RPM, pumps and motors will pass through the resonance frequency of every component. When the RPM reaches the resonance frequency for a particular component, that component will vibrate. Most VFD's have a way of locking out or bypassing a few particular speeds. However, it is impossible to lock out the resonance frequency or RPM for every component in the pump and motor.
When anyone of these components is vibrating, it is like driving a car with one unbalanced tire. Even though one tire is the only thing unbalanced, it shakes the whole car and eventually the car itself starts to break down. The same thing is true with pumps and motors. Vibration can be devastating. Vibration from anyone particular component can cause the destruction of the entire pump and motor assembly. Extra heavy-duty bearings or supports cannot help with this problem, as they may even be the part causing the resonance frequency vibration. The only cure for resonance vibration is to prevent the pump and motor from going through those particular frequencies.
Proof that mechanical vibration is caused by different RPM's, is evident when using vibration detection equipment on motors and pumps. It is basically impossible to test for vibrations in motor or pump bearings when not running at the rated constant speed or full RPM. Varying the RPM will cause detecting equipment to read vibrations, even when there is nothing wrong with the pump, motor, or bearings.
This should lead us to the conclusion that we should never vary the RPM of a pump or motor if there is another means of controlling the system. A Cycle Stop Valve or CSV will deliver constant pressure and reduce power consumption from a pump without varying the RPM. This completely eliminates problems with resonance frequency vibration. As an old joke says… "Doctor it hurts when I vary the RPM, the doctor would say, well then stop doing that and use a CSV!"