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VFD over frequency running

Stepping up motor speed with variable frequency drive (VFD) creates a problem of overheating of windings, overheating of stampings and magnetic saturation of motor stampings. If all these are taken care of, stepping up by a ratio of 2:1 may even be possible. Economics do not favor this arrangement. I do not understand why anyone would want to use variable frequency drive for high ratio seed variation when alternative methods are less expensive and more energy efficient.

There are circumstances when an over frequency VFD makes sense. Specifically, in some cases of centrifugal pumps and compressors, the RPM is determined by flow and pressure. In those cases, you may need, for example, 7000 RPM, and that number isn't negotiable. The alternative is to use a gearbox. Gearboxes are expensive and high maintenance. A well-engineered direct drive is cheaper and lower maintenance. Motors for over frequency VFD may have to be special. The variable frequency drive will be simpler than gear box assembly.

Just to throw something completely different into the mix submersible motors are often run on VFD for many reasons, level control, constant flow, the vfd pumps with internal cooling needed to maintain a minimum speed to allow for cooling and in waste water pumping there was also consideration to be given to clogging of the impellers a general rule was not to run below a certain frequency from memory around 25 or 30 hertz.

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