Notice that the output from the VFD is a “rectangular” wave form. This rectangular waveform would not be a good choice for a general purpose distribution system, but is perfectly adequate for a motor.
If we want to reduce the motor frequency to 30 Hz, then we simply switch the inverter output transistors more slowly.
A capacitor operates in a similar fashion to a reservoir or accumulator in a plumbing system.
This capacitor absorbs the ac ripple and delivers a smooth dc voltage.
In other words, the faster the frequency, the faster the RPMs go.
While this would not be practical for plumbing systems, it works very well for VFD’s.