The invention of frequency converters has created tons of opportunities to adapt the capacity of industrial machines to the demand.  If a machines is at 50% of its demand, often the partial load consumption is almost as high to 80% of its full installed power.

However, variable speed drives on air-compressors also have their drawback

First of all, power electronics also consume power. It is an illusion that the electronics do the work for free.  The variable speed drives consume on themselves easily 3-5% of the total installed power.

Secondly, due to the PWM, pulse width modulation of the voltage, the current provided by the converter, has a lot of harmonics.  Most power meters, only measure the base current and not its harmonics.   

Third, the lifetime of the power electronics is limited.  Mostly after 5 yrs, the frequency converter has a faillure.  If you are lucky that the spare parts are around or still exist, and a qualified technician can help you, you will be obliged to install a new converter, which does not necessarily match your electrical motor anymore.

For Air-compressors, the benefits are exceeding the draw-backs, if the compressor turn-down is very in-efficient, that a frequency converter should become mandatory.

Better should be to select an industrial compressor, with very low un-load losses, which can switch fast between load and unload. In that case, no expensive frequency converter needed, no losses in the frequency converter and harmonic losses in the electrical motor, and no early break-down of the electronics which is expensive to repair.

Industrial reciprocating compressors do not use frequency convertors.  Their unload losses are very low, they switch very fast between load and unload and have a very high energy efficiency at load and unload.

Could a frequency converter ever have a similar lifetime as an industrial  Oil-free piston air-compressor of 40years ?