Sizing a VFD for a Screw Air Compressor

For a variable frequency drive (VFD), a screw compressor is a constant-torque load: halving speed halves the delivered air volume and roughly halves the shaft power, with no cubic bonus. The saving comes from two places instead. First, matching air production to actual demand rather than loading and unloading a fixed-speed machine. Second, eliminating the unloaded running cycle, where a fixed-speed compressor keeps turning, consuming roughly 25 to 40% of full-load power while producing no air at all.

Select the drive from motor full-load amps, not from kilowatts, and check the compressor maker's minimum speed. Most oil-injected screws have a floor around 25 to 40% of rated speed below which lubrication and oil separation degrade. Below that floor the correct behaviour is to stop the machine and let a small receiver buffer ride through the gap, not to run the motor slower still. Also confirm whether the compressor uses a star-delta or a soft starter internally, because the VFD replaces that logic and the control wiring changes.

The one problem unique to this application is mechanical resonance. An oil-injected screw has a natural frequency, often in the 800 to 1,400 rpm band, and running continuously at that speed destroys bearings well before their design life. Enable skip frequencies in the drive and set a band around the resonance, confirmed from the compressor maker or from a vibration measurement. For oil-free or high-pressure machines, verify the motor's insulation class and cooling method as well — a shaft-mounted fan loses effectiveness as speed falls, so a separately driven fan is often required.