Try to turn the compressor by hand. It should be possible to turn it around. It should turn around smoothly, without any 'hickups' (be sure to completely shut down the air compressor!). Check the isolation of the motor windings.
[pdf] The Variable Frequency Rotary Screw Air Compressor is an energy efficient solution for industrial air compression requirements. The motor output frequency is intelligently matched to air consumption, making it more energy efficient and longer lasting than traditional air compressors.
[pdf] Environmental protection and durability; Low vibration and low noise; High efficiency, heavy load, silent. Compact design does not need to occupy a big space, working quiet. Widely used in automobile repair and services industry.
[pdf] 
