The operating principle for a vane compressor is the same as for many compressed air expansion motors. The vanes are usually manufactured of special cast alloys and most vane compressors are oil-lubricated. A rotor with radial, movable blade-shaped vanes is eccentrically mounted in a stator housing. When it rotates, the vanes are pressed against the stator walls by centrifugal force. Air is drawn in when the distance between the rotor and stator increases. The air is captured in the different compressor pockets, which decrease in volume with rotation. The air is discharged when the vanes pass the outlet port.