period two
Compressor Capacity Control
9DULDEOH6SHHG
YDULDEOHVSHHG
GULYH
Figure 41
Variable Speed
Alternatively, the capacity of a compressor can be controlled by varying the rotational speed of the compressor motor. This is accomplished using a device called an adjustable-frequency drive (AFD) or variable-speed drive.
On a reciprocating compressor, this would vary the speed at which the crankshaft rotates, thus controlling the rate at which the piston travels back and forth inside the cylinder. On a scroll compressor, this would vary the speed at which the driven scroll rotates. If applied to a helical-rotary compressor, this would vary the speed at which the rotors rotate. Applied to a centrifugal compressor, this would vary the speed at which the impeller rotates.
Although variable-speed capacity control could be applied to all four types of compressors discussed in this clinic, it is most often applied to centrifugal compressors. Because speed variation reduces both the flow rate of refrigerant through the compressor and the pressure differential created by the compressor, it is used in conjunction with inlet vanes. This requires fairly complex control strategies to balance refrigerant flow rate, pressure differential, and load.
