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TRG-TRC004-EN 29
period two
Compressor Capacity Control
notes
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.
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Figure 41