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12 TRG-TRC010-EN
notes
period one
Components
Motor
A motor is used to rotate the impeller(s). A direct-drive motor is connected
directly to the impeller shaft and the impeller rotates at the same speed as the
motor. A gear-drive motor transfers its energy to the impeller shaft using a set
of gears. This allows the impeller to rotate at a higher speed than the motor.
The direct-drive motor requires fewer bearings and does not incur gear losses.
Additionally, since the compressor rotates at a lower speed, it can be much
quieter.
Direct-drive compressors are, however, only practical in centrifugal chillers that
use low-pressure refrigerants.
Another important difference in compressor motors is the issue of hermetic
versus open. A hermetic motor is totally enclosed within the chiller’s
refrigeration system. An open motor is mounted externally—outside of the
Motor
impellers
impellers
impeller
impeller
motor
motor
motor
motor
gear-drive
direct-drive
gears
gears
Figure 18
Hermetic Motor Cooling
stator
stator
rotor
rotor
drain
drain
liquid
liquid
refrigerant
refrigerant
Figure 19
trgtrc010_book.book Page 12 Wednesday, September 1, 1999 10:39 AM