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48 TRG-TRC013-EN
notes
period three
Fan Capacity Control
Fan-Speed Control
The third method of capacity control, fan-speed control, modulates fan
capacity by varying the speed of the wheel rotation. This is commonly
accomplished using a variable-speed device on the fan motor, such as a
variable-frequency drive, a belt-speed changer, a variable-speed mechanical
drive, or an eddy current clutch.
The response of fan-speed control to system static-pressure variations is
similar to that described for inlet vanes.
Again, as the system resistance curve shifts upward and the fan begins to “ride
up” the constant-speed performance curve, toward B, from its design operating
point A, it delivers a lower airflow at a higher static pressure. The system static-
pressure controller senses this higher static pressure and sends a signal to the
fan-speed controller, instructing it to slow down the fan motor. This causes the
performance curve for the fan to shift downward until the balance point C falls
Fan-Speed Control
variable-speed
variable-speed
drive
drive
Figure 71
Fan-Speed Control
$
&
airflow
airflow
static pressure
static pressure
fan speed curves
fan speed curves
%
VAV system
VAV system
modulation
modulation
curve
curve
sensor
sensor
set point
set point
design system
design system
resistance curve
resistance curve
Figure 72