Handbook of air conditioning and refrigeration / Shan K

Inlet Vanes Modulation
AIR SYSTEMS: COMPONENTS—FANS, COILS, FILTERS, AND HUMIDIFIERS 15.21
FIGURE 15.17 AC inverter and �pt-V˙ curves of an airfoil fan at various
speeds: (a) curves at various speeds; (b) ac inverter.
be adjusted so that it is always operated near its maximum efficiency. The electronics themselves
consume little energy. They are usually about 97 to 98 percent efficient. However, because of the ac
inverter, motor efficiency degrades 5 to 7 percent, so the total energy loss is about 7 to 8 percent.
The pulse-width-modulated (PWM) inverter is the predominate design of ac inverters used in
HVAC&R systems. A PWM inverter operates from a fixed dc bus. Varying the timing on thyristors
or transistors produces pulsed output. A logic circuit and software control the switching to provide
the required variable voltage and frequency for a specific variable speed in an ac induction motor.
The advanced power electronics, called insulated gate bipolar transistors, successfully provides
high switchings with low energy lossses and a compact and quieter drive.
PWM inverters operate at a wide range of speeds and can also provide a “soft start,” which
brings the motor to the rated speed gradually to reduce abnormally high starting current. They are
now often cost-effective for large fan motors in variable-air-volume systems and pump motors in
variable-flow water systems. (They often have a simple payback of less than 3 years for motors
with a capacity equal to and greater than 25 hp.)
Variable-speed drives using adjustable-frequency ac inverters have a good operating characteristic
when they are used for centrifugal fans and pumps with impeller of backward-curved blades.
Operation of forward-curved centrifugal fans may become unstable and more difficult to control because
of their saddle-type fan characteristic curves.
As with centrifugal compressors, inlet vanes in centrifugal fans are pivoted, movable vanes installed
at the inlet of the impeller. These vanes are linked mechanically so that they can turn
simultaneously around the axis, as shown in Fig. 15.18.