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30 TRG-TRC014-EN
notes
period two
Components of a VAV System
An electronically-controlled terminal unit has an airflow-modulation device
operated by an electronic actuator that can drive the modulation device open or
closed. Electrically-actuated modulation devices typically do not have a spring
to return them to a “normal” state if power is lost – they stop at the position
they held when the power loss occurred. The actuator motor is operated with
three wires: “common,” “drive open,” and “drive closed.” To drive the device
open, the electronic controller applies 24 volts between the “common” and
“drive open” wire. To drive the device closed, it applies 24 volts between the
“common” and “drive closed” wire. To stop the actuator, no voltage is applied.
Actuator drive time is determined by the design of the electric motors and
gears, but is typically between one and six minutes.
An electronic pressure transducer enables pressure-independent control of the
VAV terminal unit. The pressure transducer is pneumatically connected to the
airflow sensor for airflow measurement and is wired to the electronic controller
along with the space temperature sensor. The electronic controller positions the
modulation device to deliver the required airflow to the space, and operates the
fan and heat source.
Minimum and maximum airflow settings are physically adjusted on the
electronic controller. A 24-volt power supply is required to power the unit
actuator and controls.
Electronic Control
▲ Electronic pressure transducer provides
pressure-independent control
▲ Electronic controller positions modulation
device, controls terminal fan and heat source
▲ Minimum and maximum airflow settings
adjusted physically on electronic controller
▲ Electric power supply operates modulation
device and electronic controller
Figure 45