Trane TRG-TRC014-EN manual Electronic Control, Components of a VAV System, period two

period two

Components of a VAV System

notes

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

An electronically-controlledterminal 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.

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