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Operating and Maintenance Instructions
5.5 Constant Volume/Constant Pressure operating mode
5.5.1 General
The MDC can be used with the DS85 differential pressure sensor to maintain a set point of constant volume or
constant pressure of the system in which the fans are installed. The options for controlling the fan array are to
maintain a constant volume or constant pressure varying the fan speed based on the voltage output from the DS85
sensor and the constant volume or constant pressure set point. The response of the system is adjusted by changing
the Proportional, Integral and Derivative (PID) control to determine how fast or slow the fan speed output signal
can change.
5.5.2 Constant volume/constant pressure mode
Figure 17 - Typical wiring diagram for Constant volume / Constant pressure operating mode
When in Constant Volume / Constant Pressure operating mode, the Power Meter screen will include a real-time
display of the volume / pressure being maintained by the fan array. From the Power Meter screen, is possible to
adjust the constant volume/pressure setpoint by pressing ENTER button twice whilst holding UP and DOWN
buttons, see H6 – Status. The setpoints can also be adjusted via the Main Menu or via a Modbus master by
changing the value of holding registers 21 (pressure) or 22 (volume). See Appendix E – Modbus Holding
Registers for more details.
It is also possible to enable a sensor alarm in the main menu, see H17 – Advanced Settings – Alarm Output
(Sensor Enabled).
If the sensor alarm is enabled, the MDC will trigger an alarm when the fans reach 100% speed and the volume /
pressure is below the constant volume / pressure setpoint. There is a time delay from when the alarm condition is
sensed to when the MDC opens the alarm relay and turns on the red LED.
Enabling the sensor alarm will also cause the MDC to open the alarm relay and turn on the red LED in case the
sensor is accidentally disconnected.
The PID control settings for the Constant Volume / Constant Pressure algorithm can be adjusted using either 'Main
Menu>Advanced Settings>PID Settings' or via Modbus - Please see Appendix E – Modbus Holding Registers
for more details.
210-OMI14229-Iss2
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Operation & Maintenance Instructions
Chelmsford Business Park Chelmsford Essex CM2 5EZ
Telephone: +44(0)1245468555 Fax: +44(0)1245466336
e-mail
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