Daikin MicroTech III Operation Manuals page 138

Modulating Return and Exhaust Fan Control (RTU,
MPS)
Four different approaches may be used to maintain the building
static pressure at acceptable levels . An analog signal is
provided to the VFD to control return or exhaust fan to:
• Maintain the speed of a return fan based on supply fan
speed (Fan Tracking)
• Maintain the building static pressure at a desired value
• Maintain a fixed speed based on a signal provided by a
Building Automation System via a network .
• Vary the speed of an exhaust fan based on OA Damper
position .
Any of the first three methods may be selected for a return
fan . Building Static Pressure, Speed, or OA Damper may be
selected for an exhaust fan .
When the unit is in the OFF or Startup state, the variable
frequency drive for the return or exhaust is driven to 0% .
After the return or exhaust fan is started, a speed signal of 5%
for exhaust fan or 33% for return fan is sent to the variable
frequency drive for the BSPCtrlDelay (Default=30 seconds)
and the VFD will increase its speed to 5% (or 33%) . Control
reverts to one of the methods below after the fan has been on
for the BSPCtrlDelay . The VFD speed is not controlled below
5% (or 33%) while the fan is operating .
During non-OFF states when the outdoor air damper is closed,
the speed of the return fan is set equal to the supply fan speed .
Under these conditions, the return fan is set to 100% if the
supply fan is a constant volume fan . To provide a "bumpless
transfer" when the outdoor air damper opens the return fan will
start to control building static pressure from its speed when the
damper was closed . Exhaust fans do not use this feature .
NOTE: Units supplied with Daikin MD2, MD3, and MD6
drives will have a user editable maximum return fan/
exhaust fan hertz setpoint (default 60 Hz) located
in the RF/EF Set Up menu . This parameter can be
changed when job site conditions require the speed
of the drive to be above 60 Hz .
Fan Tracking Control
For units with variable frequency drives, the return fan may
be controlled such that the return fan speed tracks up and
down with the supply fan speed. The user defines the position
of the return fan with respect to the supply fan for minimum
and maximum positions . When the supply fan is between the
minimum and maximum positions, the fan varies proportionally
between its minimum and maximum values . There are four
adjustable parameters to define this relationship. They are:
Supply Fan (DF) Value Return Fan (RF) Value
1 . Supply Fan Max
2 . Return Fan @ Supply Fan Max
3 . Supply Fan Min
4 . Return Fan @ Supply Fan Min
OM 920-6 • MICROTECH UNIT CONTROLLER
To set the four parameters as described, the airflow and return
vane positions are adjusted until maximum airflow and proper
indoor conditions are met . The supply fan is controlled normally
as the VAV boxes are adjusted to obtain maximum airflow. The
return fan is adjusted using the Remote RF/EF setting on the
keypad . The user then uses the key to set the Supply Fan Max
values and Return Fan @ Supply Fan Max to the observed
values. The process is repeated at minimum airflow to set
Supply Fan Min and Return Fan @ Supply Fan Min values .
Low/High Fan Differential
When a return fan is running slower than the supply fan it is
limited to a Lo Fan Diff value below the supply fan to prevent
the supply fan from overloading the return VFD or ECM motor
fan drive . When the return fan is running faster than the supply
fan it is limited to a Hi Fan Diff value above the supply fan to
prevent the return fan over pressurizing the return fan plenum .
Building Static Pressure Control
A PI control loop is used to modulate the variable frequency
drive of the exhaust fans to maintain a measured building static
pressure at a setpoint. The BSP Gain, Project Ahead Time, and
Sample Time are editable via the keypad
The Building Static Pressure setpoint may be set through
the keypad or via a network signal . The active setpoint is
changed whenever either of these values changes so it equals
whichever value was changed most recently .
Speed/Network Control
Single zone VAV control is available when the unit Control Type
is Single Zone VAV (1ZnVAV) . When single zone VAV control
is selected as the SAF capacity control method, the supply fan
VFD or ECM motor is controlled with a PI_Loop to maintain the
Control Temperature input at the Occupied Cooling Setpoint
or Occupied Heating Setpoint . This option is designed for
discharge control applications where the unit will act as a
single VAV box to control space temperature . OAT is not
allowed as the Control Temperature when SAF capacity control
method is set to 1ZnVAV . Cooling and heating discharge air
temperature control and outside air damper control will function
in the normal manner as with VAV units .
NOTE: Single zone VAV control of SAF is not available when
the Control Temperature Source is set to None .
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