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difference between proportional and PI control is that pro-
portional control is limited to a single final control element
position for each value of the controlled variable. PI control
changes the final control element position to accommodate
load changes while keeping the control point at or very near
the setpoint.
The reset action of the integral component shifts the
proportional band, as necessary, around the setpoint as the
load on the system changes. The shifting of the propor-
tional band keeps the control point at setpoint by making
further corrections in the control signal. Because offset is
eliminated, the proportional band is usually set fairly wide
to ensure system stability under all operating conditions.
Integral windup, or an excessive overshoot condition,
can occur in PI control. Integral windup is caused by the
integral function making a continued correction while wait-
ing for feedback on the effects of its correction. While
integral action keeps the control point at setpoint during
steady state conditions, large overshoots are possible at
start-up or during system upsets (e.g., setpoint changes or
large load changes). On many systems, short reset times
also cause overshoot.
PROPORTIONAL-INTEGRAL-DERIVATIVE (PID)
CONTROL
Proportional-integral-derivative (PID) control adds the
derivative function to PI control. The derivative function
opposes any change and is proportional to the rate of
change. The more quickly the control point changes, the
more corrective action the PID system provides.
If the control point moves away from the setpoint, the
derivative function outputs a corrective action to bring the
control point back more quickly than through integral
action alone. If the control point moves toward the setpoint,
the derivative function reduces the corrective action to slow
down the approach to setpoint, which reduces the possibil-
ity of overshoot.
VARIABLE AIR VOLUME FAN CONTROL
OPTIONS
The W7600 can be configured to control variable air
volume fans in three basic ways:
• Supply Fan only (no return fan) using Supply Static
Pressure Control.
• Two-Speed Supply Fan only (no return fan) using
supply and return duct velocity pressure control.
• Supply and Return Fans using Supply Static Pressure
to control the supply fan, and using supply and return
air velocities to control the return fan.
These three methods are described inthe following sec-
tions.
SUPPLY AIR FAN CONTROL-STATIC PRESSURE
The W7600 system evaluates supply duct pressure con-
ditions to determine the need for control action. Control is
based on demand from an analog static pressure sensor, and
a setpoint and control band setting measured in in. wc. On
indication of an increase or drop in pressure from the
sensor, the control references setpoint and control band to
determine necessary action. The static pressure sensor should
be located downstream of the supply fan for a representa-
tive duct static pressure reading.
NOTE: A guideline is to mount the sensor two-
thirds of the way down the supply duct. The
sensor should also be mounted in an area with
no air turbulence.
Under conditions that require action, the system will
maintain accurate static pressure by adjusting supply fan
capacity. Fan control options include floating or modulat-
ing motors. Floating motors are controlled by two relay
open and close signals to move a modulating motor con-
nected to the inlet vanes of the supply fan. The system
allows the user to program the time for motor speed to open
and close. Under conditions that require an adjustment to
duct pressure, the system uses the motor speed to calculate
the necessary open or close signal to send to the motor.
Modulating pressure control provides a true analog output
(0-10 Vdc or 0-20 mA) to directly control vane position or
fan speed. The system also includes static pressure safeties
and limits including high pressure alarm monitor, airflow
safety, fan start-up delay and system safety.
TWO-SPEED FAN-VELOCITY PRESSURE CON-
TROL
The W7600 can be configured to control supply and
return duct velocity pressure using a two-speed fan. This
option is selected by setting the FAN_TYPE Configuration
parameter to 2. Note that if this option is selectred, the
supply duct static pressure cannot be controlled. The two-
speed fan velocity pressure method of control uses low and
high fan speeds for air distribution. Control is based on the
demand from an analog velocity pressure sensor, the se-
lected low to high speed fan switch setpoint and the high to
low speed fan hysteresis configured.
When many of the VAV boxes in the system are open,
the air velocity increases as the duct static pressure de-
creases. The low speed fan is de-energized and the high
speed fan is energized when the duct velocity pressure
increases above the low to high speed fan setpoint. The
high speed fan will remain on for at least the high speed fan
minimum on time selected. After the minimum time has
elapsed, the high speed fan remains operating until the
velocity pressure drops below the low to high fan speed
switch setpoint minus the high to low speed fan hysteresis.
When the fan speed switches from high to low, the low
speed fan will not start until after the high to low speed fan
time delay has elapsed. This parameter can be selected
within the range from 0 to 30 seconds.
SUPPLY AND RETURN FANS-VELOCITY PRES-
SURE CONTROL
The supply fan is controlled to maintain supply duct
static pressure at a setpoint (same algorithm as for SUPPLY
AIR FAN CONTROL-STATIC PRESSURE.)
The Return Fan Air Flow setpoint is determined as a
percentage of Supply Fan Air Flow. For example, if supply
air is running at 4000 CFM, and the RET_CFM_OFFSET
setpoint parameter is 80%, the Return Air Flow Setpoint is
33
PM7600B
OPERATION
63-4309—2
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