iWorx® VPU2
505-010-2, Effective: April 29, 2014
31
© 2014 Taco Electronic Solutions, Inc.
To prevent the integral component from becoming too large, there is anti-wind up reset protection. This protection
clamps the integral value when all of the components add up to more than 100% or less than 0%. The following equa-
tions are used for P+I control:
K
p
=
Proportional Gain
K
i
=
Integral Gain
A separate static pressure setpoint is provided to increase the supply static pressure when an IAQ alarm condition
exists. The fan speed control maintains the IAQ alarm pressure setpoint as long as an IAQ alarm condition exists.
Programmable minimum and maximum outputs are provided for the fan speed. These settings can be reversed for
reverse air movement. Overrides are provided to assist in system air balancing during commissioning.
Soft Start Ramping
The VPU2 can be configured to soft start ramp to prevent damage to the ducts and equipment when the fan is first
energized. Setting the soft start ramp rate to 0% per second or 100% per second disables soft start ramping. The soft
start ramping clamps the P+I output to the current ramp rate. Soft start ramping ends when the static pressure is within
the deadband, greater than the pressure setpoint, or when the soft start ramp reaches 100%. Once soft start ramping
ends, the P+I loop’s integral sum is backwards calculated to create a smooth transition to the static pressure P+I loop
control.
Fan Proof
A fan status input is provided for monitoring the operation of the fan. When the fan is initially turned on, there is a 30
second delay before the fan status is checked. If at any time after the delay, the fan status indicates the fan is not run-
ning, a fan failure condition is generated. The heating and cooling stages are interlocked with the fan. When a fan fail-
ure condition exists, the heating stages, cooling stages and the fan immediately turn off. The controller must be reset to
clear this condition.
NOTE:
If not providing a fan proof switch, the dipswitch for the fan proof input must be configured with the
(0-10V) switch set to “on” and “Vth” set to “off.” After a fan failure, the controller's status LED changes from
green to solid red. To return the controller to normal operation after the failure condition is resolved, you
must reset the controller by removing and reapplying power or by using the controller reset feature on the
LCI.
VPU2 and VAVI Communications
The VPU2 polls an associated VAVI controller every 5 seconds to transfer information necessary for control.
The following information is transferred from the VPU2 to the VAVI controller:
• Operational Mode: primary cool, primary heat, and primary off
• Occupancy Mode: occupied, unoccupied, and bypass (denotes morning warm-up)
• Alarm Conditions: IAQ Mode and Heat Failed On
The following information is transferred from the VAVI to the VPU2 controller:
• Zone temperature
• Calculated Heating Setpoint
• Calculated Cooling Setpoint
• IAQ Sensor Status (safe, alarm)
Error
SupplyStaticPressure LoopSetpoint
–
=
I
I
K
i
Error
+
=
BypassPosition
K
p
Error
I
+
50.00%
+
=