McQuay MicroTech II User Manual

OM 780-2 Page 79
To set up multiple variable-speed pump logic
1. Set the Pump Control Option variable to “Multiple VFD Pumps.”
2. Set the Number of VFD Controlled Pumps variable to the number of pumps to be controlled.
3. Commission Remote I/O Module A on the Device Addressing screen. If more than two pumps are to be controlled
commission Remote I/O Modules B, if more than four pumps are to be controlled commission Remote I/O Module C.
4. Set the following variables as required for each pump stage up to the Number of VFD Controlled Pumps variable:
• Stage-Up %
• Stage-Up Delay Time
• Stage-Down %
• Stage-Down Delay Time
5. Set the Chilled Water Loop Modulation Control Option variable to “Pump VFD”(see “Pump Speed Control” below).
6. Set the Pump Status Check Delay Time as required.
7. Set the Loop Differential Pressure Setpoint as required.
8. Set the following variables as required to tune the Pump Speed Control PI Loop:
• Loop Differential Pressure Deadband
• Loop Differential Pressure Propband
• Loop Differential Pressure Sample Time
• Loop Differential Pressure Integral Time
Note: To use multiple VFD pump logic, a differential pressure transducer must be installed and connected to the CSM. In
addition, a pump status device with dry contacts should be connected to the associated Remote I/O Module for each pump.
The VFD fault output should also be wired in series with the pump status device. As described above, this alarm control
can fail-safe the system by shutting it down if all pumps fail. For more information, see the Field Wiring section of IM 781.
Pump Speed Control
The CSM can control pump speed to maintain the differential pressure at the Loop Differential Pressure Setpoint. A VFD
must control the pumps. The CSM uses a proportional-integral (PI) control loop to generate an analog pump speed signal,
which it sends to the VFDs via 0-10 Vdc analog outputs (AO 1 and AO 2 on Remote A, B, and C). The Cooling Load
Pump VFD Speed variable (Flow screen) shows the current value of this signal. The same signal is always sent to both
analog outputs.
PI Control Process
When the pressure is above the Loop Differential Pressure Setpoint, the control loop reduces pump speed. When the
pressure is below the Loop Differential Pressure Setpoint, the control loop increases pump speed. The speed can modulate
between 0% and 100%.
The PI control loop has four adjustable variables that are used for pump speed control: (1) Loop Differential Pressure
Deadband, (2) Loop Differential Pressure Propband, (3) Loop Differential Pressure Sample Time, (4) Loop Differential
Pressure Integral Time. The Loop Differential Pressure PI Function time plot is provided on the Load Flow Control screen
to assist in tuning the PI loop.
To set up pump speed control
1. Set the Chilled Water Loop Modulation Control Option variable to “Pump VFD.”
2. Commission Remote I/O Module A, B, and/or C on the Device Addressing screen. Set the Pump VFD AO Zero
variables for all analog outputs used on the I/O Config screen to match the analog input of the pump’s VFD.
3. Set the following variables as required:
• Loop Differential Pressure Setpoint
• Loop Differential Pressure Deadband
• Loop Differential Pressure Propband
• Loop Differential Sample Time
• Loop Differential Integral Time

McQuay MicroTech II Specifications

General

Brand	McQuay
Model	MicroTech II
Category	Controller
Language	English