Pump Logic: Multiple Variable-Speed Pumps - McQuay MicroTech II Operation Manual

If a pump fails, the CSM does not de-energize that pump's output when the timer expires—the output will be energized
whenever it is part of the current stage. So if the problem goes away, the pump restarts immediately and the Cooling Load
Pump #X Fail alarm will automatically be reset.
If the current pump stage is the highest stage and all pumps have failed, the No Chilled Water Flow alarm occurs and the
system shuts down. As a result, all pump outputs are de-energized. The Number of Sequenced Pump Stages variable
specifies the highest stage.
To set up sequencing constant-speed pump logic
1. Set the Pump Control Option variable to "Sequencing."
2. Set the Number of Sequenced Pump Stages variable to the number of stages in the stage table.
In a typical system, this number equals the number of pumps.
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.
Set up the stage table by setting the Stage X, Pump Y variables (1 through x , where x is the number of stages specified
4.
in step 2).
5. Set the Chilled Water Loop Modulation Control Option variable to one of the following:
• "None," if there is no loop bypass valve
• "Chilled Water Loop Bypass Valve," if there is a loop bypass valve (see "Loop Bypass Valve Control" below)
6. Set the following variables as required:
• Pump Status Check Delay Time
• Loop Differential Pressure Setpoint
• Pump Stage-Up Delay Time
• Pump Stage-Down Delay Time
7. If you're not using a bypass valve, set the Pump Stage Differential as required.
8. If you are using a bypass valve, set the following variables as required:
• Minimum Loop Bypass Valve Position
• Maximum Loop Bypass Valve Position
Note: To use sequencing 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 CSM for each pump. If pump status is not
available, jumpers can be installed, but this defeats the CSM's pump-failure alarm control. As described above, this alarm
control can fail-safe the system by shutting it down if all pumps fail. Using jumpers for pump status is not recommended.
For more information, see the Field Wiring section of IM 781.

Pump Logic: Multiple Variable-Speed Pumps

Configuration 6 uses the CSM's multiple variable-speed pump logic to control VFDs on pumps that have identical
capacities and performance. This control can be applied to a group of two to six pumps, which are identified as Cooling
Load Pump #1 through Cooling Load Pump #6 on the Flow screen. It operates one or more pumps as required to maintain
the differential pressure across the loop at the Loop Differential Pressure Setpoint. This logic is different than
Lead/Standby control of pumps with VFDs because it will run more than one pump at a time.
Sequencing and Staging
The sequencing order of multiple VFD pumps is based on pump runtime. The pump with the lowest runtime will be the 1
pump enabled, the pump with the next lowest runtime will be the 2
calculated by the CSM to be the amount of time the CSM has enabled the pump. Each pump's runtime can be reset by the
operator at the Load Flow Control screen for any number of reasons including:
• Reset to match the known runtime of an existing pump
• Reset to an artificially large number to force a problem pump to be the last to be enabled
A pump stage is defined as the number of running pumps. When the CSM turns the next pump on, it "stages up". When the
CSM turns a pump off, it "stages down". The Current Pump Stage is displayed on the Flow screen. The current Cooling
Load Pump Speed (also on the Flow screen) and interstage timers determine when a pump is staged up or staged down.
OM 780-2
nd
pump enabled, etc. Pump runtime is defined and
st
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