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ACH550-01 User's Manual
Group 81: PFA CONTROL
This group defines a Pump and Fan Alternation (PFA) mode of
operation. The major features of PFA are:
•
The ACH550 controls the motor of pump 1, varying the motor
speed to control the pump capacity. This motor is the speed
regulated motor.
•
Direct line connections power the motor of pump 2 and
pump 3, etc. The ACH550 switches pump 2 (and then
pump 3, etc.) on and off as needed. These motors are
auxiliary motors.
•
The ACH550 PID control uses two signals: a process
reference and an actual value feedback. The PID controller
adjusts the speed (frequency) of the first pump so that the
actual value follows the process reference.
•
When demand (defined by the process reference) exceeds
the first motor's capacity (user defined as a frequency limit),
the PFA automatically starts an auxiliary pump. The PFA also
reduces the speed of the first pump to account for the
auxiliary pump's addition to total output. Then, as before, the
PID controller adjusts the speed (frequency) of the first pump
so that the actual value follows the process reference. If
demand continues to increase, PFA adds additional auxiliary
pumps, using the same process.
•
When demand drops, so that the first pump speed falls below
a minimum limit (user defined by a frequency limit), the PFA
automatically stops an auxiliary pump. The PFA also
increases the speed of the first pump to account for the
auxiliary pump's missing output.
•
An Interlock function (when enabled) identifies off-line (out of
service) motors, and the PFA skips to the next available
motor in the sequence.
•
An Autochange function (when enabled and with the
appropriate switchgear) equalises duty time between the
pump motors. Autochange periodically increments the
position of each motor in the rotation – the speed-regulated
motor becomes the last auxiliary motor, the first auxiliary
motor becomes the speed regulated motor, etc.
Parameter listing and descriptions
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