Actual signals and parameters 261
Group 81: PFA control
This group defines a Pump and Fan Alternation (PFA) mode of operation. The major
features of PFA are:
• The ACS320 controls the motor of pump no. 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 no. 2 and pump no. 3, etc. The
ACS320 switches pump no. 2 (and then pump no. 3, etc.) on and off as needed.
These motors are auxiliary motors. Use of the MREL-01 relay output module is
required for control of pump no. 2 and no. 3.
• The ACS320 PID control uses two signals: a process reference and an actual
value feedback. The PID controller adjusts the speed (frequency) of the first pump
such 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 such 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, such 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)
equalizes 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.
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