ABB DriveIT ACH550-01 User Manual page 296

Group 81: PFA
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 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 ACH550 switches pump no. 2 (and then pump no. 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 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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ACH550 User's Manual
Parameter listing and descriptions