Program features 155
PFC and SPFC control
PFC control
Pump and fan control (PFC) switches auxiliary pumps on and off as required by
capacity changes. Autochange function alternates between pumps to keep the duty
times of the pumps equal. Interlocks function enables the drive to detect if any of the
pumps are unavailable (eg switched off for maintenance), in which case the next
available pump is started instead.
The drive 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 motors of pump 2 and pump 3, etc. The drive
switches pump 2 (and then pump 3, etc.) on and off as needed. These motors are
auxiliary motors.
The drive 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 PFC control automatically starts an auxiliary
pump. The PFC 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, PFC adds additional auxiliary pumps, using the
same process.
When demand drops, such that the first pump speed falls below a minimum limit
(user defined as a frequency limit), the PFC control automatically stops an auxiliary
pump. The PFC 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
PFC control 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.
See also section PFC control macro on page 110.
SPFC control
Soft pump and fan control (SPFC) is used for pump and fan alternation applications
where lower pressure peaks are desirable when a new auxiliary motor is connected
on-line. SPFC is an easy way to implement soft starting of direct on line (auxiliary)
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