Program features 155PFC and SPFC control PFC controlPump and fan control (PFC) switches auxiliary pumps on and off as required bycapacity changes. Autochange function alternates between pumps to keep the dutytimes of the pumps equal. Interlocks function enables the drive to detect if any of thepumps are unavailable (eg switched off for maintenance), in which case the nextavailable pump is started instead.The drive controls the motor of pump 1, varying the motor speed to control the pumpcapacity. This motor is the speed regulated motor.Direct line connections power the motors of pump 2 and pump 3, etc. The driveswitches pump 2 (and then pump 3, etc.) on and off as needed. These motors areauxiliary motors.The drive PID control uses two signals: a process reference and an actual valuefeedback. The PID controller adjusts the speed (frequency) of the first pump such thatthe 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 auxiliarypump. The PFC also reduces the speed of the first pump to account for the auxiliarypump’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 thesame 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 auxiliarypump. The PFC also increases the speed of the first pump to account for the auxiliarypump’s missing output.An Interlock function (when enabled) identifies off-line (out of service) motors, and thePFC 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 incrementsthe position of each motor in the rotation – the speed regulated motor becomes thelast auxiliary motor, the first auxiliary motor becomes the speed regulated motor, etc.See also section PFC control macro on page 110. SPFC controlSoft pump and fan control (SPFC) is used for pump and fan alternation applicationswhere lower pressure peaks are desirable when a new auxiliary motor is connectedon-line. SPFC is an easy way to implement soft starting of direct on line (auxiliary)
