Program features 63 Single pump control (PFC)The Single pump control (PFC) is used in pump systems consisting of one drive andmultiple pumps. The drive controls the speed of one of the pumps and in additionconnects (and disconnects) the other pumps directly to the supply network throughcontactors.The PFC control logic switches auxiliary motors on and off as required by thecapacity changes of the process. In a pump application for example, the drivecontrols the motor of the first pump, varying the motor speed to control the output ofthe pump. This pump is the speed regulated pump. When the demand (representedby the process PID reference) exceeds the capacity of the first pump (a user definedspeed/frequency limit), the PFC logic automatically starts an auxiliary pump. Thelogic also reduces the speed of the first pump, controlled by the drive, to account forthe addition to the total system output by the auxiliary pump. Then, as before, the PIDcontroller adjusts the speed/frequency of the first pump in such a way that the systemoutput meets the process needs. If the demand continues to increase, the PFC logicadds further auxiliary pumps, in a similar manner as just described.As the demand drops, making the speed of the first pump fall below a minimum limit(user defined as a speed/frequency limit), the PFC logic automatically stops anauxiliary pump. The PFC logic also increases the speed of the drive controlled pumpto account for the missing output of the stopped auxiliary pump.The Single pump control (PFC) is supported in external control location EXT2 only.AutochangeAutomatic rotation of the start order, or Autochange functionality, serves two mainpurposes in many PFC type setups. One is to keep the run times of the pumps equalover time to even their wear. The other is to prevent any pump from standing still fortoo long, which would clog up the unit. In some cases it is desirable to rotate the startorder only when all units are stopped, for example to minimize the impact on theprocess.The Autochange can also be triggered by the Timed function (see page 72).InterlockThere is an option to define interlock signals for each motor in the PFC system. Whenthe interlock signal of a motor is Available, the motor participates in the PFC startingsequence. If the signal is Interlocked, the motor is excluded. This feature can be usedfor informing the PFC logic that a motor is not available (for example due tomaintenance or manual direct-on-line starting).
