Danfoss VLT AQUA Drive FC 202 Design Manual

Also see for VLT AQUA Drive FC 202: Service guide Installation guide Installation guide Operating guide Installation guide

Contents

5.5 Basic Cascade Controller OverviewThe basic cascade controller is used for pump applicationswhere a certain pressure (head) or level must bemaintained over a wide dynamic range. Running a largepump at variable speed over a wide range is not an idealsolution because of low pump efficiency at lower speed. Ina practical way, the limit is 25% of the rated full-loadspeed for the pump.In the basic cascade controller, the drive controls a variablespeed (lead) motor as the variable speed pump and canstage up to 2 more constant speed pumps on and off.Connect the additional constant speed pumps directly tomains or via soft starters. By varying the speed of theinitial pump, variable speed control of the entire system isprovided. The variable speed maintains constant pressure,which results in reduced system stress, and quieteroperation in pumping systems.Constant SpeedPumps (2)Variable SpeedPumps (1)Motor starter Frequency ConverterwithCascade ControllerPressure Sensor130BA362.10Illustration 5.10 Basic Cascade ControllerFixed lead pumpThe motors must be of equal size. The basic cascadecontroller allows the drive to control up to 3 equal pumpsusing the 2 built-in relays in the drive. When the variablepump (lead) is connected directly to the drive, the 2 built-in relays control the other 2 pumps. When lead pumpalternations are enabled, pumps are connected to thebuilt-in relays and the drive can operate 2 pumps.Lead pump alternationThe motors must be of equal size. This function makes itpossible to cycle the drive between the pumps in thesystem (maximum of 2 pumps). In this operation, the run-time between pumps is equalized, reducing the requiredpump maintenance and increasing reliability and lifetimeof the system. The alternation of the lead pump can takeplace at a command signal or at staging (adding anotherpump).The command can be a manual alternation or analternation event signal. If the alternation event is selected,the lead pump alternation takes place every time theevent occurs. Selections include:• Whenever an alternation timer expires.• At a predefined time of day.• When the lead pump goes into sleep mode.The actual system load determines staging.A separate parameter limits alternation only to take place iftotal capacity required is >50%. Total pump capacity isdetermined as lead pump plus fixed speed pumpscapacities.Bandwidth managementIn cascade control systems, to avoid frequent switching offixed-speed pumps, the desired system pressure is keptwithin a bandwidth rather than at a constant level. Thestaging bandwidth provides the required bandwidth foroperation. When a large and quick change in systempressure occurs, the override bandwidth overrides thestaging bandwidth to prevent immediate response to ashort duration pressure change. An override bandwidthtimer can be programmed to prevent staging until thesystem pressure has stabilized and normal control isestablished.When the cascade controller is enabled and the driveissues a trip alarm, the system head is maintained bystaging and destaging fixed-speed pumps. To preventfrequent staging and destaging and minimize pressurefluctuations, a wider fixed speed bandwidth is used insteadof the staging bandwidth.5.5.1.1 Pump Staging with Lead PumpAlternationWith lead pump alternation enabled, a maximum of 2pumps are controlled. At an alternation command, the PIDstops, the lead pump ramps to minimum frequency (fmin)and, after a delay, it ramps to maximum frequency (fmax).When the speed of the lead pump reaches the destagingfrequency, the fixed-speed pump is cut out (destaged). Thelead pump continues to ramp up and then ramps down toa stop and the 2 relays are cut out.TimeTimeMains operationDestaging freq.Alternationcommand/PID stopsStaging freq. Mains operationPID contr.starts130BA364.105sf maxf maxf minIllustration 5.11 Lead Pump AlternationAfter a time delay, the relay for the fixed-speed pump cutsin (staged) and this pump becomes the new lead pump.Product Features VLT® AQUA Drive FC 20222 Danfoss A/S © 05/2017 All rights reserved. MG22B10255