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What are the possible causes of water flow fluctuation, while operating a pump through ATV212 VFD?

The water flow fluctuation can be due to any of the following three reasons,

1. The speed reference (from PLC or external PID controller) to VFD's analog input fluctuation 
2. High fluctuation in load current due to mechanical jamming in pump
3. Improper PID tuning in ATV212 VFD.

1. The speed reference (from PLC or external PID controller) to VFD's analog input fluctuation: You can check the below points,

a)Check the speed reference voltage in drive terminals VIA & CC with multi-meter. It should be steady, if the reference is unstable the speed of the VFD also will be unstable. If so, you should correct the PLC program to correct the speed reference.
b)Remove and isolate the wires from VIA & CC in drive. Connect a wire between PP & VIA( means, short terminals VIA & PP). Now you can give run command now and check the speed. The motor should run at maximum speed setting at VFD. If it is running at full speed without any speed variation, it indicates that the VFD programming in VFD is okay. Hence you should check the PLC programming for the unstable speed reference output from PLC.
c)You can select the local / remote button in VFD to local mode and start the motor form start button in VFD keypad and increase the speed through up (arrow) key in keypad. If the motor is running steadily, there would not be any problem in drive.

2. High fluctuation in load current due to mechanical jamming in pump:
The ATV212 VFD current is limited with current limit parameter. If there is a jamming in pump, there would be fluctuation in load current due to un-even loading on the motor. The un-even loading also can cause speed fluctuation in pump. Hence you should check and correct the mechanical problem in pump to solve the fluctuation in water flow.

3. Improper PID tunning in ATV212 VFD: 
You may use pressure or speed feed back sensor to control the flow. If you are using PID configuration in VFD with pressure or flow feedback, there is a possibility of speed fluctuations due to improper PID parameter tuning.
The pump water flow fluctuation can be due to following PID parameter settings. The following parameters should be fine tuned to get optimum stability in speed. 

 
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