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Hello to all, I have this project that my supervisor wants to control the inverter (model is GRUNDFOS CUE) of a pump that supplies with cold water a tank.

I want the plc to be able to control the revs of the motor proportionally with a setpoint (temperature) and send more or less cold water to the tank.

I think that this can be done via a PID control loop, but I don't know the way and I couldn't find any related examples or topics in the forum.

Has someone any experience with similar system or any clue how to achieve this? Any suggestion is more than welcome.

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Geia sou Panagiwti,


I have successfully completed similar applications in the past.

Since the PID process should be based on temperature measurement, your temp. sensor input ( For example a K-type thermocouple) would be the PID process input.

I think the easiest way to control a VFD is to use an analog output.

In the PID configuration, set min PID output register to 0 and max PID output register to 4095 (This is for 12-bit analog output)

Then just run PID process and store the PID result value to your analog output linked register.

In case you choose the analog way, you need to check your VFD's analog input requirements and select PLC hardware that suits your application.


Another option is to use a communication protocol like Modbus RTU but of course you need to make sure that your VFD supports this Feature.

In this case, you set 0 to Min PID output and 500 to Max PID output register (If this is the resolution for 0-50Hz of your VFD) and then write this value to the frequency setting address of your VFD.


In any case, your minimum ouput value might not be 0Hz and maximum value might not be 50Hz.

It depends from the application, i just gave an example.

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