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ToothFairy

How to control AC drive speed according the analog input value

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I have an analog input which measures a fluid pressure in pipe. I want to control the pump speed to make sure the pressure is held in preset value. What would be the best way to do this?

I tried to do a simple program which did a simple add/subtract to frequency output value depending whether the measured pressure value were higher or lower than the set value. This works, but it is very slow. Also the response time this way is way too slow. For example, if I close a valve in the output pipe, it takes a lot of time to adjust the pump seed low enough.

Is there some standard way to do this? So I don't need to invent the wheel again.. 

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This is a good application for PID instruction in your PLC.

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A PID instruction might work, but it might also be even slower to respond. If you are working with liquids a fairly small change in flow can result in a large change in pressure. You might want to also consider an accumulator tank to slow down the required response time.

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Another factor that might be affecting the speed of response are the settings in the AC VSD.  For example it may have rate-of-change (or slew rate) settings that prevent the output frequency changing as quickly as the speed setpoint from your PLC.

But as Dick has said above, the ability of your proposed system to work properly will depend ultimately on the characteristics of your process.  Centrifugal pumps have a very non-linear speed/flow/pressure relationship.  Can you post a P&I Diagram or sketch to explain what you are trying to achieve?

Ian

Edited by Thermo
Added a bit...

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