I am new to process control, and my question is how do we define the required pump signal? Do we find it via P&ID? Or just by knowing the pump type? or it can be found via the control loop (then how)?
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A little more information before i can help.
1. If it's a big motor, is it connected to a VFD or a motor starter?
2. If connected to a VFD, look in the VFD manual and wiring diagrams. It'll show you what kind of signal it requires to control its speed. (i.e. 0-10VDC, 4-20mA, 0-5VDC etc)
3. If connected to a motor starter, you'll need a relay with the contacts connected to the "Auto" side of the starter switch.
Take a step back and ask 'what is the pump intended to do'
within a process environment.
A simple example is topping up a water storage tank - auto relay as above to run the filling pump at a fixed speed for short periods.
Another example is a pump required to top up a wet well to maintain constant water level throughout the day when the process throughput varies in outflow. The pump flowrate is proportional to pump speed but is not linear. As above, have the pump motor connected to a variable speed drive, and use a P&ID with parameters such to cater for the non-linear characteristics of the pump. Your process variable in this case is water level, using say an ultrasonic meter.