VSD Speed control with Pressure

Jethro Baidoo

Member
J
Join Date
Jun 2015
Location
Accra
Posts
35
Can I get the logic for controlling pump speed using pressure in the line?
When pressure is high the speed should slow down and vice versa.
I am using S7 300 PLC.
Thanks.
 
you are trying to control pressure
It sounds like you are looking for a PID control
set the desired pressure and let the PID set the speed required
 
Hi all,
I'm new to this so treat me as a complete novice I have been tasked to create a dosing system, the system has a water meter to establish flow rate and gives out 1L/min =1 pulse and another chemical needs to flow at 20% of this flow rate.
I can do the basics on off etc but this has me completely baffled any help would be greatly appreciated.
 
@Jethro please give us a sketch of the process. Where is the pressure being measured? Where is the pump relative to where the pressure is measured? Where are any flow elements (valves, tanks, other control systems) that will cause the measure pressure to change if the pump's motor speed stays constant? What kind of fluid is being pumped?


Also please provide your skill level e.g.

  • Do you know how to get the measured pressure into the PLC?
  • Do you know how to get the PLC to control the VFD that sets the speed for the motor that turns the pump?
  • Do you know how to do linear scaling?
 
Pretty easy use a PID loop ,use the input from the pressure transducer via 0/10 volt input the output of the loop 0/10 the drive the speed control, the loop set point will be the pressure set point, this will all fit on one page using function blocks
 
This is not going to be easy since the flow is been measured in chunks and not a steady state , really been better with a 0/10 volts flow rate measuring device not pulses, how accurate has this got to be, draw the system out for us and then we can help, start on another new thread
 
This is not going to be easy since the flow is been measured in chunks and not a steady state , really been better with a 0/10 volts flow rate measuring device not pulses, how accurate has this got to be, draw the system out for us and then we can help, start on another new thread
 
Hello Steve, I have analog input and output channels.
I want to control the speed of the pump whiles monitoring the pressure in the pipeline whiles pump is running.
I can scale the analog input signal(0-27648) to get the desired integer output.
But my problem is how do I get 4-20mA to the VSD to serve as speed reference?
 
drbitboy said:
@Jethro please give us a sketch of the process. Where is the pressure being measured? Where is the pump relative to where the pressure is measured? Where are any flow elements (valves, tanks, other control systems) that will cause the measure pressure to change if the pump's motor speed stays constant? What kind of fluid is being pumped?


Also please provide your skill level e.g.

  • Do you know how to get the measured pressure into the PLC?
  • Do you know how to get the PLC to control the VFD that sets the speed for the motor that turns the pump?
  • Do you know how to do linear scaling?
Click to expand...
Hello drbitboy, I have analog input and output channels.
I want to control the speed of the pump whiles monitoring the pressure in the pipeline when pump is running.
I can scale the analog input signal(0-27648) to get the desired integer output.
But my problem is how do I get 4-20mA to the VSD to serve as speed reference?
 
But my problem is how do I get 4-20mA to the VSD to serve as speed reference?
Click to expand...
The smart-a$$ answer is to use a twisted, shielded pair and connect it to the correct terminals on both the PLCs analog output module and the the drive.
There will also be a parameter in your VFD that you will need to set to configure it to accept the 4 -20 mA signal as its speed command.
 
Rcoxcms said:
and gives out 1L/min =1 pulse
Click to expand...

Is this correct? If 1 pulse equals 1L/min rather than 1L you should start off from replacing the flowmeter/transmitter as it will be impossible to have any kind of precision on your measurement to determine actual flow and you're likely to require some additional hardware depending on the real flow rate of the system.
 

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