Issues regarding DCS control of variable-frequency pumps~
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I understand the principle of frequency converters~ Now I would like to ask how to control pumps equipped with frequency converters using a DCS system? It’s fine as long as you can give a general idea; the specific configurations and wiring aren’t an issue. I just want to know the general process, or rather, the principle behind the entire control system. Last edited by fourkingsgrubby on 2009-2-9 13:57.]
Create a manual operator on the DCS that outputs a 4-20Ma signal; then the frequency converter can receive it and thus enable control
You can simply treat the signal going to the inverter as the signal going to the control valve!
If we understand it in terms of the 3rd floor, then it’s simple~ Is there any small difference between this and the signal from the control valve?
Also, could someone roughly describe the circuit diagram for controlling a pump using an inverter? A textual description will suffice~ Thank you
Here is an example of a pump controlled by an inverter: http://bbs.hcbbs.com/viewthread.php?tid=331275
Generally, the control of water pumps relies on PLCs, frequency converters, and PID controllers. What the original poster means is to use a DCS to replace the PLCs and PID controllers in order to achieve variable-frequency control of the pumps
Thank you to the friend who provided the connection~ There’s also the 7th floor; yes, no PLC control is needed – the signals can be sent directly to the DCS~
Generally, there are 4 signals connected to the frequency converter: start/stop, status feedback, alarm feedback, and settings. If necessary, other signals such as current and speed can also be connected
If controlling one pump is simple, the wiring for multiple pumps becomes much more complex, involving issues such as adding or removing pumps, as well as operating at fixed frequency or variable frequency
The speed of a power-frequency pump is fixed, as is its flow rate; therefore, the flow rate has to be controlled through control valves. As a result, the pump’s energy consumption remains at its maximum level. With an inverter-driven pump, however, the pump’s speed can be controlled using 4–20mA signals generated by a DCS or PLC, or other controllers. The signal that was originally used to control the opening and closing of the control valve can now be used to control the pump’s speed – in other words, the signal sent to the inverter can be treated as a signal for controlling the control valve. Of course, switchers need to be added as required during implementation.
Variable frequency drives are typically controlled by a 4-20mA current output from a PLC or DCS system; the drive outputs a voltage corresponding to the magnitude of this control current (4-20mA) in order to regulate the motor speed. The DCS system creates the various modules that need to be controlled according to process requirements, using configuration software developed specifically for this purpose, and then implements human-machine interface control through designed control screens. A foolish opinion
A simple approach is to treat the frequency converter as an AO, being sure to use an isolation barrier or signal isolator; otherwise, it may interfere with the proper operation of the DCS. When control becomes more complex, some DOs are needed for frequency conversion; depending on the requirements and the functions of the frequency converter, signals can be received or sent by the DCS.
Create a PID control loop for the pump’s outlet pressure or flow rate, and use the output to control the frequency converter.
Implement a flow control circuit; the controller outputs a proportional signal, with the upper range being controlled by an inverter, and the outlet valve is closed starting from the lower limit set by the inverter in order to regulate the flow rate.
The common controls for frequency converters that I use include: DO: Start/Stop; 1DI: Ready status (for motors with normally open switches, 1 indicates the switch is closed and 0 indicates it is open); 2DI: Start/Stop feedback; 3DI: Frequency converter alarm. AO: 4-20mA signal output; AI: 4-20mA input (usually not used; an alarm is displayed when there is a problem with the frequency converter). When the switch trips, the motor turns red; when there is a frequency converter alarm, it flashes yellow. Normal operation is indicated by green, while the stop state is indicated by white