Control System Application Scenario Requirements…
In order to start – one has first to decide what the scenario of the control system is. Various factors are to be considered prior to parameterization in order to avoid ad-hoc applications. These include the following;
PID Action Selection: How will the control system work? What role will the feedback and its summation point with the input play? In other words, Will this be a forward action control system or a backward action control system. This has to do with the PV (Process Variable) value relation with the SP (Setpoint) value – and the VSD’s response thereof.
Also, where does the controller reside. Will the VSD react as the entire control system – or will it be a mere actuation device, controlled by and external PID system?
What type of input signals are used, as well as which input terminals are utilized on the VSD for this purpose?
Based on these questions we can set up a basic VSD PID control system.
Primary PID Control System Function Basic Configuration
Here we opted for a standard Forward Action Control System – with Pr.133 representing the SP value and the Terminal 4 analog input representing the PV value (measured value). (Note that if a reverse action control is required Pr.128 = 20.
The SP value is static (user-predefined) – and thus is stored on Pr.133 as a range between 0-100% of the engineering value. (Note that the engineering values of the SP and PV needs to be of the same type and unit of measure.)
Terminal 4 is used for PV value. Pr.610 is to be set accordingly. In addition – the function of the Terminal 4 is to be established, and defined via Pr.858.
The Control System Model (unique to this application example.)
In this Control System Model – we have opted to have the control system PID computation to occur within the VSD itself.
Pr.128 = 21 (Measured Value) and Pr.133 = Predefined user-defined Setpoint.
Control System – VSD Connection Diagram
It should be clear that there are 3 separate sets of connections pertaining to a VSD, which is the power circuitry (input from the source. and output to motor). These are collectively known as switch gear circuits in industry. The control circuit (signals wired into the VSD’s control board) for interlocking and control purposes. The last set of inter-connections are that of the networks for bus interfacing (this will not be covered in this procedure.)
Notice the wiring for the PV into terminal 4 – as well as the type of transmitters (process measurement devices) which can be used for Process variable measurement. Both 2-wire and 3-wire instrumentation are accommodated.
Pr.128 – PID Action Selection
Pr.128 = 21
This allows for VSD Forward action control – with the VSD acting as the controller and the actuator.
Input Terminal 4 Specifications
For this applications we will use Terminal 4 as the Process Measured Value input, as a 4-20mA signal.
Note that Pr.126 C5, C6, C7 can be used for signal calibration purposes.
Pr.241 – Analog Input display unit specifications
Pr.241=0 is being set to allow for a analog signal input of type mA, range 0-20mA, which is scaled from 0-100%, with 0.1% resolution accuracy.
Pr.610 – PID Measured value Input selection
Pr.610 = 3 allows for an direct analog signal input into the terminal 4 terminal – to represent the measured value input.