Essential Tips for Upgrading Industrial Control Systems: Perfect PLC and Inverter Coordination

I remember when I first joined the industry, I took over a project to upgrade an injection molding machine. The communication between the inverter and the PLC was always intermittent. After several days of troubleshooting, I discovered that the grounding was not done correctly, which caused interference. Today, I will share my experiences and techniques for controlling inverters with PLCs, focusing on those easily overlooked but critical details.

1. Choosing Communication Methods

Hardware Connection Schemes

Just like mobile phones have two internet access methods: Wi-Fi and 4G, PLCs controlling inverters also have multiple options:

// Analog control method
AI/AO connection:
- PLC analog output (0-10V/4-20mA)
- Inverter analog input terminal
- Shielded wire must be routed separately

// Communication control method
RS485 interface:
- PLC: RS485 communication module
- Inverter: 485 terminals A/B
- Communication protocol: Modbus-RTU

Wiring Requirements

// Inverter terminal definitions
+10V: Reference voltage output
AI1: Analog input 1 (0-10V)
AI2: Analog input 2 (4-20mA)
GND: Analog ground
A+: RS485 communication A line
B-: RS485 communication B line

// PLC output terminals
Q0.0: Run enable
Q0.1: Fault reset
Q0.2: Forward command
Q0.3: Reverse command
QW64: Analog output (frequency set)

2. Inverter Parameter Settings

Basic Parameter Configuration

// Essential parameters for the inverter
P00.01 = 2    // Control method selection (communication control)
P00.06 = 8    // Frequency set method (communication set)
P14.00 = 1    // Communication protocol selection (Modbus-RTU)
P14.01 = 2    // Baud rate setting (19200bps)
P14.02 = 1    // Data format (RTU)
P14.03 = 1    // Local address (station 1)

// Communication timeout protection
P14.11 = 2    // Communication timeout action (free stop)
P14.10 = 1.0  // Communication timeout duration (1 second)

Control Program Example

// PLC control program
FUNCTION "VFD_Control" : VOID
VAR_INPUT
    Enable : BOOL;    // Enable signal
    Speed  : REAL;    // Speed set
END_VAR
VAR
    ModbusCmd : MB_MASTER;  // Modbus master
    SendData  : ARRAY[0..3] OF WORD;  // Send data
END_VAR

BEGIN
    // Data packaging
    SendData[0] := REAL_TO_WORD(#Speed * 100.0);
    
    // Modbus communication
    ModbusCmd(
        REQ := #Enable,
        MB_ADDR := 1,        // Slave address
        MODE := 0,           // Write single register
        DATA_ADDR := 2000,   // Frequency set address
        DATA_LEN := 1,       // Data length
        DATA_PTR := #SendData// Data pointer
    );
END_FUNCTION

3. Fault Handling Design

Fault Monitoring Program

// Fault monitoring function block
FUNCTION_BLOCK "Fault_Monitor"
VAR_INPUT
    Run_Status : BOOL;    // Running status
    Fault_Code : WORD;    // Fault code
END_VAR
VAR
    Fault_Timer : TON;    // Fault timer
    Retry_Count : INT;    // Retry count
END_VAR

BEGIN
    // Fault detection
    IF #Fault_Code &lt;&gt; 0 THEN
        // Delay confirmation
        #Fault_Timer(IN := TRUE,
                     PT := T#1S);
        
        IF #Fault_Timer.Q THEN
            // Fault handling
            IF #Retry_Count &lt; 3 THEN
                // Automatic reset
                "Reset_VFD" := TRUE;
                #Retry_Count := #Retry_Count + 1;
            ELSE
                // Alarm and stop
                "System_Alarm" := TRUE;
            END_IF;
        END_IF;
    END_IF;
END_FUNCTION_BLOCK

4. Debugging Optimization Techniques

Quick Debugging Program

// Inverter debugging program
FUNCTION "VFD_Debug" : VOID
VAR_TEMP
    Status : WORD;    // Communication status
    Speed  : REAL;    // Actual speed
END_VAR

BEGIN
    // Automatic parameter check
    FOR #i := 0 TO 10 DO
        // Read parameters
        #Status := READ_PAR(
            VFD_ADDR := 1,
            PAR_ADDR := #i,
            PAR_VAL =&gt; #ParValue
        );
        
        // Parameter verification
        IF #Status &lt;&gt; 0 THEN
            "Debug_Error" := TRUE;
            RETURN;
        END_IF;
    END_FOR;
    
    // Speed ramp test
    FOR #Speed := 0.0 TO 50.0 BY 5.0 DO
        "VFD_Control"(
            Enable := TRUE,
            Speed := #Speed
        );
        WAIT TIME := T#1S;
    END_FOR;
END_FUNCTION

Practical Suggestions

1. Key Points in System Design:

• Control method selection
• Grounding system design
• Electromagnetic Compatibility
• Protection functions

2. Debugging Tools:

• Oscilloscope
• Multimeter
• Inverter debugging software
• Communication analyzer

3. Key Technologies:

• Inverter principles
• Communication protocols
• PID tuning
• Fault handling

4. Precautions:

• Grounding standards
• Cable laying
• Parameter backup
• Motor Matching

Key Capability Requirements:

1. Hardware Technology:

• Inverter principles
• Wiring standards
• Motor characteristics
• EMC design

2. Software Development:

• PLC programming
• Communication programming
• Debugging methods
• Fault diagnosis

3. On-Site Skills:

• Wiring construction
• Parameter tuning
• Fault troubleshooting
• Performance optimization

Implementation Steps Suggestions:

1. Preliminary Preparation:

• Motor parameter measurement
• Inverter selection
• Control scheme design
• Wiring diagram drawing

2. System Installation:

• Electrical cabinet layout
• Cable laying
• Grounding construction
• Power-on testing

3. Debugging Optimization:

• Parameter settings
• Communication testing
• Performance tuning
• Reliability verification

Follow the solutions provided in this article to practice, starting with simple analog control and gradually achieving advanced communication control functions. Establish complete technical documentation, including wiring diagrams, parameter tables, and debugging records. Pay attention to electromagnetic compatibility design, which is the foundation for the reliable operation of inverter systems. Regularly perform equipment maintenance and parameter backups to maintain stable system operation. When matching motors, pay attention to power margins, and it is recommended to choose a larger capacity for the inverter. When developing programs, consider fault protection and automatic recovery to improve system reliability.