PLC and inverter communication wiring method

The memory of a PLC programmable controller can be divided into three types: system program memory, user program memory and working data memory.

1. System program memory

The system program memory is used to store the system program written by the programmable controller manufacturer and is fixed in the ROM. The user cannot change it directly. The quality of the system program determines the performance of the PLC to a large extent.

Its content mainly includes three parts: the first part is the system management program, which mainly controls the operation of the programmable controller and makes the entire programmable controller work step by step; the second part is the user instruction interpreter, through which the programming language of the programmable controller is converted into machine language instructions, and then the CPU executes these instructions; the third part is the standard program module and the system call program.

2. User program memory

The application program compiled according to the control requirements is called the user program. The user program memory is used to store various user programs written by the user in the specified programmable controller programming language for specific control tasks.

Currently, the more advanced programmable controllers use flash memory that can be read and written at any time as user program memory. Flash memory does not require a backup battery and data will not be lost when the TV is turned off.

3. Working data storage

The working data memory is used to store working data, i.e., the ON/OFF status and numerical data used in the user program. The component image register and data table are opened in the working data area. The component image register is used to store the switch quantity, output status, and the ON/OFF status of internal devices such as timers, counters, and auxiliary relays. The data table is used to store various data, which stores certain variable parameter values ​​when the user program is executed, the digital quantity obtained by A/D conversion, and the results of digital operations.

Basic structure diagram of inverter

The inverter is a device that converts the industrial frequency power supply (50Hz or 60Hz) into AC power supply of various frequencies to achieve variable speed operation of the motor. The control circuit completes the control of the main circuit, the rectifier circuit converts AC power into DC power, the DC intermediate circuit smoothes and filters the output of the rectifier circuit, and the inverter circuit converts DC power into AC power. For inverters that require a lot of calculations, such as vector control inverters, a CPU for torque calculation and some corresponding circuits are sometimes required.

There are generally three ways to connect PLC and inverter

① Use the analog output module of the PLC to control the inverter. The analog output module of the PLC outputs a 0-5V voltage signal or a 4-20mA current signal as the analog input signal of the inverter to control the output frequency of the inverter. This control method has simple wiring, but it is necessary to select a PLC output module that matches the input impedance of the inverter, and the analog output module of the PLC is relatively expensive. In addition, voltage division measures must be taken to make the inverter adapt to the voltage signal range of the PLC. When connecting, pay attention to separate the wiring to ensure that the noise on one side of the main circuit is not transmitted to the control circuit.

② Use the switch output of PLC to control the frequency converter. The switch output of PLC can generally be directly connected to the switch input terminal of the frequency converter. This control method has simple wiring and strong anti-interference ability. The switch output of PLC can be used to control the start/stop, forward/reverse, jog, speed and addition and subtraction time of the frequency converter, which can achieve more complex control requirements, but can only be adjusted in steps.

When using relay contacts for connection, there may be erroneous operation due to poor contact. When using transistors for connection, it is necessary to consider factors such as the voltage and current capacity of the transistor itself to ensure the reliability of the system. In addition, when designing the input signal circuit of the inverter, it should be noted that improper connection of the input signal circuit may sometimes cause malfunction of the inverter. For example, when the input signal circuit uses inductive loads such as relays, the noise caused by the surge current generated when the relay is opened and closed may cause malfunction of the inverter, which should be avoided as much as possible.

③ Connection between PLC and RS-485 communication interface. All standard Siemens inverters have an RS-485 serial interface (some also provide RS-232 interface), which adopts two-wire connection and its design standard is suitable for application objects in industrial environment. A single RS-485 link can connect up to 30 inverters, and the inverter that needs to communicate can be found according to the address of each inverter or by broadcast information. There needs to be a main controller (master station) in the link, and each inverter is a subordinate control object (slave station).

PLC inverter control motor forward and reverse wiring diagram

1. After connecting the wires according to the wiring diagram, start the power supply and prepare to set the inverter parameters.

2. Press the "MODE" key to enter the parameter setting mode, and set Pr.79 to "2": external operation mode, the start signal is input by the external terminal (STF, STR), and the speed adjustment is input by the external terminal (between 2 and 5, between 4 and 5, multi-terminal speed).

3. Press the "MODE" button continuously to exit the parameter setting mode.

4. Press the forward button and the motor starts running in the forward direction.

5. Press the stop button to stop the motor.

6. Press the reverse button and the motor will start running in reverse.

7. Press the stop button and the motor stops.

8. If the reverse button is pressed when the motor is rotating forward, the motor will stop first and then reverse; conversely, if the forward button is pressed when the motor is rotating reversely, the motor will stop first and then rotate forward.

Wiring diagram of PLC and inverter

PLC and inverter communication method

1. PLC switch signal controls the frequency converter

The output point and COM point of PLC (MR or MT type) are directly connected to the STF (forward start), RH (high speed), RM (medium speed), RL (low speed), input terminal SG and other ports of the inverter. PLC can control the start, stop and reset of the inverter through program; it can also control the different combinations of high speed, medium speed and low speed terminals of the inverter to achieve multi-speed operation. However, because it uses switch quantity to implement control, its speed regulation curve is not a continuous and smooth curve, and it is impossible to achieve fine speed regulation.

2. PLC analog signal controls the frequency converter

Hardware: FX1N and FX2N PLC host, equipped with 1-channel simple FX1N-1DA-BD expansion analog output board; or analog input and output mixed module FX0N-3A; or 2-channel output FX2N-2DA; or 4-channel output FX2N-4DA module, etc. Advantages: PLC program compilation is simple and convenient, speed regulation curve is smooth and continuous, and operation is stable.

Disadvantages: In large-scale production lines, the control cable is relatively long. Especially when the DA module uses voltage signal output, the line has a large voltage drop, which affects the stability and reliability of the system.

3. PLC uses RS-485 communication method to control the inverter

This is the most commonly used method. PLC uses RS serial communication instructions for programming. Advantages: simple hardware, lowest cost, can control 32 inverters. Disadvantages: large programming workload.

4. PLC uses RS-485 Modbus-RTU communication method to control the inverter

Mitsubishi's new F700 series inverter uses RS-485 terminals to communicate with PLC using Modbus-RTU protocol. Advantages: PLC programming with Modbus communication is simpler and more convenient than RS-485 without protocol. Disadvantages: PLC programming workload is still large.

5. PLC uses fieldbus to control the inverter

Mitsubishi inverters can be equipped with various communication options, such as the FR-A5NC option for CC-Link fieldbus, the FR-A5AP (A) option for Profibus DP fieldbus, the FR-A5ND option for DeviceNet fieldbus, etc. Mitsubishi FX series PLCs have corresponding communication interface modules for docking.

Advantages: fast speed, long distance, high efficiency, stable operation, simple programming, and can connect a large number of inverters. Disadvantages: high cost.

6. Use extended memory

Advantages: low cost, easy to learn and use, reliable performance Disadvantages: can only be used in systems with no more than 8 inverters.

PLC and inverter communication wiring diagram

Mitsubishi PLC control inverter case analysis

There are three ways to directly operate the inverter without an external controller (such as PLC):

①Buttons on the operation panel;

② Parts connected to the operating terminals (such as buttons and potentiometers);

③Compound operation (such as setting the frequency on the operation panel and starting/stopping the buttons connected to the terminals). In order to facilitate operation and make full use of the inverter, PLC can also be used to control the inverter.

There are three basic ways for PLC to control inverter:

①Controlled by switch quantity;

②Control by analog quantity;

③Controlled by RS485 communication.