Both PLC and inverter have a relationship of inclusion and inclusion. Both PLC and inverter can complete some specific instructions to control motors. PLC is a kind of program input and execution hardware, and inverter is one of them. , But PLC covers a larger range than frequency converters, and can also be used to control more things. It has a wider application field and more powerful performance. Of course, the control accuracy of PLC is also greater.
The inverter cannot be programmed to change the frequency, voltage and other parameters of the power supply. Its output frequency can be set to a fixed value, or it can be dynamically controlled by the PLC.
PLC can be programmed to control electrical components or complete functions, communication and other tasks.
The communication between the PLC and the inverter needs to follow the universal serial interface protocol (USS), and the method of access is determined according to the principle of master-slave communication of the serial bus. A master station and up to 31 slave stations can be connected to the bus. The master station selects the slave station to transmit data according to the address characters in the communication message. When the master station does not require it to communicate, the slave station itself cannot send data first , The transmission of information between each slave station can not be carried out directly.
PLC basic structure diagram
The memory of 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 it is solidified in the ROM, and the user cannot directly modify it. 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, so that the entire programmable controller works step by step. , The second part is the user instruction interpretation program, through the user instruction interpretation program, the programming language of the programmable controller is changed 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 requirement 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. At present, more advanced programmable controllers use flash memory that can be read and written at any time as the user program memory. The flash memory does not need a backup battery, and the data will not be lost even if the TV is dropped.
3. Working data storage
The working data memory is used to store working data, which is the ON/OFF status and numerical data used in the user program. There are component mapping registers and data tables in the work data area. Among them, the component image register is used to store the switch value, 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. It stores some variable parameter values ​​during the execution of the user program and the digital quantity obtained by A/D conversion and the result of digital operation.
Basic structure diagram of inverter
Inverter is a device that converts industrial frequency power (50Hz or 60Hz) into AC power of various frequencies to achieve variable-speed operation of the motor. The control circuit completes the control of the main circuit, and the rectifier circuit converts AC power into DC power. The circuit smoothes and filters the output of the rectifier circuit, and the inverter circuit inverts the direct current into alternating current. For frequency converters such as vector control frequency converters that require a large amount of calculations, sometimes a CPU for torque calculation and some corresponding circuits are needed.
There are generally three connection methods between PLC and inverter
①Using the PLC analog output module to control the inverter. The PLC analog output module outputs a 0～5V voltage signal or 4～20mA current signal as the analog input signal of the inverter to control the output frequency of the inverter. The wiring of this control method is simple, but it is necessary to select a PLC output module that matches the input impedance of the inverter, and the price of the PLC analog output module is relatively expensive. In addition, it is necessary to take voltage division measures 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 the side of the main circuit is not transmitted to the control circuit.
â‘¡Using the switch output of PLC to control the inverter. The switch output of PLC can generally be directly connected with the switch input 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 rotation, jog, rotation speed and addition and subtraction time of the frequency converter, which can realize more complicated control requirements, but can only regulate the speed in stages.
When the relay contacts are used for connection, there may be malfunctions due to poor contact. When a transistor is used 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 the inverter to malfunction. For example, when an inductive load such as a relay is used in the input signal circuit, when the relay is switched on and off, the noise caused by the generated surge current may cause the inverter to malfunction, which should be avoided as much as possible.
â‘¢The connection between PLC and RS-485 communication interface. All standard Siemens inverters have an RS-485 serial interface (some also provide RS-232 interface), using two-wire connection, and its design standards are suitable for application objects in an industrial environment. A single RS-485 link can connect up to 30 inverters, and according to the address of each inverter or the use of broadcast information, the inverter that needs to be communicated can be found. A master controller (master station) is required in the link, and each inverter is a slave control object (slave station).
PLC inverter control motor forward and reverse wiring diagram
1. After connecting the wires according to the wiring diagram, turn on the power supply and prepare to set the parameters of the inverter.
2. Press the "MODE" key to enter the parameter setting mode, set Pr.79 to "2": External operation mode, the start signal is input from the external terminals (STF, STR), and the speed adjustment is from the external terminals (between 2, 5, 4, 4). 5, multi-terminal speed) input.
3. Press the "MODE" button continuously to exit the parameter setting mode.
4. Press the forward button, the motor will start to run forward.
5. Press the stop button to stop the motor.
6. Press the reverse button to start the motor reversely.
7. Press the stop button to stop the motor.
8. If the reverse button is pressed during the electric forward rotation, the motor stops first and then reverses; on the contrary, if the forward button is pressed during the reverse rotation of the motor, the motor stops first and then rotates forward.
Wiring diagram of PLC and inverter
Communication between PLC and inverter
1. PLC's switch signal control frequency converter
The output points and COM points of PLC (type MR or MT) are directly connected to the STF (forward rotation start), RH (high speed), RM (medium speed), RL (low speed), input terminal SG and other ports of the inverter. The PLC can control the start, stop, and reset of the inverter through the program; it can also control the different combinations of the inverter's high-speed, medium-speed, and low-speed terminals to achieve multi-speed operation. However, because it uses a switch to implement control, its speed control curve is not a continuous and smooth curve, and it cannot achieve fine speed adjustment.
2. PLC analog signal control frequency converter
Hardware: FX1N, FX2N PLC host, equipped with a simple FX1N-1DA-BD extended analog output board; or analog input and output mixed module FX0N-3A; or two-output FX2N-2DA; or four-channel Output FX2N-4DA module, etc. Advantages: PLC programming is simple and convenient, the speed control curve is smooth and continuous, and the work is stable.
Disadvantages: In a large-scale production line, the control cable is 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 frequency converter
This is the most commonly used method. PLC uses RS serial communication instructions to program. Advantages: The hardware is simple, the cost is the lowest, and 32 inverters can be controlled. Disadvantages: The programming workload is relatively large.
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 in Modbus communication mode is simpler and more convenient than RS-485 protocol-free mode. Disadvantages: PLC programming workload is still relatively large.
5. The PLC uses the field bus method to control the frequency converter
Mitsubishi inverters can be built-in various types of communication options, such as FR-A5NC option for CC-Link fieldbus; FR-A5AP(A) option for Profibus DP fieldbus; For DeviceNet fieldbus FR-A5ND option and so on. Mitsubishi FX series PLC has corresponding communication interface module to connect with it.
Advantages: fast speed, long distance, high efficiency, stable work, simple programming, and a large number of inverters that can be connected. Disadvantages: higher cost.
6. Use extended memory
Advantages: low cost, easy to learn, easy to use, reliable performance Disadvantages: can only be used in systems with no more than 8 inverters.
PLC and inverter communication wiring diagram
Case analysis of Mitsubishi PLC console inverter
There are three ways to directly operate the inverter without an external controller (such as PLC):
â‘ Keys on the operation panel;
â‘¡Operate the components connected to the terminal (such as buttons and potentiometers);
â‘¢Composite operation (such as setting the frequency on the operation panel, operating the button connected to the terminal block for start/stop control). In order to facilitate the operation and make full use of the frequency converter, PLC can also be used to control the frequency converter.
There are three basic ways for PLC to control the inverter:
â‘ Control by switching value;
â‘¡Control by analog quantity;
â‘¢Controlled by RS485 communication.
PLC controls the hardware connection of the frequency converter in the switch mode
The inverter has many switching terminals, such as forward, reverse and multi-speed speed control terminals. When PLC is not used, just connect these terminals with switches to control the inverter forward, reverse and multi-speed speed. . When using a PLC to control the inverter, if the PLC controls the inverter in a switch mode, you need to connect the switch output terminal of the PLC to the switch input terminal of the inverter. In order to detect certain states of the inverter, you can also The switch output terminal of the frequency converter is connected with the switch input terminal of the PLC.
The hardware connection of the PLC to control the frequency converter in switch mode is shown in the figure below. When the internal hard contact of the Y001 terminal is closed when the PLC internal program runs, it is equivalent to the external switch of the STF terminal of the inverter is closed, and the STF terminal input is ON. The input voltage of terminal 2 can be changed, thereby changing the frequency of the inverter output power, and then changing the speed of the motor. If there is an abnormality inside the inverter, the internal contact between the A and C terminals is closed, which is equivalent to that the external switch of the X001 terminal of the PLC is closed, and the input of the X001 terminal is ON.
PLC controls the hardware connection of the frequency converter by analog quantity
The inverter has some voltage and current analog input terminals. Changing the voltage or current input value of these terminals can change the speed of the motor. If these terminals are connected with the analog output terminals of the PLC, the PLC can be used to control the inverter to adjust the motor.的rpm。 The analog quantity is a continuously changing quantity. Using the analog quantity control function, the motor speed can be continuously changed (infinitely variable speed).
The hardware connection of the PLC to control the inverter in analog mode is shown in the figure below. Since Mitsubishi FX2N-32MR PLC has no analog output function, it needs to be connected to an analog output module (such as FX2N-4DA), and then the analog output module The output terminal of is connected with the analog input terminal of the inverter. When the external switch of the STF terminal of the inverter is closed, the terminal input is ON, the inverter starts the motor forward, and the digital data generated when the internal program of the PLC is running is sent to the analog output module (DA module) through the connecting cable. Convert to 0～5V or 0～10V voltage (analog) and send it to terminals 2, 5 of the inverter to control the frequency of the output power of the inverter, and then control the speed of the motor. If the DA module outputs to the inverter 2, 5 When the voltage of the terminal changes, the frequency of the output power of the inverter will also change, and the speed of the motor will change.
When the PLC controls the analog input terminals of the inverter in the analog mode, it can also control the digital input terminals of the inverter in the switch mode at the same time.
PLC uses RS485 communication to control the hardware connection of the inverter
When PLC controls the inverter in switch mode, it needs to occupy more output terminals to connect to the input terminals of the corresponding functions of the inverter in order to control the inverter in forward rotation, reverse rotation and stop; PLC controls the inverter in analog mode When it is time, the DA module is needed to control the frequency and speed of the inverter. If PLC uses RS485 communication mode to control the inverter, only one RS485 communication cable (including 5 core wires) is needed to directly send various control and frequency modulation commands to the inverter, and the inverter will send it through the RS485 communication cable according to the PLC. The instruction can execute the corresponding function control.
RS485 communication is a communication method widely used in industrial control at present. It has strong anti-interference ability, and its communication distance can reach tens of meters to thousands of meters. Using RS485 communication can not only connect two devices for communication, but also connect multiple devices (up to 32 devices in parallel) to form a distributed system for mutual communication. Help more people learn more! Thank you for your support!
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