PLC and frequency converter have a relationship of inclusion and inclusion. Both PLC and frequency converter can complete some specific instructions to control the motor. PLC is a kind of program input execution hardware, and frequency converter is one of them. .
However, PLC has a larger coverage than frequency converters and can be used to control more things. It has wider application fields and more powerful performance. Of course, PLC's control accuracy is also greater. The frequency converter 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 dynamically controlled by a PLC.
PLC can be programmed to control electrical components or complete functions, communication and other tasks.
The communication between PLC and frequency converter needs to follow the universal serial interface protocol (USS), and the access method is determined according to the master-slave communication principle of the serial bus. One master station and up to 31 slave stations can be connected to the bus. The master station selects the slave station to transmit data based on 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. , information cannot be transmitted directly between slave stations.
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 is solidified in the ROM, which cannot be changed directly by the user. 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 interpreter program, which uses the user instruction interpreter program to 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 system call program.
2. User program memory
Application programs compiled according to control requirements are called user programs. The user program memory is used to store various user programs written by users in specified programmable controller programming languages for specific control tasks.
Currently, 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 if the TV is turned off.
3. Working data storage
The working data memory is used to store working data, that is, ON/OFF status, numerical data, etc. used in user programs. Component image registers and data tables are opened in the working data area. The component image register is used to store switching values, 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 certain variable parameter values when the user program is executed, digital quantities obtained by A/D conversion, and the results of digital operations.
Frequency converter
The 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 DC intermediate The circuit smoothes and filters the output of the rectifier circuit, and the inverter circuit inverts the DC power into AC power. For inverters that require a lot of calculations, such as vector control inverters, sometimes a CPU for torque calculation and some corresponding circuits are needed.
There are generally three connection methods between PLC and frequency converter.
①Use 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, which is used as the analog input signal of the inverter to control the output frequency of the inverter. This control method has simple wiring, but it needs to select a PLC output module that matches the input impedance of the frequency converter, and the analog output module of PLC is relatively expensive. In addition, voltage dividing measures need to be taken to make the frequency converter adapt to the voltage signal range of the PLC. When connecting, be sure to separate the wiring to ensure that noise on the main circuit side is not transmitted to the control circuit.
② Use the switching output of PLC to control the frequency converter. The switching output of the PLC can generally be directly connected to the switching input of the frequency converter. This control method has simple wiring and strong anti-interference ability. The switching output of PLC can be used to control the start/stop, forward/reverse rotation, jog, rotation speed, addition and subtraction time, etc. of the frequency converter. It can achieve more complex control requirements, but it can only regulate speed in steps.
When using relay contacts for connection, there may be misoperation due to poor contact. When using transistors for connection, you need 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 frequency converter, you should also pay attention to improper connection of the input signal circuit, which sometimes causes malfunction of the frequency converter. For example, when the input signal circuit uses an inductive load such as a relay, and the noise caused by the surge current generated when the relay is opened and closed may cause malfunction of the frequency converter, this should be avoided as much as possible.
③Connection of 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 their design standards are suitable for applications in industrial environments. A single RS-485 link can connect up to 30 inverters, and the inverter that needs communication can be found based on the address of each inverter or using broadcast information. There needs to be a master controller (master station) in the link, and each inverter is a slave control object (slave station).
PLC inverter controlled 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 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 is adjusted by the external terminals (between 2, 5, 4, 5, multi-terminal speed) input.
3. Press the "MODE" button continuously to exit parameter setting mode.
4. Press the forward button and the motor will start running forward.
5. Press the stop button and the motor stops.
6. Press the reverse button and the motor will start running in reverse.
7．Press the stop button and the motor stops.
8. If you press the reverse button when the motor is rotating forward, the motor will stop first and then reverse; conversely, if you press the forward button when the motor is rotating reversely, the motor will stop first and then rotate forward.
Wiring diagram of plc and frequency converter
Communication method between PLC and frequency converter
1. PLC switching signal controls the frequency converter
The output point and COM point of the PLC (MR type 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 respectively. PLC can control the start, stop, and reset of the inverter through programs; it can also control different combinations of high-speed, medium-speed, and low-speed terminals of the inverter to achieve multi-speed operation. However, because it uses switching values to implement control, its speed regulation curve is not a continuous and smooth curve, and it cannot achieve fine speed regulation.
2. PLC’s analog signal controls the frequency converter
Hardware: FX1N type, FX2N type PLC host, equipped with 1 simple FX1N-1DA-BD extended analog output board; or analog input and output hybrid module FX0N-3A; or FX2N-2DA with two outputs; or four outputs 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 large-scale production lines, the control cables are 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, and can control 32 frequency converters. Disadvantages: The programming workload is large.
4. PLC uses RS-485 Modbus-RTU communication method to control the frequency converter
Mitsubishi's new F700 series inverter uses the RS-485 terminal to communicate with the PLC using the Modbus-RTU protocol. Advantages: PLC programming using Modbus communication method is simpler and more convenient than RS-485 protocol-free method. Disadvantages: PLC programming workload is still large.
5. PLC uses fieldbus to control the frequency converter
Mitsubishi inverters can have various types of built-in communication options, such as the FR-A5NC option for CC-Link fieldbus; the FR-A5AP (A) option for Profibus DP fieldbus; the FR-A5AP (A) option for DeviceNet fieldbus FR-A5ND option and more. Mitsubishi FX series PLC has corresponding communication interface modules to connect with it.
Advantages: fast speed, long distance, high efficiency, stable operation, simple programming, and a large number of connected inverters. Disadvantages: higher 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.
There are three ways to directly operate the inverter without connecting an external controller (such as PLC):
①Buttons on the operation panel;
② Operate the components connected to the terminal blocks (such as buttons and potentiometers);
③ Compound operation (such as setting the frequency on the operation panel and operating the button connected to the terminal block for start/stop control). In order to facilitate 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:
①Controlled by switching quantity;
②Control by analog quantity;
③Controlled by RS485 communication method.
PLC controls the hardware connection of the frequency converter in a switching manner
The frequency converter has many switching terminals, such as forward rotation, reverse rotation, and multi-speed speed control terminals. When a PLC is not used, just connect switches to these terminals to control the frequency converter for forward rotation, reverse rotation, and multi-speed speed control. . When using a PLC to control the frequency converter, if the PLC controls the frequency in a switching manner, it is necessary to connect the switching output terminals of the PLC to the switching input terminals of the frequency converter. In order to detect certain states of the frequency converter, you can also The switching output terminal of the frequency converter is connected to the switching input terminal of the PLC.
The hardware connection of the PLC controlling the frequency converter in a switching mode is shown in the figure below. When the PLC internal program runs and the internal hard contact of the Y001 terminal is closed, it is equivalent to the external switch of the STF terminal of the frequency converter being closed, the STF terminal input is ON, the frequency converter starts the motor to rotate forward, and adjusts the potentiometers connected to terminals 10, 2, and 5. The input voltage of terminal 2 can be changed, thereby changing the frequency of the inverter's output power supply, and thus changing the speed of the motor. If an abnormality occurs inside the inverter, the internal contact between terminals A and C is closed, which is equivalent to the external switch of terminal X001 of the PLC being closed, and the input of terminal X001 is ON.
PLC controls the hardware connection of the frequency converter in analog mode
The frequency converter has some voltage and current analog input terminals. Changing the voltage or current input value of these terminals can change the motor speed. If these terminals are connected to the analog output terminals of the PLC, the PLC can be used to control the frequency converter to adjust the motor. of rotational speed. Analog quantity is a continuously changing quantity. Using the analog quantity control function, the motor speed can be continuously changed (continuously variable speed)
The hardware connection of PLC controlling the frequency converter in analog mode is shown in the figure below. Since Mitsubishi FX2N-32MR PLC does not have an 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 is connected to the analog input terminal of the frequency converter. When the external switch of the STF terminal of the frequency converter is closed, the terminal input is ON, the frequency converter starts the motor to rotate forward, and the digital data generated when the PLC internal program is running is sent to the analog output module (DA module) through the connecting cable, and is processed by it. Convert it into a voltage (analog quantity) in the range of 0~5V or 0~10V and send it to terminals 2 and 5 of the inverter to control the frequency of the inverter output power supply and then control the speed of the motor. If the DA module outputs to the inverter 2 and 5 When the voltage of the terminal changes, the output power frequency of the inverter will also change, and the motor speed will change.
When the PLC controls the analog input terminals of the frequency converter in analog mode, it can also control the switching input terminals of the frequency converter in switching mode at the same time.
PLC uses RS485 communication to control the hardware connection of the inverter
When PLC controls the frequency converter in the switching mode, it needs to occupy more output terminals to connect the input terminals of the corresponding functions of the frequency converter in order to control the frequency converter such as forward rotation, reverse rotation and stop; PLC controls the frequency converter in the analog mode. When, the DA module is needed to control the frequency speed of the inverter. If the PLC controls the inverter through RS485 communication, only one RS485 communication cable (containing 5 core wires) is needed to directly send various control and frequency modulation commands to the inverter. The inverter will respond to the commands sent by the PLC through the RS485 communication cable. The instructions 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, not only can two devices be connected for communication, but multiple devices (up to 32 devices can be connected in parallel) can be connected to form a distributed system for mutual communication.
1. RS485 communication port of the frequency converter
Mitsubishi FR500 series inverter has a PU port for connecting the operation panel. This interface can be used as an RS485 communication port. When using RS485 to communicate with other devices, you need to pull out the operation panel plug (RJ45 plug) from the PU port. Then insert one end of the RS485 communication cable into the PU port, and connect the other end of the communication cable to a PLC or other device. The appearance of the PU port of Mitsubishi FR500 series inverter and the function description of each pin are shown in the figure below.
Mitsubishi FR500 series inverters have only one RS485 communication port (PU port). Panel operation and RS485 communication cannot be performed at the same time. In addition to a PU interface, Mitsubishi FR700 series inverters are also equipped with a separate RS485 communication port (wiring strip). Dedicated to RS485 communication. The appearance of the RS485 communication port of Mitsubishi FR700 series inverter and the function description of each pin are shown in the figure below. Each functional terminal of the communication port has 2. One terminal is connected to an RS485 communication device, and the other terminal is connected to an RS485 communication device. , if there is no next device, the terminal resistance switch should be moved to the "100Ω" side.
2.PLC’s RS485 communication port
Mitsubishi FX PLC generally does not have an RS485 communication port. If you want to communicate with the inverter through RS485, you must install the FX2N-485BD communication board on the PLC. The appearance and terminals of the 485BD communication board are shown in Figure (a) below. The installation method of the communication board is shown in Figure (b) below.
(b)Installation method
3. RS485 communication connection between inverter and PLC
(1) RS485 communication connection between a single inverter and PLC
The RS485 communication connection between a single frequency converter and a PLC is shown in the figure below. When the two are connected, the sending terminal (+-) of one device should be connected to the receiving terminal (+-) of the other device, and the receiving terminal ( +-) should be connected to the sending terminal (+-) of another device respectively.
(2) RS485 communication connection between multiple inverters and PLC
The RS485 communication connection between multiple inverters and PLC is shown in the figure below. It can realize the operation of multiple inverters controlled by one PLC.
PLC controls the circuit, program and parameter setting of the forward and reverse rotation of the frequency converter driven motor
1.Hardware connection circuit diagram of PLC and frequency converter
The circuit diagram of PLC controlling the forward and reverse rotation of the inverter-driven motor in a switching manner is shown in the figure below.
2. Parameter settings of the frequency converter
When using PLC to control the frequency converter, it is necessary to set the relevant parameters of the frequency converter.
3.Write PLC control program
After the relevant parameters of the frequency converter are set, the corresponding PLC control program must be written using programming software and downloaded to the PLC.
PLC controls the circuit, program and parameter setting of the inverter-driven motor running at multiple speeds
The frequency converter can adjust speed continuously or in stages. The FR-500 series frequency converter has three control terminals: RH (high speed), RM (medium speed) and RL (low speed). Through the combined input of these three terminals, Can achieve 7-speed speed control. If the output terminals of the PLC are connected to these terminals of the frequency converter, the PLC can be used to control the frequency converter to drive the motor to run at multiple speeds.
1.Hardware connection circuit diagram of PLC and frequency converter
The circuit diagram of PLC controlling the multi-speed operation of the inverter-driven motor in a switching manner is shown in the figure below.
3.Write PLC control program
The PLC program that uses PLC to control the multi-speed operation of the inverter-driven motor is as shown below.
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