However, PLCs have a wider scope than frequency converters, can control more things, have broader applications, and are more powerful. Of course, PLCs also offer greater control precision. Frequency converters cannot be programmed to change parameters such as power supply frequency and voltage; their output frequency can be set to a fixed value or dynamically controlled by a PLC.
PLCs are programmable and used to control electrical components or perform functions, communication, and other tasks.
Communication between the PLC and the frequency converter must 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. A slave station cannot send data first unless requested to do so by the master station, and slave stations cannot directly transmit information to each other.
PLC basic structure diagram
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 stores the system program written by the programmable logic controller (PLC) manufacturer and is permanently stored in ROM; users cannot directly modify it. The quality of the system program largely determines the performance of the PLC.
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, which converts the programmable controller's programming language 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 developed according to control requirements are called user programs. The user program memory is used to store various user programs written by the user for specific control tasks using the specified programmable controller programming language.
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 when power is off.
3. Working data storage
The working data memory stores working data, such as ON/OFF states and numerical data used in the user program. Component image registers and data tables are allocated within the working data area. The component image registers store switching quantities, output states, and the ON/OFF states of internal devices such as timers, counters, and auxiliary relays. The data table stores various data, including variable parameter values during user program execution, digital quantities obtained from A/D conversion, and the results of digital calculations.
Basic structure diagram of frequency converter
A frequency converter is a device that transforms mains frequency power (50Hz or 60Hz) into AC power of various frequencies to enable variable speed operation of a motor. The control circuit controls the main circuit, the rectifier circuit converts AC to DC, the DC intermediate circuit smooths and filters the output of the rectifier circuit, and the inverter circuit converts the DC back into AC. For frequency converters like vector control frequency converters that require a large amount of computation, a CPU for torque calculation and other corresponding circuits are sometimes also needed.
There are generally three ways to connect a PLC to a frequency converter.
① Using the analog output module of a PLC to control a frequency converter: The PLC's analog output module outputs a 0-5V voltage signal or a 4-20mA current signal as the analog input signal to the frequency converter, controlling its output frequency. This control method has simple wiring, but requires selecting a PLC output module that matches the frequency converter's input impedance. Furthermore, PLC analog output modules are relatively expensive. Additionally, voltage divider measures are needed to adapt the frequency converter to the PLC's voltage signal range. During connection, care must be taken to separate the wiring to ensure that noise from the main circuit side does not transmit to the control circuit.
② Controlling the frequency converter using the PLC's digital outputs. The PLC's digital outputs can generally be directly connected to the frequency converter's digital inputs. This control method has simple wiring and strong anti-interference capabilities. Using the PLC's digital outputs, the frequency converter's start/stop, forward/reverse, jogging, speed, and acceleration/deceleration time can be controlled, achieving relatively complex control requirements, but only stepped speed regulation is possible.
When using relay contacts for connection, malfunctions can sometimes occur due to poor contact. When using transistors for connection, factors such as the transistor's voltage and current capacity must be considered to ensure system reliability. Furthermore, when designing the inverter's input signal circuit, improper connection can also cause malfunctions. For example, when the input signal circuit uses inductive loads such as relays, the surge current generated when the relay opens and closes can cause noise that may lead to inverter malfunctions; this should be avoided as much as possible.
③ PLC and RS-485 communication interface connection. All standard Siemens frequency converters have an RS-485 serial interface (some also provide an RS-232 interface), using a two-wire connection. Its design standard is suitable for industrial applications. A single RS-485 link can connect up to 30 frequency converters, and the required communication frequency converter can be located based on its address or by using broadcast information. The link requires a master controller (master station), and each frequency converter is a subordinate control object (slave station).
PLC-controlled motor forward and reverse rotation wiring diagram
1. After connecting the wires according to the wiring diagram, turn on 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 from the external terminals (STF, STR), and the speed adjustment is input from the external terminals (between 2 and 5, between 4 and 5, and multi-speed).
3. Press the "MODE" button repeatedly to exit the parameter setting mode.
4. Press the forward rotation button to start the motor in forward rotation.
5. Press the stop button to stop the motor.
6. Press the reverse button to start the motor in reverse.
7. Press the stop button to stop the motor.
8. If the reverse button is pressed while the motor is rotating forward, the motor will stop first and then rotate in reverse; conversely, if the forward button is pressed while the motor is rotating in reverse, the motor will stop first and then rotate in forward.
Communication methods between PLC and frequency converter
1. PLC's digital input signals control the frequency converter
The output and COM points of the PLC (MR or MT type) are directly connected to the inverter's STF (forward start), RH (high speed), RM (medium speed), RL (low speed), and input SG ports, respectively. The PLC can control the inverter's start, stop, and reset via a program; it can also control different combinations of the inverter's high-speed, medium-speed, and low-speed terminals to achieve multi-speed operation. However, because it uses switching signals for control, its speed regulation curve is not a continuous and smooth curve, and it cannot achieve precise speed adjustment.
2. PLC analog signal control of frequency converter
Hardware: FX1N or FX2N PLC main unit, configured with a simple FX1N-1DA-BD extended analog output board; or an FX0N-3A analog input/output hybrid module; or a two-output FX2N-2DA; or a four-output FX2N-4DA module, etc. Advantages: Simple and convenient PLC programming, smooth and continuous speed control curve, and stable operation.
Disadvantages: In large-scale production lines, control cables are long, especially when the DA module uses voltage signal output, resulting in a large voltage drop in the line, which affects the stability and reliability of the system.
3. The PLC uses RS-485 communication to control the frequency converter.
This is the most common method, where the PLC is programmed using RS serial communication instructions. Advantages: simple hardware, lowest cost, and can control up to 32 frequency converters. Disadvantages: relatively large programming workload.
4. The PLC uses RS-485 Modbus-RTU communication to control the frequency converter.
Mitsubishi's new F700 series inverters use RS-485 terminals to communicate with PLCs via the Modbus-RTU protocol. Advantages: PLC programming using Modbus communication is simpler and more convenient than the RS-485 protocol-less method. Disadvantages: PLC programming still requires a significant amount of work.
5. The PLC uses a fieldbus method to control the frequency converter.
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; and the FR-A5ND option for DeviceNet fieldbus, etc. Mitsubishi FX series PLCs have corresponding communication interface modules for interface with them.
Advantages: High speed, long distance, high efficiency, stable operation, simple programming, and can connect to a large number of frequency converters. 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 frequency converters.
PLC and frequency converter communication wiring diagram
Mitsubishi PLC control console inverter case study
There are three ways to directly operate a frequency converter without an external controller (such as a PLC):
① Buttons on the control panel;
② Components connected to the operating terminals (such as buttons and potentiometers);
③Combined operation (such as setting the frequency on the operation panel and controlling the start/stop by operating the buttons connected to the terminal blocks). For ease of operation and to make full use of the frequency converter, a PLC can also be used to control the frequency converter.
There are three basic ways to control a frequency converter using a PLC:
① Controlled by on/off signals;
② Controlled using analog signals;
③ Controlled via RS485 communication.
The PLC controls the inverter via a switch signal method.
Inverters have many digital input/output terminals, such as forward, reverse, and multi-speed control terminals. Without a PLC, simply connecting switches to these terminals allows for forward, reverse, and multi-speed control of the inverter. When using a PLC to control the inverter, if the PLC controls the inverter via digital inputs, the PLC's digital output terminals need to be connected to the inverter's digital input terminals. To detect certain states of the inverter, the inverter's digital output terminals can also be connected to the PLC's digital input terminals.
The hardware connection for PLC control of the frequency converter via digital input/output is shown in the diagram below. When the PLC's internal program closes the internal hard contact of terminal Y001, it is equivalent to closing the external switch of the frequency converter's STF terminal, making the STF terminal input ON. The frequency converter starts the motor and rotates forward. Adjusting the potentiometer connected to terminals 10, 2, and 5 changes the input voltage at terminal 2, thereby changing the frequency of the frequency converter's output power supply and thus changing the motor speed. If an internal malfunction occurs in the frequency converter, the internal contact between terminals A and C closes, which is equivalent to closing the external switch of the PLC's X001 terminal, making the X001 terminal input ON.
The PLC controls the inverter via analog signals.
Frequency converters have analog voltage and current input terminals. Changing the voltage or current input values of these terminals can change the speed of the motor. If these terminals are connected to the analog output terminals of a PLC, the PLC can be used to control the frequency converter to adjust the speed of the motor. Analog signals are continuously changing quantities, and analog signal control functions can be used to continuously change the speed of the motor (stepless speed regulation).
The hardware connection for PLC to control the frequency converter via analog signals is shown in the diagram below. Since the Mitsubishi FX2N-32MR PLC lacks analog output functionality, an analog output module (such as FX2N-4DA) needs to be connected to it. The output terminals of the analog output module are then connected to the analog input terminals of the frequency converter. When the external switch of the frequency converter's STF terminal is closed, the input of that terminal is ON, and the frequency converter starts the motor and rotates forward. The digital data generated by the PLC's internal program is sent to the analog output module (DA module) via the connecting cable. The DA module converts this data into a voltage (analog signal) within the range of 0-5V or 0-10V, which is then sent to terminals 2 and 5 of the frequency converter to control the frequency of the frequency converter's output power, thereby controlling the motor speed. If the voltage output from the DA module to terminals 2 and 5 of the frequency converter changes, the frequency of the frequency converter's output power will also change, and the motor speed will change accordingly.
When a PLC controls the analog input terminals of a frequency converter in analog mode, it can also simultaneously control the digital input terminals of the frequency converter in digital mode.
The PLC controls the inverter via RS485 communication.
When a PLC controls a frequency converter using digital inputs, it requires numerous output terminals to connect to the corresponding input terminals of the frequency converter to control functions such as forward rotation, reverse rotation, and stop. When a PLC controls a frequency converter using analog inputs, a DA module is needed for frequency and speed control. However, if the PLC controls the frequency converter via RS485 communication, only one RS485 communication cable (containing 5 wires) is needed to directly send various control and frequency adjustment commands to the frequency converter. The frequency converter can then execute the corresponding function control based on the instructions sent by the PLC through the RS485 communication cable.
RS485 communication is a widely used communication method in industrial control, with strong anti-interference capabilities and a communication distance of tens to thousands of meters. 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.
1. RS485 communication port of the frequency converter
The Mitsubishi FR500 series inverter has a PU port for connecting the operation panel. This port can also be used as an RS485 communication port. When communicating with other devices via RS485, the operation panel plug (RJ45 plug) needs to be unplugged from the PU port, and one end of the RS485 communication cable needs to be inserted into the PU port. The other end of the communication cable is connected to the PLC or other equipment. The appearance of the Mitsubishi FR500 series inverter PU port and the functions of each pin are shown in the figure below.
The Mitsubishi FR500 series inverter has only one RS485 communication port (PU port), meaning panel operation and RS485 communication cannot be performed simultaneously. The Mitsubishi FR700 series inverter, in addition to a PU interface, is also equipped with a separate RS485 communication port (terminal block) dedicated to RS485 communication. The appearance and functions of the RS485 communication port on the Mitsubishi FR700 series inverter are shown in the figure below. Each function terminal of the communication port has two terminals: one connects to one RS485 communication device, and the other connects to the next RS485 communication device. If there is no next device, the terminating resistor switch should be set to the "100Ω" side.
2. PLC's RS485 communication port
Mitsubishi FX PLCs generally do not have an RS485 communication port. To communicate with a frequency converter via RS485, an FX2N-485BD communication board must be installed on the PLC. The appearance and terminals of the 485BD communication board are shown in Figure (a) below, and the installation method of the communication board is shown in Figure (b) below.
(a) External shape
(b) Installation method
3. RS485 communication connection between the frequency converter and the PLC
(1) RS485 communication connection between a single frequency converter and the PLC
The RS485 communication connection between a single frequency converter and a PLC is shown in the figure below. When connecting the two, the transmitting terminal (+-) of one device should be connected to the receiving terminal (+-) of the other device, and the receiving terminal (+-) should be connected to the transmitting terminal (+-) of the other device.
(2) RS485 communication connection between multiple frequency converters and PLC
The RS485 communication connection between multiple frequency converters and the PLC is shown in the figure below. It can enable one PLC to control the operation of multiple frequency converters.
Circuit, program, and parameter settings for PLC control of frequency converter to drive motor forward and reverse rotation
1. Hardware connection diagram between PLC and frequency converter
The circuit diagram below shows how a PLC controls the forward and reverse rotation of a frequency converter-driven motor using a switching method.
2. Parameter settings for the frequency converter
When using a PLC to control a frequency converter, it is necessary to set relevant parameters for the frequency converter, as shown in the table below.
3. Write the PLC control program
After setting the relevant parameters of the frequency converter, the corresponding PLC control program needs to be written using programming software and downloaded to the PLC. The PLC program for controlling the frequency converter to drive the motor in forward and reverse rotation is shown in the figure below.
Circuit, program, and parameter settings for PLC control of inverter-driven motors operating at multiple speeds.
The frequency converter can provide continuous speed control or speed adjustment in multiple speed ranges. The FR-500 series frequency converter has three control terminals: RH (high speed), RM (medium speed), and RL (low speed). By combining the inputs of these three terminals, seven speed ranges can be controlled. If the output terminals of a PLC are connected to these terminals of the frequency converter, the PLC can control the frequency converter to drive the motor at multiple speeds.
1. Hardware connection diagram between PLC and frequency converter
The circuit diagram below shows how a PLC controls a frequency converter to drive a motor at multiple speeds using a switching method.
2. Write the PLC control program
The following figure shows a PLC program that uses switching signals to control the inverter-driven motor to operate at multiple speeds.
