PLCs and frequency converters have a relationship of inclusion and being included. Both PLCs and frequency converters can execute specific instructions to control motors. A PLC is a program input and execution hardware, while a frequency converter is one of them. However, the scope of a PLC is larger than that of a frequency converter. It can be used to control more things, has a wider range of applications, and more powerful performance. Of course, the control precision of a PLC is also greater.
Inverters 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 can be programmed 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 direct information transmission between slave stations is also prohibited.
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.
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1. System program memory
The system program memory stores the system program written by the programmable controller (PLC) manufacturer and is permanently stored in ROM, which cannot be directly modified by the user. 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 PLC, ensuring the entire PLC works systematically; the second part is the user instruction interpreter, which translates the PLC's programming language into machine language instructions, which are then executed by the CPU; and the third part consists of standard program modules and system call programs.
2. User program memory
The application program developed according to control requirements is called the user program. 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 the user program memory. Flash memory does not require a backup battery and will not lose data even if the TV is out of service.
3. Working data storage
The working data memory stores working data, including 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 extensive 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. The PLC's digital outputs can be used to control the frequency converter's start/stop, forward/reverse, jogging, speed, and acceleration/deceleration time, 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, 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.
Wiring diagram of PLC and frequency converter
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 ProfibusDP 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
