Abstract Frequency converters are now widely used in industrial control. In practical applications, frequency converters are generally not independent systems and need to be used in conjunction with DCS or PLCs to achieve remote control. Hollysys LK series PLCs can easily achieve remote control of frequency converters via communication methods.
Keywords : Hollysys LK series PLC; ModBus; RS485
1 Introduction
With enterprises placing greater emphasis on energy conservation and emission reduction, and the continuous improvement of industrial automation, PLCs and frequency converters have been widely used in the field of industrial control. As a control layer device, the PLC acquires and controls the frequency converter. This article mainly introduces the applications of Hollysys LK series PLCs in these areas. The LK series PLC is a large-scale PLC launched by Hollysys, whose powerful expansion functions and communication interfaces can meet the requirements of various industrial control applications.
2. PLC and Inverter Communication System Design
The PLC-inverter communication system consists of a PLC, an inverter, and other peripheral devices. The PLC part mainly includes a backplane, CPU, I/O modules, and a communication module. The inverter can be remotely controlled in two ways. The first method connects the inverter's control signals directly to the PLC's I/O module via a connecting cable. The second method uses an RS485 interface with the ModBus communication protocol for remote control. The advantage of the second method is that it eliminates the need for cable wiring and avoids occupying I/O channels, thus effectively reducing costs. This article introduces the design and implementation method of the second method. The hardware structure diagram of the PLC-inverter communication system is shown in Figure 1, where the addresses of the two inverters are set as #2 and #3, respectively.
Figure 1. Hardware structure diagram of the PLC and frequency converter communication system
3. PLC and Inverter Communication Parameter Settings
The PLC and frequency converter use ModBus communication, with the PLC acting as the master and the frequency converter as the slave, and their communication parameters must be consistent. In this system, the communication rate is 19200bps, the data bits are 8 bits, the parity is no parity, and the stop bit is 1. The PLC and frequency converter communication parameter setting instructions are shown in Figure 2, and the communication call instructions are shown in Figure 3. The communication call instruction HS_ModbusMaster_COM2 does not support multiple calls within the program. In practical applications, it is sometimes necessary to read and write data to different devices. In such cases, intermediate registers can be established, and the control of different devices can be achieved by continuously changing the values of these registers.
Figure 2. Communication parameter setting instructions between PLC and frequency converter
Figure 3 Communication and call instructions between PLC and frequency converter
4. Application Features
The application of serial communication control for frequency converters is becoming increasingly widespread. Compared with the traditional cable wiring control method, this system has the following main advantages.
(1) The control circuit of the frequency converter is simplified.
Most industrial communication physical layers use RS485 interfaces. Communication between the PLC and the frequency converter can be achieved using ordinary shielded twisted-pair cables, saving the cost and labor of connecting control cables and avoiding wiring errors.
(2) Direct digital exchange between PLC and frequency converter
The two communicate via direct digital communication. The CPU directly reads and writes the inverter's parameters, which not only saves on the cost of I/O modules but also offers higher accuracy.
References
[1] Hollysys PLC Technical Data, Beijing Hollysys Automation Drive Technology Co., Ltd.
