(I) Analog Control
In industrial automation control, analog control is a common method for PLC to control inverters. PLC outputs 0-10V or 4-20mA analog signals to the analog input terminals of the inverter through analog output modules. For example, in a conveying system, PLC outputs corresponding analog signals to the inverter according to the material flow detected by the sensor, and the inverter adjusts the motor speed to match the conveyor belt speed with the material flow.
The inverter adjusts the output frequency according to the size of the received analog signal to control the motor speed. Usually, the wiring of this control method is relatively simple. However, it should be noted that the PLC output module that matches the input impedance of the inverter should be selected, and the analog output module of the PLC is relatively expensive. In addition, voltage division measures should be taken to make the inverter adapt to the voltage signal range of the PLC. When connecting, care should be taken to separate the wiring to ensure that the noise on the main circuit side is not transmitted to the control circuit. For example, when the output signal voltage range of the PLC is 0-10V and the input voltage signal range of the inverter is 0-5V, due to the limitations of the allowable voltage and current of the inverter and transistor, it is necessary to connect the current limiting resistor and the voltage divider circuit in series, adjust the inverter parameters and jumpers to change the inverter voltage and analog signal to ensure that the corresponding capacity of the PLC and inverter interface circuit is not exceeded during opening and closing.
(II) Digital Control
The control of the inverter can be realized by connecting the digital output point of the PLC to the digital input terminal of the inverter. For example, the start and stop, forward and reverse rotation, and multi-speed operation of the inverter can be controlled by digital signals. Taking the fan control system as an example, when the indoor temperature is high, the PLC outputs a digital signal to make the inverter drive the motor to run at high speed; when the temperature is moderate, it switches to medium speed; when the temperature is low, it switches to low speed.
When using relay contacts for connection, there is sometimes a phenomenon of misoperation due to poor contact. When using transistors 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 also be noted that improper connection of the input signal circuit may sometimes cause malfunction of the inverter. For example, when the input signal circuit uses an inductive load such as a relay, the noise caused by the surge current generated when the relay is opened and closed may cause malfunction of the inverter, which should be avoided as much as possible.
(III) Communication control
Communication control is the use of communication protocols to achieve data exchange and control instruction transmission between PLC and inverter. Common communication protocols include Modbus RTU, Modbus TCP, Profibus-DP, etc. In a large automated production line, multiple PLCs and multiple inverters are connected through a communication network to achieve centralized control and management.
For example, in an automated production line in an automobile manufacturing plant, multiple PLCs and multiple inverters are connected through a communication network. PLCs can read the operating parameters of the inverter in real time, such as frequency, current, voltage, etc., and remotely set and control the inverter. This control method has many advantages, such as remote control, centralized management, real-time monitoring, etc. However, there are also some disadvantages, such as a large programming workload and the need for in-depth understanding of communication protocols.
