A Programmable Logic Controller (PLC) is a digital control device for computation and operation. As a replacement for traditional relays, PLCs are widely used in various fields of industrial control. Because PLCs can modify the control process using software and have advantages such as small size, flexible assembly, simple programming, strong anti-interference ability, and high reliability, they are particularly suitable for operation in harsh environments. When using frequency converters to construct automatic control systems, PLCs and frequency converters are often used in conjunction, such as the automatic soot blowing system in our factory's secondary catalytic converter. A PLC provides control signals and on/off signals for instructions. A PLC system consists of three parts: a central processing unit, input/output modules, and a programming unit. This article introduces the precautions required when using frequency converters and PLCs together.
1. Input of switch command signal
The input signals of a frequency converter include switching command signals that control operating states such as run/stop, forward/reverse rotation, and micro-motion. Frequency converters typically connect to a PLC using relay contacts or components with relay contact switching characteristics (such as transistors) to obtain operating status commands, as shown in Figure 1. When using relay contacts, malfunctions often occur due to poor contact; when using transistors, factors such as the transistor's voltage and current capacity must be considered to ensure system reliability. When designing the input signal circuit of a frequency converter, it should also be noted that improper connection of the input signal circuit can sometimes cause malfunctions. For example, when the input signal circuit uses inductive loads such as relays, the noise from the inrush current generated by the relay opening and closing may cause malfunctions of the frequency converter, which should be avoided as much as possible. Figures 2 and 3 show examples of correct and incorrect wiring. When the input switching signal enters the frequency converter, crosstalk sometimes occurs between the external power supply and the frequency converter control power supply (DC24V). The correct connection is to use the PLC power supply and connect the collector of the external transistor to the PLC through a diode, as shown in Figure 4.
Figure 1 Connection method of operation signal
Figure 2 Inverter input signal access method
Figure 3 shows an incorrect connection method for the input signal.
Figure 4 shows the connection method for input signal anti-interference.
2. Input of numerical signals
Inverters also have numerical (such as frequency, voltage, etc.) command signal inputs, which can be divided into digital inputs and analog inputs. Digital inputs are mostly given through the keyboard operation and serial interface on the inverter panel; analog inputs are given externally through wiring terminals, usually through a voltage signal of 0-10V/5V or a current signal of 0/4-20mA. Since the interface circuit varies depending on the input signal, the PLC output module must be selected according to the input impedance of the inverter. When the voltage signal ranges of the inverter and PLC are different, such as when the inverter's input signal is 0-10V and the PLC's output voltage signal range is 0-5V; or when the PLC's output signal voltage range is 0-10V and the inverter's input voltage signal range is 0-5V, due to the limitations of the inverter and transistor's allowable voltage and current, a current-limiting resistor and a voltage divider must be connected in series to ensure that the switching does not exceed the corresponding capacity of the PLC and inverter. In addition, care should be taken to separate the wiring during connection to ensure that noise from the main circuit side does not reach the control circuit. Typically, the frequency converter also outputs corresponding monitoring analog signals to the outside via terminals. The range of electrical signals is usually 0-10V/5V and 0/4-20mA current signals. In either case, it is important to ensure that the input impedance on the PLC side is sufficient to prevent the voltage and current in the circuit from exceeding the circuit's allowable values, thus ensuring system reliability and reducing errors. Furthermore, since the components of these monitoring systems differ, any unclear points should be consulted with the manufacturer. Additionally, when using a PLC for sequential control, a certain time delay exists due to the CPU's data processing time; this should be considered for more precise control. Because frequency converters generate strong electromagnetic interference during operation, to ensure that the PLC does not malfunction due to noise from the frequency converter's main circuit circuit breaker and switching devices, the following points should be noted when connecting the frequency converter and PLC:
(1) The PLC itself should be grounded according to the specified wiring standards and grounding conditions. It should also be noted that it should avoid using the same grounding wire as the frequency converter, and the two should be kept as separate as possible when grounding.
(2) When the power supply conditions are not good, noise filters and transformers for noise reduction should be connected to the power supply lines of the PLC power supply module and input/output module. In addition, if necessary, corresponding measures should also be taken on the inverter side.
(3) When the frequency converter and PLC are installed in the same control cabinet, the wires related to the frequency converter and the PLC should be kept separate as much as possible.
(4) The level of noise interference can be improved by using shielded wires and twisted pairs.
3. Conclusion
When connecting PLCs and frequency converters, since both involve controlling high-voltage electricity with low-voltage electricity, attention should be paid to interference during the connection to avoid malfunctions of the frequency converter due to interference, or damage to the PLC or frequency converter due to improper connection.
