A Programmable Logic Controller (PLC) is a digital control device that performs calculations and operations. As a replacement for traditional relays, PLCs are widely used in various fields of industrial control. Because PLCs can be modified by 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 are often used in conjunction with the frequency converters, such as the automatic soot blowing system in our factory's secondary catalytic converter. A PLC provides control signals and on/off signals for commands. A PLC system consists of three parts: a central processing unit, input/output modules, and a programming unit. This article introduces the precautions to take when using frequency converters and PLCs in conjunction.
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 (Programmable Logic Controller) using relay contacts or components with relay contact switching characteristics (such as transistors) to obtain operating status commands, as shown in Figure 1.
Figure 1 Connection method of operation signal
When using relay contacts, malfunctions often 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 the reliability of the system.
When designing the input signal circuit of a frequency converter, it is important to note 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 generated by the inrush current produced by the relay's opening and closing may cause malfunctions in the frequency converter, which should be avoided as much as possible. Figures 2 and 3 show examples of correct and incorrect wiring.
Figure 2 Inverter input signal access method
Figure 3 shows an incorrect connection method for the input signal.
When input switch signals enter 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 4 shows the connection method for input signal interference prevention.
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. Figure 5 shows the signal connection diagram between the PLC and the inverter.
When the voltage signal ranges of the frequency converter and the PLC are different—for example, the input signal of the frequency converter is 0–10V, while the output voltage signal range of the PLC is 0–5V; or the output signal voltage range of one side of the PLC is 0–10V while the input voltage signal range of the frequency converter is 0–5V—due to the limitations of the allowable voltage and current of the frequency converter and transistors, a current-limiting resistor and a voltage divider must be connected in series to ensure that the switching operation does not exceed the corresponding capacity of the PLC and the frequency converter. Furthermore, care should be taken to separate the wiring during connection to ensure that noise from the main circuit side does not travel to the control circuit.
Frequency converters typically also output corresponding analog monitoring signals to the outside via wiring terminals. The electrical signal range is usually 0–10V/5V and the current signal range is 0/4–20mA. 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 guaranteeing system reliability and minimizing errors. Furthermore, since the components of these monitoring systems differ, any unclear points should be addressed by consulting the manufacturer.
In addition, when using a PLC for sequential control, there is a certain time delay because the CPU needs time to process data. This should be taken into account when using 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 the 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.
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.
