I. Three connection methods
1. Switching input/output connection
The inverter is controlled using the PLC 's digital outputs. The PLC's digital outputs can generally be directly connected to the inverter's digital inputs. This control method is simple to wire and has strong anti-interference capabilities. The PLC's digital outputs can control the inverter's start/stop, forward/reverse, jogging, multi-speed, and time adjustment, achieving relatively complex control requirements, but only stepped speed regulation is possible.
2. Analog signal online
The PLC's analog output module controls the frequency converter. The PLC 's analog output module outputs a 0-10V voltage signal or a 4-20mA current signal as the analog input signal to the frequency converter, controlling the frequency converter's output frequency.
This control method has simple wiring, but it requires selecting a PLC output module that matches the input impedance of the frequency converter. The analog output module of the PLC is relatively expensive. In addition, voltage division measures are required to adapt the frequency converter to the voltage signal range of the PLC. When connecting, care should be taken to separate the wiring.
3. Communication connection
The PLC and the frequency converter are connected via an RS-485 communication interface. A large number of frequency converters have an RS-485 serial interface (some also provide an RS-232C interface), which uses 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 converters that need to communicate can be located based on their addresses or by using broadcast information. The link requires a master controller (master station), while each frequency converter is a subordinate control object (slave station).
Advantages of using serial communication
① It greatly reduces the amount of wiring.
② Control functions can be changed without rewiring.
③ The parameters of the frequency converter can be set and modified through the serial interface.
④ It can continuously monitor and control the characteristics of the frequency converter.
Precautions for dual-unit system
1. Precautions for switch signals
Inverters typically connect to PLCs using relay contacts or components with relay contact switching characteristics (such as transistors) to obtain operating status commands, as shown in Figures 1(A) and (B).
Figure 1A shows the operation mode of connecting the relay type PLC output to the frequency converter.
Figure 1B shows the operation mode of connecting the transistor-type PLC output to the frequency converter.
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 also cause malfunctions. For example, when the input signal circuit uses inductive loads such as relays, the surge current generated by the relay's opening and closing may damage or cause failure of internal components of the frequency converter, leading to malfunctions. Therefore, this situation should be avoided as much as possible. Figures 2 and 3 show examples of correct and incorrect wiring.
Figure 2. Inverter input signal connection method
Figure 3 Incorrect connection of inverter input signal
When the input switch 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 4. Anti-interference connection method for input signal
2. Precautions for analog signals
Analog inputs are provided externally via terminals, typically through voltage signals of 0–10V/5V or current signals of 0/4–20mA. Since the interface circuit varies depending on the input signal, the PLC's output module must be selected based on the inverter's input impedance.
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—resistors need to be connected in parallel or series to limit the current or divert part of the voltage due to the limitations of the allowable voltage and current of the frequency converter and transistors. This ensures 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 control circuit from the main circuit during wiring. Shielded cables should ideally be used for the control circuit to prevent noise from the main circuit from being transmitted to the control circuit.
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. This is crucial for ensuring system reliability and minimizing errors. Furthermore, when using a PLC for sequential control, data processing time, as well as variations in the program's order and the use of instructions, can all lead to time delays during system operation. Therefore, these factors 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 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, reactors and devices that can reduce noise 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 input side of the frequency converter.
(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) Improve the level of noise interference by using shielded wires and twisted pairs.
