In current production conditions, when using frequency converters to construct automatic control systems, it is often necessary to use a PLC in conjunction with the frequency converter , such as in bearing cleaning, packaging paper printing, and PCB board fabrication. The PLC can provide various control signals and on/off signals of commands through output points or communication. Below are some points to note 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, jog, and slow-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. 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's 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 inrush 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. 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, connecting the collector of the external transistor to the PLC through a diode.
2. Input of numerical signals
Inverters also have numerical command signal inputs (such as frequency and voltage), which can be divided into analog inputs and analog outputs. Analog inputs are given externally through terminals, typically via 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 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 some 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 reaching the control circuit.
Note: The input impedance on the PLC side must be such that the voltage and current in the circuit do not exceed the circuit's allowable values to ensure system reliability and reduce errors.
In addition, when using PLC for sequential control, the time required for data processing, as well as the different order of program writing and the different use of instructions, will cause a certain time delay in the system operation. Therefore, these factors should be considered when performing more precise control.
Because frequency converters generate strong electromagnetic interference during operation, it is important to ensure that the PLC does not malfunction due to noise generated by the frequency converter's main circuit circuit breaker and switching devices.
Therefore, the following points should be noted when connecting the frequency converter to 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) The level of noise interference can be improved by using shielded wires and twisted pairs.
