When a Variable Frequency Drive (hereinafter referred to as VFD) is used in an automatic control system, in most cases it will be used together with a PLC (Programmable Logic Controller). Therefore, how to ensure the correct connection between the VFD and the PLC becomes a key step in the success of the entire variable frequency control system.

1. PLC Switch Command Signal Input
The input signals of the VFD include switch-type command signals for operation status such as Run/Stop, Forward/Reverse, and Jog. The VFD usually connects to the PLC using relay contacts or electrical components with relay contact switching characteristics (such as transistors), to execute operational commands.
When using relay contacts, poor contact often causes malfunctions; when connecting via transistors, one must consider factors such as the voltage and current capacity of the transistor itself to ensure system reliability.
In designing the VFD's input signal circuit, attention should also be paid to the fact that improper wiring may lead to misoperation of the inverter. For example, if inductive loads such as relays are used in the input signal circuit, the surge current caused by relay switching may generate noise that leads to incorrect operation of the inverter. This situation should be avoided as much as possible.
When external switch signals enter the inverter, interference may occur between the external power supply and the inverter's control power supply (DC24V). The correct wiring method is to use the PLC power supply and connect an external collector diode to the PLC.

2. Digital Signal Input of the VFD
There are also digital instruction signals input into the VFD (such as frequency, voltage, etc.), which can be divided into digital inputs and analog inputs. Digital inputs are mostly provided through keypad operations and serial interfaces on the VFD panel; for analog inputs, the signal is typically given externally through terminal blocks, generally as 0-10V / 5V voltage signals or 0/4-20mA current signals. Since the interface circuits vary depending on the input signal, the input impedance of the VFD must be considered when selecting the PLC output module.
When the voltage signal ranges of the VFD and the PLC differ, for example, if the input signal is 0-10V but the PLC outputs only 0-5V, or vice versa, then factors such as the allowable voltage and current of the VFD and transistor must be taken into account. A current-limiting resistor or voltage divider should be connected in series to ensure that the switching does not exceed the respective capacities of the PLC and the VFD. Additionally, during wiring, separate cabling should be used to prevent noise from the main circuit side from being transmitted to the control circuit.
Generally, the VFD also outputs corresponding monitoring analog signals through terminal blocks. These signals are usually in the range of 0-10V / 5V or 0/4-20mA current signals. In either case, you should note: the input impedance on the PLC side should ensure that the voltage and current in the circuit do not exceed the allowable values, to guarantee system reliability and reduce errors. Moreover, these monitoring components vary across models — if uncertain, consult the manufacturer.

Additionally, when the PLC is used for sequential control, the CPU requires time to process data, resulting in some time delay. Therefore, more precise control considerations must be made. During its operation, the VFD generates strong electromagnetic interference. To ensure the PLC does not malfunction due to noise generated by the VFD’s main circuit breaker and switching devices, the following points should be noted:

1. The PLC itself should be wired according to specified wiring standards and conditions. Pay attention to avoid sharing wiring with the VFD, and keep them separated as much as possible during installation.
2. When the power supply condition is poor, a noise filter and noise suppression transformer should be connected to the PLC power module and I/O module power lines. If necessary, appropriate measures should also be taken on the VFD side.
3. When the VFD and PLC are installed in the same control cabinet, related wiring and PLC-related wiring should be kept as separate as possible.
4. Use shielded cables and twisted pair cables to increase resistance to noise interference.

Note: When connecting a PLC and a VFD, since this involves the theory and technology of controlling high-power circuits with low-voltage signals, attention must be paid to avoiding interference. Improper connections may potentially damage the PLC or the VFD. Changsha Sunye Electric Co., Ltd. (established in 2010) and Shenzhen Sunye Electric Co., Ltd. (established in 2002) are national high-tech enterprises specializing in R&D, manufacturing, and sales of inverters, industry-specific integrated machines, servo drives, and new energy products.
Sunye Electric owns independent intellectual property rights, with products certified by national authorities and multiple software copyrights and IP certificates. Significant investment is made annually in new technologies and product development, with numerous patents and software copyrights approved. The company possesses core platform technologies such as construction hoist drivers, high-performance vector inverters, servos, and permanent magnet synchronous motor controls. Medium- and low-voltage inverters and servo drives are widely used in industries such as lifting, stone processing, HVAC, machine tools, metal products, wire and cable, plastics, printing and packaging, textile and chemical fibers, building materials, metallurgy, coal mining, municipal services, and automotive manufacturing. Guided by the philosophy of “Innovation, Technology, Strength,” driven by the corporate spirit of “Unity, Progress, Pragmatism, Innovation,” and adhering to the business principle of “Sincerity and Excellence, Mutual Benefit and Coexistence,” the company aims to provide industry-leading products through comprehensive quality management, continuous improvement, and pursuit of zero defects.
Address: Sunye Electric Industrial Park, No. 669 Xinsheng Road, High-Tech Zone, Changsha City
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