Under current production conditions, when using an inverter to create an automated control system, it is common to integrate it with a PLC for various applications such as bearing cleaning, label printing, and PCB manufacturing. The PLC can generate multiple control signals and on/off commands through its output points or via communication. When working with inverters and PLCs together, there are several important considerations to ensure smooth operation and system reliability.
First, the input of switch command signals plays a critical role. The inverter typically receives switch-type signals that control operations like start/stop, forward/reverse, multi-speed, and jogging. These signals are often connected via relay contacts or components with similar characteristics, such as transistors. However, relay contacts may cause malfunctions due to poor contact, while transistor connections require careful attention to voltage and current ratings to maintain system stability.
Additionally, improper wiring of the inverter's input circuit can lead to malfunctions. For example, if the input uses an inductive load like a relay, the inrush current during switching might damage internal components of the inverter. To avoid this, it’s essential to use proper circuit design, including diodes and resistors where necessary. Also, crosstalk between the external power supply and the inverter’s control power (usually DC 24V) should be prevented by correctly connecting the PLC power supply to the transistor’s collector.
Second, numerical signals such as frequency or voltage are also commonly used in inverter systems. These can be either analog inputs or outputs. Analog inputs usually come from terminal blocks and may involve voltage signals (0–10V or 5V) or current signals (0–20mA). The PLC’s output module must match the inverter’s input impedance. If the voltage ranges differ between the PLC and inverter, resistors may need to be added to limit current or divide voltage, ensuring neither device exceeds its capacity.
It's also important to separate the control and main circuits to prevent noise interference. Shielded cables are recommended for the control circuit to reduce the impact of electrical noise from the main circuit.
When using a PLC for sequential control, program structure and instruction usage can affect response time. This may result in delays, especially in applications requiring high precision. Therefore, these factors should be carefully considered during system design.
Moreover, inverters generate significant electromagnetic interference (EMI) during operation. To protect the PLC from EMI caused by the inverter’s main circuit and switching devices, proper grounding, filtering, and shielding techniques should be implemented.
When connecting the inverter to the PLC, the following guidelines should be followed:
1. Ensure the PLC is grounded according to standard procedures, avoiding shared grounding with the inverter to minimize interference.
2. If the power supply is unstable, install noise filters, reactors, or other noise-reduction devices on both the PLC and inverter power lines.
3. When installing the inverter and PLC in the same cabinet, keep their wiring as separate as possible to reduce crosstalk.
4. Use shielded cables and twisted pairs to enhance resistance against noise interference.
By following these best practices, you can significantly improve the performance, reliability, and safety of your inverter-PLC system.
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