The combined use of variable frequency drives (VFDs) and programmable logic controllers (PLCs) is very common in industrial automation systems. There are various methods for connecting and communicating between them. Below are some primary methods for their combined use:
I. Connection Methods
1. Analog Output Control
● The PLC provides a 0–5V voltage signal or a 4–20mA current signal to the VFD via its analog output module. This serves as the VFD's analog input, thereby controlling its output frequency.
● Advantages: Simple wiring, smooth and continuous speed curve, stable operation, and straightforward PLC programming.
● Disadvantages: Requires impedance matching between the PLC output module and the VFD input; relatively high cost; voltage division measures are needed to accommodate the PLC's voltage signal range; long control cables may cause voltage drop, affecting system stability.
2. Digital Output Control
● The PLC's digital outputs can be directly connected to the VFD's digital inputs to implement control functions such as start/stop, forward/reverse, jog, speed, and acceleration/deceleration timing.
● Advantages: Simple wiring and strong interference resistance.
● Disadvantages: Only supports stepped speed regulation; relay contact connections require attention to potential misoperation from poor contact; transistor connections necessitate consideration of voltage and current capacity.
3. RS-485 Communication Interface Connection
● Utilizes the RS-485 serial interface on both the PLC and VFD (some VFDs also offer RS-232 interfaces) for communication via a two-wire connection.
● Advantages: Simple hardware, lower cost, capable of controlling multiple inverters (e.g., up to 30 units), and can accurately locate the target inverter for communication via address or broadcast messages.
● Note: Communication protocol compatibility must be considered. If the PLC and inverter share the same protocol, direct wiring enables reading/writing inverter registers. If they differ, PLC programs must be written to handle the VFD's communication format.
II. Communication Methods
Beyond the communication methods mentioned in the connection types above, the following common communication methods exist:
1. Modbus-RTU Communication Control: Some VFDs support the Modbus-RTU protocol, communicating with PLCs via RS-485 terminals. This method simplifies PLC programming compared to the protocol-free RS-485 approach.
2. Fieldbus Control: PLCs connect to VFDs via fieldbuses (e.g., CC-Link, Profibus DP, DeviceNet) for high-speed, long-distance, and efficient communication. This method offers speed, extended range, and stable operation but incurs higher costs.
3. Expanded Memory Control: Suitable for systems with no more than 8 inverters, this method utilizes expanded memory for control. It is low-cost, easy to learn and use, but has limited application scope.
III. Considerations in Practical Applications
In practical applications, the choice of connection and communication method requires comprehensive consideration of specific application scenarios, control requirements, and cost budgets. For instance: - Analog output control is suitable for applications requiring smooth speed regulation and high control precision. - Digital output control is appropriate for scenarios with relatively simple control requirements and cost constraints. - RS-485 communication interfaces or fieldbus control are ideal for connecting multiple inverters over extended distances.
In summary, the integration methods between VFDs and PLCs are diverse. Engineers should select appropriate connection and communication methods based on actual requirements to ensure stable and efficient operation of industrial automation systems.