The combination of PLC and frequency converter（VFD） is very common in industrial control,mainly used to achieve precise control and adjustment of motors.The following are some methods and key points for using PLC with frequency converter:
1.How PLC controls VFD?
1.1 Output signal control
Direct connection:Directly connect to the start and stop signal of the frequency converter through the output point of PLC,and PLC can also output analog signal to adjust the speed of the frequency converter.
Signal transmission:Make sure that the com port of PLC is connected to the com port of frequency converter to ensure smooth signal transmission.
1.2 485 communication control
Advantages:PLC controls frequency converter through 485 communication,and only two control cables are needed,which can control multiple frequency converters at the same time without interfering with each other.
Data reading:Not only can the start and stop and speed adjustment of frequency converter be easily realized,but also the current,voltage,frequency,power,speed,temperature and fault code data can be read in real time.
2.Precautions for use with the inverter
2.1 Input of switch command signal
Connection method:The inverter usually uses relay contacts or components with relay contact switching characteristics such as transistors to connect to the PLC to obtain the operating status command.
Anti-misoperation:When using relay contacts,try to avoid misoperation caused by poor contact;when using transistors for connection,you need to consider the voltage and current capacity of the transistor itself to ensure the reliability of the system.
2.2 Input of numerical signal
Signal type:There are some numerical command signal inputs such as frequency and voltage in the inverter,which can be divided into digital input and analog input.
Interface selection:When the voltage signal range of the inverter and PLC is different,the output module of the PLC needs to be selected according to the input impedance of the inverter.
3.Selection of control method
3.1 Switching method
Hardware connection:Connect the switching output terminal of the PLC with the switching input terminal of the inverter,and at the same time,the switching output terminal of the inverter can be connected with the switching input terminal of the PLC.
3.2 Analog method
Hardware connection:Connect the analog output terminal of the PLC(it may be necessary to connect the analog output module)to the analog input terminal of the inverter,and use the analog signal to adjust the speed of the motor.
3.3 Communication method
Hardware connection:Taking RS485 as an example,connect the PLC and the inverter through a communication cable to achieve remote control and data transmission.
