How to control the frequency of PLC inverter

1. How to control the inverter frequency through PLC

Use PLC to control the frequency of the frequency converter, and use the photoelectric encoder to feedback the speed signal to achieve the precise control of the motor speed. The plc is the main body of the control, which is the command and speed setting center, and the frequency converter is the slave device, which is the lower mechanism that accepts the command and speed, and will feed back some status of the main body to the plc.

Second, PlC and inverter communication method:

1. The switch signal of PLC controls the frequency converter

The output point and COM point of PLC (MR type or MT type) are directly connected with the STF of the frequency converter ( Forward rotation start), RH (high speed), RM (medium speed), RL (low speed), input terminal SG and other ports are connected respectively. PLC can control the start, stop and reset of the inverter through the program; it can also control the different combinations of high-speed, medium-speed and low-speed terminals of the inverter to achieve multi-speed operation. However, because it is controlled by a switch, its speed regulation curve is not a continuous and smooth curve, nor can it achieve fine speed regulation.

2. PLC analog signal controls frequency converter

Hardware: FX1N type, FX2N type PLC mainframe, equipped with 1-way simple FX1N-1DA-BD extended analog output board; or analog input and output hybrid module FX0N-3A; or two-way output FX2N- 2DA; or four-way output FX2N-4DA module, etc. Advantages: PLC programming is simple and convenient, the speed regulation curve is smooth and continuous, and the work is stable.

Disadvantages: In large-scale production lines, the control cable is long, especially when the DA module uses voltage signal output, the line has a large voltage drop, which affects the stability and reliability of the system.

3. PLC adopts RS-485 communication method to control frequency converter

This is the most commonly used method. PLC adopts RS serial communication command to program. Advantages: The hardware is simple, the cost is the lowest, and it can control 32 frequency converters. Disadvantages: The programming workload is relatively large.

4. PLC uses RS-485 Modbus-RTU communication method to control the inverter

Mitsubishi’s new F700 series inverter uses RS-485 terminal to communicate with PLC through Modbus-RTU protocol. Advantages: PLC programming in Modbus communication mode is simpler and more convenient than RS-485 non-protocol mode. Disadvantages: PLC programming workload is still relatively large.

5. PLC adopts field bus to control the frequency converter

Mitsubishi frequency converter can be built with various types of communication options, such as FR-A5NC option for CC-Link field bus; FR-A5AP (A) option for ProfibusDP field bus; FR-A5ND option for DeviceNet field bus and so on. Mitsubishi FX series PLC has a corresponding communication interface module to connect with it.

Advantages: fast speed, long distance, high efficiency, stable work, simple programming, and a large number of inverters that can be connected. Disadvantages: higher cost.

6. Using extended memory

Advantages: low cost, easy to learn and use, reliable performance Disadvantages: it can only be used in a system with no more than 8 inverters.

There are three ways to directly operate the inverter without an external controller (such as PLC):

①The buttons on the operation panel;

②Operate the terminal blocks Connected components (such as buttons and potentiometers);

③ Compound operations (such as operating the panel to set the frequency, operating the buttons connected to the terminal for start/stop control). In order to facilitate operation and make full use of the frequency converter, PLC can also be used to control the frequency converter.

Third, the parameters that need to be adjusted by the plc control inverter

The internal parameter setting of the inverter depends on your inverter brand and model, there are 2 important parameters that must be set:

1. First, choose whether the panel is valid or the terminal efficient! It means that you tune the frequency from the handheld terminal or receive the signal frequency from the control terminal

2. Then there is “external frequency input”: “choose voltage + current” to realize potentiometer frequency modulation and current frequency modulation; “current effective”: 4~20mA frequency modulation.