This paper introduces a multi-branch communication network composed of plc and frequency conversion governor, expounds the advantages of this network-controlled speed regulation system compared with the general analog control speed regulation system, and gives the system block diagram and plc program.
[Keywords: ]plc inverter multi-motor control network communication protocol
I. Introduction
Taking frequency conversion governor as speed controller
Synchronous control system, proportional control system and synchronous speed regulation system have been widely used in metallurgy, machinery, textile, chemical industry and other industries. Taking the proportional control system as an example, the general system structure is shown in figure 1.
When working, the operator sets the proportional operation parameters through the control machine (which can be plc or industrial computer), and then the control machine sends out speed instructions, and the d/a conversion module controls the variable frequency governors, so that each variable frequency governor drives the motor to run at a certain speed ratio. This scheme is more suitable for the application system with few motors and concentrated motor distribution. But for the automatic production line, on the one hand, there are many motors, on the other hand, the motors are distributed far away. When this control scheme is adopted, the stability and reliability of the whole system are reduced due to the attenuation of speed command signals in long-distance transmission and external interference. At the same time, a large number of d/a conversion modules increase the system cost. Therefore, we propose a multi-branch communication control network composed of plc and frequency conversion governor. The system has low cost, long signal transmission distance and strong anti-interference ability, and is especially suitable for long-distance and multi-motor control.
Second, the system hardware structure
The hardware structure of the system is shown in Figure 2, which mainly consists of the following parts:
1 and fx0n—24mr are the basic units of plc, and the execution system and user software are the core of the system.
2. fx0n-485ADP is the communication adapter of FX0N system plc. The main function of this module is to receive the information sent by the computer to plc, as a substation in the computer -plc communication system, or as a network adapter when multiple plcs form an n:n network. Generally, it is only used as the receiving unit of the specified protocol. Based on the analysis of its structure, the author uses it as the communication master station to complete the transmission of the control signal of the variable frequency governor.
3.FR-Cu03 is the computer connection unit of FR-A044 series proportional governors, which conforms to the communication specification of RS-422/RS-485 and is used to realize the networking between computers and multiple variable frequency governors. Through this unit, the functions of operation control (such as starting, stopping, frequency setting), parameter setting, state monitoring and so on of the inverter can be realized on the network, and it is the network interface of the inverter.
4.FR-A044 inverter realizes motor speed regulation.
In the 1:n (1:3 in this paper) multi-branch communication network, each frequency converter is a substation, and each substation has a station number, which is preset by the parameter setting unit. During the working process, after plc sends relevant command information through fx0n—485adp, each substation receives this information, and then each substation judges whether the station number and address of this information are consistent with the station number of this station. If yes, processing the information and returning the response information; If not, give up information processing and ensure that only one substation exchanges information with the main station on the network at the same time.
Third, software design.
1, communication protocol
FR-CU03 specifies the communication process between the computer and the frequency converter, as shown in Figure 3.
This process can be divided into five stages at most. ? The computer sends a communication request; ? Frequency converter processing waiting; ? Frequency converter responds; ? , computer processing waiting; ? The computer replied. Complete the corresponding process according to different communication requirements, such as writing the start-stop control command of frequency converter? ~? Three processes; When monitoring the frequency of the inverter? ~? Five processes. Whether writing data or reading data, the computer sends a request, and the inverter only passively accepts the request and responds. The data format of each stage is different. Fig. 4 shows data formats for writing inverter control commands and inverter operating frequencies, respectively.
2, plc programming
In order to control the frequency converter, plc must be programmed to realize the control of information exchange between plc and frequency converter through the program. The plc program should complete the initialization of fx0n—485adp communication adapter, the combination of control command words, code conversion and the processing of frequency converter response information. The Plc ladder diagram program (part of the program) is shown in Figure 5.
The communication sending buffer in the program is d127 ~ d149; The acceptance buffer is d 150~d 160. The start and stop of the motor 1 are controlled by the rising edge and falling edge of x0 respectively; The start and stop of the motor 2 are controlled by the rising edge and falling edge of x 1 respectively; The start and stop of that motor 3 are respectively control by the rising edge and the fall edge of x2. The communication protocol of fx0n—485adp is initialized by the system start pulse m8002. Then the start and stop signals are processed. Take the start of motor 1 as an example, attract the rising edge m50 of x0, send the station number of inverter 1 to d 130, and send the operation command word to d 135. Enq, the control word and waiting time of the write operation command are written into d 13 1 and d65438 in advance by the programmer. Then check the sum and send it to d 136 and d137; ; Finally, set m8 122 to allow rs instruction to send control information to. The frequency converter returns the response information immediately after receiving the signal. This information fx0n—485adp receives POSM8132, and plc finishes the program after making corresponding processing according to the situation.
Fourth, the train of thought and new method of inverter braking
In the traditional variable frequency speed regulation system composed of general frequency converter, asynchronous motor and mechanical load, when the potential energy load driven by the motor decreases, the motor may be in regenerative braking state; Or when the motor decelerates from high speed to low speed (including stopping), the frequency may suddenly decrease, but due to the mechanical inertia of the motor, the motor may be in a regenerative power generation state, and the mechanical energy stored in the transmission system is converted into electrical energy by the motor and sent back to the DC circuit of the inverter through the six freewheeling diodes of the inverter. At this time, the inverter is in a rectifying state. At this time, if the inverter does not take measures to consume energy, this part of energy will lead to the voltage increase of the energy storage capacitor in the intermediate loop. If the braking is too fast or the mechanical load is lifted, this part of energy may.
It will damage the frequency converter, so this part of energy should be considered.
In general frequency converters, there are two common ways to deal with regenerative energy: (1), which is dissipated into the artificially set "braking resistance" in DC circuit in parallel with the capacitor, which is called dynamic braking state; (2) Feed it back to the power grid, which is called feedback braking state (also called regenerative braking state). There is also a braking mode, that is, DC braking, which can be used in situations where accurate parking is needed or the braking motor rotates irregularly due to external factors before starting.
Many experts have talked about the design and application of variable frequency braking in books and publications, especially in recent years, there have been many articles about "energy feedback braking". Today, the author provides a new braking method, which not only has the advantages of "feedback braking" with four quadrants and high efficiency, but also has the advantages of "energy consumption braking" with no pollution to the power grid and high reliability.
1, energy consumption braking
The way to absorb the regenerative electric energy of the motor by using the braking resistor arranged in the DC circuit is called energy consumption braking.
Its advantages are simple structure; No pollution to the power grid (compared with the feedback system) and low cost; The disadvantage is that the operation efficiency is low, especially when braking frequently, it will consume a lot of energy and increase the braking resistance capacity.
Generally, in general frequency converters, low-power frequency converters (below 22kw) have built-in braking units, and only need to increase the braking resistance. High-power inverter (above 22kw) needs external braking unit and braking resistor.
2. Feedback braking
To realize energy feedback braking, it is necessary to control the voltage in the same frequency and phase and the feedback current. It uses active inverter technology to invert the regenerated electric energy into alternating current with the same frequency and phase as the power grid and send it back to the power grid, thus realizing braking.
The advantage of feedback braking is that it can run in four quadrants, and electric energy feedback improves the efficiency of the system. Its disadvantage is: (1), this feedback braking method can only be used when the grid voltage is stable (the grid voltage fluctuation does not exceed 10%). Because the fault time of grid voltage is more than 2ms during power generation braking operation, commutation failure may occur, resulting in equipment damage. (2) When feedback, there is harmonic pollution to the power grid. (3) The control is complicated and the cost is high.
3. New braking mode (capacitor feedback braking)
3. 1 principle of main circuit
The rectifier part adopts common uncontrollable rectifier bridge for rectification, the filter circuit adopts common electrolytic capacitor, and the delay circuit adopts contactor or thyristor. The charging and feedback loop consists of power module igbt, charging and feedback reactor L and large electrolytic capacitor C (the capacity is about a few tenths of a farad, which can be determined according to the working condition system where the frequency converter is located). The inverter part is composed of power module igbt. The protection circuit consists of igbt and power resistor.
(1) Motor power generation operation state
The cpu monitors the input AC voltage and DC loop voltage νd in real time, and decides whether to send the charging signal to vt 1. Once νd is higher than the DC voltage corresponding to the input AC voltage (for example, 380vac—530vdc) to a certain value, the cpu will turn off vt3 and realize the charging process of electrolytic capacitor C by turning on a pulse to vt 1. At this time, the reactor L and the electrolytic capacitor C are divided to ensure that the electrolytic capacitor C works in a safe range. When the voltage on the electrolytic capacitor C is close to the dangerous value (for example, 370v), and the system is still in the power generation state, and the electric energy is continuously fed back to the DC circuit through the inverter, the safety circuit will play a role to realize energy consumption braking (resistance braking) and control the turn-off and turn-on of vt3, so as to realize that the resistor R consumes redundant energy, which generally does not happen.
(2) the electrical operation state of the motor
When the cpu finds that the system is no longer charging, it conducts a pulse on vt3, so that an instantaneous Zuo Zheng right negative voltage is generated on the reactor L. The voltage on the electrolytic capacitor C can realize the energy feedback process from the capacitor to the DC loop. The cpu controls the switching frequency and duty ratio of vt3 by detecting the voltage on electrolytic capacitor C and the voltage of DC loop, so as to control the feedback current and ensure that the DC loop voltage νd is not too high.
3.2 System difficulties
Selection of (1) reactor
(1) Considering the particularity of working conditions, we assume that there is some fault in the system, which leads to the free acceleration of the potential energy load carried by the motor. At this time, the motor is in the power generation operation state, and the regenerative energy is sent back to the DC circuit through six freewheeling diodes, so that νd rises and the inverter is in the charging state soon. At this time, the current will be very large. Therefore, the conductor diameter of the selected reactor should be large enough to pass current at this time.
(b) In the feedback loop, in order to make the electrolytic capacitor release as much electric energy as possible before the next charging, it is impossible to choose an ordinary iron core (silicon steel sheet). It is best to choose ferrite core. Look at the current value considered above, and you will know how big the core is. I don't know if there is such a big ferrite core on the market, and if there is, the price will certainly not be very low.
Therefore, the author suggests using a reactor in the charging loop and the feedback loop.
(2) difficulty in control
(a) In the DC circuit of frequency converter, the voltage νd is generally higher than 500vdc, while the withstand voltage of electrolytic capacitor C is only 400vdc. It can be seen that the control of this charging process is not like the control mode of energy braking (resistance braking). The instantaneous voltage drop generated on the reactor, the instantaneous charging voltage of electrolytic capacitor C is ν c =νd-νl. In order to ensure the electrolytic capacitor to work in a safe range (≤400v), it is necessary to effectively control the voltage drop ν L on the reactor, which depends on the instantaneous change rate of inductance and current.
(b) In the feedback process, it is also necessary to prevent the electric energy released by electrolytic capacitor C from passing through the reactor, which will cause the DC loop voltage to be too high and the system to generate overvoltage protection.
3.3 Main application occasions and application examples
It is precisely because of the advantages of this new braking method (capacitive feedback braking) that in recent years, many users have proposed to equip this system in combination with the characteristics of their own equipment. Due to some technical difficulties, is there such a braking method abroad? At present, only Shandong Fengguang Electronics Co., Ltd. has replaced the previous frequency converters with feedback braking (two of which are still in normal operation) with a new series of mine hoists with this capacitive feedback braking method. So far, this kind of capacitor feedback braking frequency converter has been running normally for a long time in Baoan Coal Mine in Ningyang, Shandong Province and Taiyuan, Shanxi Province, which fills the gap in China.
With the broadening of the application field of frequency converter, this application technology will have great development prospects. Specifically, it is mainly used in mine hoists (manned or loaded), inclined harvesters (single cylinder or double cylinder), hoisting machinery and other industries. In short, the energy feedback device can be used in any situation.
Verb (abbreviation of verb) conclusion
1. The practical application shows that the scheme can realize the operation control, parameter setting and operation state monitoring of the variable frequency governor by plc through the network.
2. The system can control up to 32 variable frequency governors with the maximum distance of 500m.
3. The cost of controlling multiple inverters is obviously lower than that of d/a control mode.
4. With the increase of frequency converter, the communication delay increases, and the response speed of the system is lower than that of d/a control mode.
refer to
1, Han. General frequency converter and its application (2nd edition) [m]. Beijing: Machinery Industry Press,
2. Liu Wenbing (198 1-) has been engaged in the application of frequency converters, and now Taizhou Fuling Electromechanical Manufacturing Co., Ltd. is engaged in the design and manufacture of frequency converters.
Express one's gratitude formally
When the thesis is completed, I sincerely thank Mr. Xun Yanlong and his colleagues who helped me in the design, so that I can finish my graduation thesis on time and benefit for life.
In the process of completing the thesis, the teachers in the department gave me great help. Thank you very much Other students also gave me a lot of care and help, and I really appreciate it.