Research on energy saving control of oil pump moto

Research on energy-saving control of hydraulic injection molding machine oil pump motor

Abstract: Based on the analysis of the working process and energy consumption of hydraulic injection molding machine oil pump motor and the comparison of motor energy-saving schemes, an energy-saving scheme of hydraulic injection molding machine is proposed to adjust the motor torque to achieve efficiency control. This paper introduces the energy-saving system that realizes the above scheme by controlling semiconductor power switch with single chip microcomputer and its practical application test results

key words: hydraulic injection molding machine; Torque control; Energy saving

1 working process and energy consumption analysis of hydraulic injection molding machine

hydraulic injection molding machine has been widely used in injection molding processing. It uses hydraulic oil pump to suck oil from the oil tank, pressurize and output, and uses proportional valve to control the pressure, flow and direction of oil, so as to meet the pressure, flow and direction required by the working mechanism at different stages of the injection molding process. In the hydraulic injection molding machine, the oil pump motor uses the most electricity. Therefore, the research on energy saving of hydraulic injection molding machine should start with the energy saving of oil pump

when the load changes, the working characteristic change curve of the injection molding machine is shown in MJ (j=1, 2) in Figure 1. Set the oil pump motor to work at point a in Figure 1. When the injection molding machine works in the process of pressure maintaining and cooling, the motor will work at point B in Figure 1, and the speed will increase slightly. The corresponding torque and speed are T2 and N2 respectively, and its power is corresponding to rectangular bn20t2 in the figure. At this time, the injection molding machine has no speed-up requirements, its speed should be N1, the torque should be T3, and the required power should be cn10t3 in the figure. It can be seen that the dotted line in the figure is wasted power. If the motor can work at C (N1, T3), it will save wasted energy and realize efficient operation

Figure 1 Analysis of working process changes and energy consumption of injection molding machine

2 Analysis of energy-saving ways of hydraulic injection molding machine motor

the injection molding process of injection molding machine is a circular process. A complete injection molding process is composed of mold closing, injection seat advance, injection, pressure maintaining, cooling, plasticizing, mold opening, ejection products and other procedures. In this continuous process, the pressure and time required for each stage are different, among which: the mold closing process requires slightly higher pressure, and the duration is not long; The injection process requires high pressure and lasts very short; During the pressure maintaining process, the core material for heat preservation should also be fire-resistant B1 material, which requires high pressure and does not last long; The cooling and plasticizing process only needs low pressure and lasts a long time, accounting for about 40% - 60% of an injection molding cycle; The pressure required for mold opening and ejection is slightly higher, and the duration is not long [1]

in addition to the high pressure required in the injection process, the load of the injection molding machine in other stages is relatively light, especially in the cooling process. The oil pump is basically in the unloading state and lasts for a long time. The pressure control of the injection molding machine can be realized by controlling the output power of the oil pump motor

2.1 variable frequency speed regulation is used for energy saving of hydraulic injection molding machine

due to the different hydraulic flow required by the oil pump of hydraulic injection molding machine in each process, the environment has also become better q=nq (where q is the flow of oil pump, n is the motor speed, q is the displacement of oil pump), so the frequency converter can be used to control the change of motor speed of oil pump, so that the flow provided by the oil pump in each process stage can meet the process requirements [2]

however, since the change of motor speed is affected by the system mechanical time constant [4], and the system mechanical time constant TM is large, it is impossible to obtain a fast pressure dynamic response speed only by using v/f control strategy. Set the injection molding machine to work in the mold closing process with light load, and the motor operates at a low speed. When the injection molding machine is injecting, sudden loading requires the pressure to immediately reach the pressure required for injection, and the rising process of speed requires a certain acceleration time. In this case, the injection pressure may not be enough to meet the requirements of precision injection molding for the dynamic response speed of pressure. This is also the main reason why many enterprises failed to transform injection molding machines with frequency converters to save energy

if vector control, direct torque control and other advanced control strategies are adopted, this part is used for: torque control rather than speed control of the injection molding machine can well meet the process requirements of the injection molding machine and achieve energy saving, but on the one hand, the main function of frequency converter is not used, and the high equipment cost is contrary to the main goal of saving resources. Therefore, it can be considered that the frequency converter is not suitable for energy saving of precision injection molding machine

2.2 energy saving of injection molding machine based on torque control strategy

the purpose of energy saving can be achieved by controlling the motor torque to follow the change of load torque through appropriate control strategy. Torque control can be realized by controlling the input voltage of the motor according to the fact that the torque T of the motor is proportional to the square of the voltage U [4]. The control structure is shown in Figure 2. In this scheme, the torque change is controlled only according to the load condition, the speed remains basically unchanged, and the dynamic response speed of pressure is fast, which can meet the energy-saving requirements of various injection molding machines

Figure 2 dynamic structure diagram based on torque control strategy

3 design of new intelligent energy-saving system

3.1 design idea

the core of energy saving is to improve the efficiency of the system, so the energy-saving system should be an efficiency control system. However, the measurement of efficiency is a very complex process, and complex measurement also increases the cost of the system, which is contrary to the original intention of saving resources. Motor cos φ At about 0.85, the motor works in the high efficiency area, as shown in Figure 3 [4]. Therefore, the efficiency control of the motor can be realized by controlling the power factor

Figure 3 motor efficiency and power factor curve

3.2 system realization

power control adopts anti parallel thyristor and phase control strategy to realize motor power control, and the control function is realized by single chip microcomputer system. The single chip microcomputer adopts Motorola mc68hc908gp32[5]. The software design completely adopts single-chip microcomputer assembly language programming. The energy-saving system is integrated into a control box, which can be put into operation after being connected to the power supply and the oil pump motor. The installation is simple. In addition to the described energy-saving function, the system also has relatively perfect protection measures

3.3 experiment and result analysis

the developed intelligent energy-saving system is actually applied to the injection molding machine in a mold factory. The test wiring diagram is shown in Figure 4. The main technical indicators of the injection molding machine are shown in Table 1. The test process is as follows: first, use the original control equipment to close the contactor C, and carry out production without connecting the energy-saving equipment. After a certain period of production, read out the degree of the watt hour meter and record it. Then disconnect C, put the energy-saving system into operation, produce the same product within the same time, read out the degree of the watt hour meter and record it. The experimental results are shown in Table 2

Figure 4 field application wiring diagram

Table 1 notes that the main technical indicators of the plastic machine are on the road of entrepreneurship and innovation

Table 2 experimental results

the test results show that the developed intelligent energy-saving system has no impact on the output and quality of injection molding products and has achieved a certain energy-saving effect. The proportion of saving electric energy varies with the production of different products. This is because the raw materials and processing processes (pressure and time required by each process) used in the products are different, and the actual electric energy required and wasted are also different. Energy saving can only save wasted electric energy

4 conclusion

injection molding machine has a waste of energy because its injection molding process includes a light load process. In the strategy of controlling speed and torque, the frequency converter based on speed control cannot meet the requirements of pressure rapidity for precision injection molding because of its insufficient pressure response speed. The pressure regulation strategy based on torque control can meet all the requirements of injection molding and achieve energy saving. The energy saving effect will change with the different processed products

references

[1] Peng Hongtao Research on energy saving control of injection molding machine motor based on efficiency soft sensing [d] Master Thesis of South China University of technology, 2004:14

[2] Huang Fangping, Xu Bing, Yang Huayong, etc Application of variable frequency hydraulic technology in injection molding machine [j] Hydraulic pneumatic and sealing, 2004,3

[3] Luo Fei Motion control system [m] Beijing: Chemical Industry Press, 2001

[4] Mai Chongyi, Lin Liangyang, Weng kaichao, etc Fundamentals of electrics and drag [m] Guangzhou: South China University of Technology Press, 1998

[5] Zhang Youde, Tu Shiliang, Chen Zhanglong Principle and application of m68hc08 series single chip microcomputer [m] Shanghai: Fudan University Press, 2001 (end)