Summary
This article introduced the energy saving transformation of Zhongke Electric vector inverter on the variable displacement pump injection molding machine, discussed the control system and functional requirements of the variable displacement pump injection molding machine, and discussed the differences between the injection molding machine and the quantitative pump injection molding machine.
Key words
Zhongke Electric Drive Variable Pump Injection Molding Machine Energy Saving Reform
I. Introduction
Plastics processing industry is one of the industries with rapid development in the light industry in recent years. With the increase in the number of plastics companies, reducing production costs and improving product competitiveness have become common concerns for many plastics manufacturers. In the processing cost, the electricity fee occupies a considerable proportion, and a large number of injection molding machines are the most power-consuming equipment. Therefore, many plastics companies have made large-scale energy-saving technological transformations to the quantitative pump injection molding machines to achieve energy conservation and improve the company’s Competitiveness, but for variable pump injection molding machines many companies, including many professional energy-saving companies are helpless, so for the large number of variable pump injection molding machine on the market how to carry out energy-saving transformation has become a hot spot for many plastics companies, Shenzhen Zhongke Electric Technology Co., Ltd. The company through the in-depth study of the variable pump injection molding machine, combined with the latest vector control frequency conversion technology to successfully realize the energy-saving transformation of the variable pump injection molding machine, and achieved a 10-20% energy-saving effect.
Second, quantitative pump and variable pump injection molding machine introduced
1) Quantitative pump injection molding machine
At present, most of the remodeled injection molding machines are hydraulic injection molding machines. The power in the hydraulic transmission system is provided by the motor driven oil pump. The hydraulic oil pump is a vane pump. The fuel supply is proportional to the rotation speed of the oil pump, and the oil pump motor is 50HZ. At constant speed, the oil supply to the oil pump is constant, so it is called a dosing pump. The system diagram is as follows:
As shown in Figure 1:
A1: cylinder rodless cavity piston area F: working load Q1: working flow P1: working pressure Δp: differential pressure across the proportional valve
Qy: proportional relief valve flow Pb: oil pump pressure, Qb: oil pump flow Vb: oil pump displacement n: motor speed
As shown in Figure 1, the system uses a quantitative pump to supply oil, the pump speed n and the pump displacement Vb are not adjustable, and the flow and pressure in the oil circuit are:
Pump flow rate Qb=Vb×n is fixed value;
Pump pressure Pb is set by proportional relief valve;
Work flow Q1 is set by the proportional speed control valve;
Working pressure P1 = F/A1 depends on the workload F;
In the production process of a general injection molding machine, there are the following steps: clamping, injection, melting, pressure keeping, cooling, and mold opening. Injection molding machine needs different flow and pressure under different working procedures. It must depend on the flow valve and pressure valve to adjust the flow and pressure required by different processes. When the flow required for the work is small, the oil supply of the oil pump is greater than that required by the actuator. The excess hydraulic oil under high pressure is returned through the overflow valve, and a large amount of energy is consumed through the temperature rise of the oil. This not only exacerbates the wear of various valves, but also causes the oil temperature to be too high, and in order to reduce the oil temperature, an additional cooling water circulation is often required, resulting in further waste of energy. Therefore, the conversion of the general quantitative pump injection molding machine uses a frequency converter to control the speed of the oil pump motor to regulate the flow rate. The amount of oil pumped in a specified time just meets the needs of each action link, and the overflow flow can be controlled to a minimum state, thereby saving Energy
2) Variable pump injection molding machine
At present, the variable-displacement pump injection molding machine uses a proportional variable displacement pump as a driving element, which is a typical volumetric speed control system, and has a relatively obvious energy-saving effect compared to the injection quantitative injection pump injection molding machine. As shown in Figure 2, the system consists of load-sensitive proportional variable plunger pumps, proportional relief valves, proportional speed control valves, pressure feedback valves, and flow feedback valves.
Figure II oil system schematic
A1: Cylinder rodless cavity Piston area F: Workload P1: Working pressure Q1: Workflow ΔP: Pressure difference across the proportional control valve Pb: Oil pump pressure Qb: Oil pump flow n: Motor speed
working principle
(1) When the system is in the flow control state, the oil pump pressure Pb is greater than the working pressure P1 and less than the setting pressure of the molding machine, and the proportional relief valve will be reliably closed. When the load changes, the pressure difference ΔP between Pb and P1 will fluctuate accordingly, and the working flow rate Q1 will change. The above changes are passed to the adjusting piston of the variable displacement pump through the flow and pressure feedback valve. The piston then pushes the plunger pump swashplate and changes the displacement of the pump, ultimately stabilizing the pump output flow Qb. It can be seen from this that the system achieves pressure adaptive control and there is basically no overflow loss, but in order to control the operating flow Q1, there is still a certain throttling loss on the speed control valve.
(2) When the system is under pressure control, the swash plate angle of the variable piston pump is very small, and only a small amount of hydraulic oil flows through the proportional relief valve to ensure a certain system pressure is formed. By changing the input signal of the proportional relief valve, the corresponding pump output pressure can be obtained. Compared with the traditional fixed-displacement pump injection molding machine, this control method reduces the overflow loss to some extent.
Third, variable pump injection molding machine energy saving principle
From the above introduction, we can know that the variable displacement injection molding machine has reduced a large amount of overflow loss compared to the injection pump injection molding machine, making the pump motor load lighter, current is reduced, and light load conditions. Variable pump injection molding machine starts from the perspective of hydraulic components, and taps the energy-saving transformation space on the oil road, thereby improving the working efficiency of the oil circuit system. However, due to the limitations of its control method, the system cannot completely eliminate throttling and overflow losses.
1) The variable pump injection molding machine still has space for waste. The oil from the oil pump still needs to be adjusted by the proportional speed control valve and the proportional relief valve. There is still a certain power loss on the valve.
1) The variable pump injection molding machine still has space for waste. The oil from the oil pump still needs to be adjusted by the proportional speed control valve and the proportional relief valve. There is still a certain power loss on the valve.
2) The motor speed of the oil pump still cannot be adjusted. Although the load of the oil pump is lighter than that of the fixed pump, compared with adjusting the speed to adjust the oil amount, the former is obviously more energy-consuming. Especially when the system is under pressure control, the pressure required by the system is large but the flow rate is small, and the speed of the pump motor cannot be adjusted. If the output flow rate, output pressure, displacement, and speed of the pump are expressed as Q, P, Vb, and n, respectively, the output power N of the pump is expressed by the formula:
N = Q*P = Vb*n*P
Injection molding machine is a mechanical device to adjust the displacement of the oil pump Vb, obviously its adjustable linear range is limited, can not achieve zero displacement output. In any case, the motor runs at full speed and there is a lot of unnecessary energy loss.
3) The starting of the oil pump motor is still the start of the star triangle, and the power factor is low.
IV. Power Saving System of Zhongke Electric Company
Zhongke Electric Technology Co., Ltd. is a vector inverter technology applied to the injection molding machine motor drive, through the speed adjustment and pressure PID closed-loop control functions to complete the precise control of the injection molding machine. In the oil path, the proportional speed regulating valve is artificially hard to adjust the flow to the maximum and to maintain or remove, and at the same time cut off the control of the displacement, pressure feedback valve to the piston of the variable displacement pump. Instead, the vector variable frequency PID control maintains the pressure constant and automatically realizes the pressure adaptive control. This system eliminates the throttling and overflow energy loss in the oil circuit from the source, so that the injection molding machine can obtain the best energy-saving effect. The system looks like this:
working principle:
When the oil pump flow Qb approaches or reaches the setting value of the press, the oil system enters the flow control state, and the output frequency of the inverter is proportional to the set value of the flow rate of the press. In this process, the oil pump flow rate Qb=Vb×n is always adapted to the needs of the working circuit. No proportional valve is required for speed regulation, and no excess flow is allowed to overflow, which can fundamentally eliminate throttling and overflow energy losses. When the pump is working under pressure, as shown in the figure below:
The frequency control system is equipped with a smart conversion card. The inverter will monitor this pressure change in real time through the pressure sensor and compare it with the pressure setting value of the press. When the difference is small, the inverter's smart conversion card will switch the inverter to the pressure control state in time. , start PID control and keep system pressure constant through PID function. At this time, the output frequency of the inverter will be proportional to the pressure difference. Because the difference is small, the output frequency of the inverter and the motor speed will rapidly decrease, and the oil pump only maintains a small flow output to compensate for various leakage losses in the oil circuit. The electrical system diagram is as follows:
Zhongke Electric Technology Co., Ltd. Inverter is open-loop vector mode, starting torque, 0.5HZ can reach 150% of the rated torque; fast dynamic response, step torque response time is less than 200MS; steady speed accuracy of 0.5%. Through the closed-loop vector speed control function of the inverter, the oil pump speed n of the frequency conversion injection molding machine can achieve a linear adjustment range from 0 to the rated speed, especially when the press stands by and no action (such as the cooling phase), the speed of the motor and the pump Can be reduced to 0, basically no energy loss. It is worth noting that the volumetric efficiency of the oil pump will decrease at low speeds, which will affect the power saving effect to some extent. In this regard, we will improve this by increasing the low frequency gain of the drive. Moreover, the actual production of less than 20% of the flow is very rare, so the oil pump speed may not be too low.
V. Cost and maintenance analysis
From the example of some variable pumping machines of a plastics factory in our company in Guangdong Province, the frequency conversion energy saving transformation of the horizontal variable displacement pumping machine with a clamping force of 120 tons still has a saving rate of 20%. It can be seen that the power saving rate of variable frequency injection molding machines is higher than that of variable pump injection molding machines, and the maintenance analysis is as follows:
Compare items
Quantitative pump injection molding machine
Variable pump injection molding machine
Frequency injection molding machine
Start method
Y/Δ start, large starting current, low motor power factor, large impact on power grid and oil circuit
Y/Δ start, large starting current, low motor power factor, large impact on power grid and machinery
Frequency conversion soft start, small starting current to improve the motor power factor to the grid and mechanical shock
Maintenance
Quantitative pumps and universal proportional valves have low failure rates and low maintenance costs
Variable pump and supporting proportional valve failure rate high pump maintenance costs
Quantitative pump, frequency converter and proportional valve failure rate in repair costs
Mechanical component loss
Motors and oil pumps always run at full speed. Flow and excess supply of mechanical components
The motor and the oil pump always run at full speed. The oil pump displacement adaptively adjusts the loss of the mechanical components.
Motor, oil pump shifting and stop running, acceleration and deceleration ramp, low loss of adjustable mechanical components
Oil temperature and cooling water
The hydraulic oil always has a large circulating oil temperature and consumes large cooling water
Hydraulic oil variable circulating oil temperature, cooling water consumption
Hydraulic oil variable and stop cycle Low oil temperature, low cooling water consumption
Average noise
Motor and oil pump running smoothly average noise
Pump has mechanical adjustment device with high average noise
The motor does not rotate when the motor soft start does not operate and the average noise is low
Hydraulic oil cost
Low requirements for hydraulic oil Replacement frequency is low
High requirements for hydraulic oil
Low requirements for hydraulic oil Replacement frequency is low
Power saving effect
Not saving electricity
Saving electricity
Power saving
Sixth, concluding remarks
Zhongke Electric Technology Co., Ltd. has accumulated a lot of valuable experience in many years of practical application, developed a variety of energy-saving programs in the injection molding machinery upconverter applications, this time developed a variable frequency pump energy-saving inverter injection molding machine, will Leading the new trend of energy-saving transformation of presses