Frequently asked questions about injection molding machine inverter modification
1. Solutions to the possible extension of the production cycle
Injection molding When the machine undergoes frequency conversion transformation, you may often find that the production cycle will be 0.3s~1.2s slower when the machine is working with frequency conversion. And the greater the power of the frequency converter, the more complex the product working conditions are, and the cycle will increase accordingly. This is mainly because when switching between different process sections, the frequency converter needs to continuously change the output frequency according to the different pressures and quantities required by each process, so that the motor is often in the process of acceleration and deceleration.
Although the acceleration and deceleration time of the frequency converter can be adjusted very short, the on-off speed of the solenoid valve has a certain lag compared with the power frequency operation, which results in unit processing under frequency conversion control under the same process parameters. The cycle has been extended and production efficiency has been affected to a certain extent. For this problem, on the one hand, the selected frequency converter is required to have good dynamic response performance to shorten the response time as much as possible; on the other hand, some process parameters of the injection molding machine can be adjusted according to the actual situation, such as increasing the pressure setting value. wait. Of course, the hydraulic system and control system can also be improved to solve the problem. For example, the hydraulic system design of an injection molding machine under power frequency should take into account the hazards caused by hydraulic shock, and use many buffer circuits in the hydraulic circuit. If the response time of the hydraulic circuit can be improved, the lag caused by the frequency conversion acceleration process can be compensated. As for the frequency converter control system, if two-way signal control of flow and pressure can be used, this problem can also be solved better.
2. Reasons for insufficient pressure and corresponding measures
In some cases, due to the quality of the components used in the machine is not up to standard, or the maintenance is not good, the hydraulic circuit The wear and tear of some hydraulic components in the hydraulic system will lead to a decrease in its pressure-flow characteristics, resulting in large changes in control pressure with flow. At this time, it shows that sometimes the force is sufficient and sometimes not strong enough. Since the flow rate is constant during power frequency production, the problem is not exposed. Insufficient pressure will appear when using variable frequency control.
In order to solve this problem, two methods are adopted: one is to adjust the process parameters of the injection molding machine to set the pressure and flow rate of the injection molding machine higher than that of the power frequency when working at variable frequency; the other is to adjust the process parameters of the injection molding machine. The first is to think of ways to control the pressure, such as using closed-loop pressure control based on the speed link to ensure that the pressure meets the requirements.
3. Measures to deal with the interference of frequency converters on injection molding machines
Since the input and output currents of the frequency converter will have a certain amount of high-order harmonics during operation, they will interfere with the control system of the injection molding machine, and in severe cases may affect the normal operation of the injection molding machine. . In this regard, the following measures can be taken:
(1) Install special filters, reactors, etc. for the frequency converter on the input and output sides to reduce high-order harmonic current;
(2) Use shielded cables for control signal lines, and connect one end of the shielding layer to the public input terminal of the inverter;
(3) The input and output cables of the inverter are routed separately from the control signal lines , they should be at least 20cm apart and as far apart as possible;
(4) Reduce the carrier frequency of the inverter;
(5) Other equipment under the same power supply should be at least It is best to isolate the power supply.
4. Problems found in practical applications
The effectiveness of the energy-saving frequency converter is closely related to the setting of injection process parameters. This involves the weight of plastic products and the clamping force of the injection molding machine.
(1) The weight of the product is large and should be processed by an injection molding machine with correspondingly larger clamping force and larger injection volume (generally above 300 tons). The processing and molding cycle of this type of product is long, especially the cooling stage. Reliable mold clamping force and instantaneous pressure increase are required during the injection action. Through some practical applications, quantitative pumps with variable frequency speed regulation can meet this working condition, and the energy-saving effect of variable frequency speed regulation can be fully exerted.
(2) For injection molding factories that process all small products, the maximum clamping force of the injection molding machine does not exceed 180t, there is almost no cooling process in the process parameter setting, and the energy saving effect is not significant.
Benefits of using frequency converters in the transformation of injection molding machines
1. Speed regulation and energy saving
According to The process requirements of the injection molding machine require that the switching signals of the total pressure valve, low pressure valve, first pressure valve, and second pressure valve are converted and added to the input end of the frequency converter as the frequency given signal of the frequency converter. The frequency converter performs real-time The sample is sampled and processed by the CPU to make the output frequency change linearly with the analog signal of the proportional valve. In processes that require small pressure and flow, the motor speed is reduced, thereby reducing the output power of the motor. During the cooling and semi-finished product placement processes The motor can be stopped to minimize the energy loss of the motor in the entire load range.
2. Improve power factor and save energy
Reactive power not only increases line losses and equipment heating, but more importantly, the reduction in power factor causes the power grid to Reduction of active power. From the formula S2=P2+Q2, Q=S*SINφ, P=S*COSφ, where: S-apparent power. Q-reactive power, P-active power, COSφ-power factor. It can be seen that when COSφ is larger,The greater the active power P. The COSφ value of ordinary quantitative pump injection molding machines is between 0.6-0.8. However, after using the variable frequency speed regulation device, due to the compensation effect of the filter capacitor in the frequency converter, the COSφ≈1, thereby reducing the reactive power loss and increasing the power grid. Active power.
3. Soft start section
Since the original motor is a direct start or Y/Δ start, the starting current is equal to (3-7) times the rated current. This will have a serious impact on the electromechanical equipment and the power supply grid, and will also increase the grid capacity requirements. The large current and vibration generated during startup are extremely detrimental to the service life of the equipment. After using the variable frequency energy-saving device, the soft start function of the frequency converter will cause the starting current to start from zero, and the maximum value will be limited to the current limiting level during acceleration set by the frequency converter, which generally does not exceed 1.7 times the rated current, which reduces the impact on the current. The impact of the power grid and the requirements for power grid capacity extend the service life of equipment and molds.
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