Application of frequency conversion speed regulati

  • Detail

Application of frequency conversion and speed regulation technology in dry white powder quantitative packaging scale introduction white powder is a powdery chemical product. In the past, the feeding device of dry white powder quantitative packaging scale used for finished product packaging mostly used double screw feeder driven by motor. In order to ensure the accuracy and speed of the control system, the double screw feeder has two stages of screw feeding, large and small, and the initial state is in the stop position. When entering the automatic operation state, when the material weight reaches the set value of large feeding, stop the large feeding motor and continue to keep the small feeding motor running. When the material weight reaches the final set value, close the small feeding motor. Due to the fluidity of dry white powder, the mechanical inertia of screw feeder, and the uneven material transportation, the dynamic accuracy of quantitative packaging scale is difficult to control stably. Based on the above problems, we used frequency converter to transform the feeding device of dry white powder quantitative packaging scale, and changed the form of double screw feeder to single screw feeder, which not only reduced the production cost, but also improved the performance of dry white powder quantitative packaging scale

2 structural composition of dry white powder quantitative packaging scale

dry white powder quantitative packaging scale is a gross weight weighing method, and the structural composition is shown in Figure 1: it is mainly composed of storage bin, screw feeder, cutting device, weighing bag clamping device, pneumatic part and electrical control cabinet

Figure 1 Schematic diagram of structural composition

3 main technical indicators and parameters

(1) description of quantitative packaging scale

4 selection of frequency converter

when selecting frequency converter, in addition to the motor category and the production efficiency and driving power of screw feeder, the actual motor current should also be used as the basis for selection, and the rated power of motor can only be used as a reference. In addition, it should be fully considered that the output of the frequency converter contains rich high-order harmonics, which will deteriorate the power factor and efficiency of the motor. Therefore, the current of the motor will increase by 10% and the temperature rise will increase by 20% compared with the power frequency power supply. Therefore, when selecting motor and frequency converter, this situation should be taken into account and appropriate allowance should be left to prevent excessive temperature rise from affecting the service life of screw feeder

after technical comparison, we selected if-4.0k frequency converter of Sanken company, which adopts advanced drive control technology. It is a low-noise and high-performance frequency converter with the best control voltage and current vector, which can maximize the output torque of the motor, and is fully applicable to the technical requirements of quantitative packaging control of dry white powder

5 hardware composition of variable frequency speed regulation

the electrical schematic diagram is shown in Figure 2. The core components are if-4.0k frequency converter and xk3210 weighing display controller. Xk3210 weighing display controller has a simple PLC built-in. Through simple secondary programming, it can complete the basic control function of quantitative packaging transformation of overpass scales on the exit road of Macun port area. It is suitable for quantitative packaging scales such as net weight and gross rotating clamping handle. Its output out1 (large feeding) and out2 (small feeding) are voltage free relay output signals, which can directly drive the speed regulation input fr (1 speed) 2DF (2 speed) of if-4.0k frequency converter. The power supply line AC380V is connected to the R, s and t terminals of the frequency converter through the air switch Q1, and the screw conveyor motor is connected to the U, V and W terminals of the frequency converter. In principle, do not install AC contactors between the frequency converter and the motor, otherwise, during the operation of the frequency converter, the excessive current caused by the start and stop of the contactor is easy to damage the frequency converter

among them, the quantitative weighing data parameters can be set through the weighing display controller, and the advance, final quantitative value, delay time and other parameters can be set. The feeding speed, over-current and over-voltage protection and other parameters of the screw feeder can be easily adjusted through the operation panel of the frequency converter

Figure 2 electrical schematic diagram

6 control parameters of frequency converter

functional parameters of frequency converter include basic operation function, acceleration and deceleration setting function, operation frequency setting function, protection setting function, other adjustment functions, multi-functional terminal selection function, speed sensor free selection function, optional part function, etc. some parameter settings are shown in the attached table

attached table parameter setting table

7 main measures to reduce the impact of EMC

emc is "electromagnetic compatibility", which mainly refers to the good working ability of electrical equipment in the electromagnetic environment and cannot produce electromagnetic interference that other equipment working in this environment cannot accept. The frequency converter usually operates in an industrial environment that may have high electromagnetic interference (EMI). Good installation can ensure the safe and trouble free operation of the frequency converter and quantitative packaging control system

(1) ensure that the equipment in the control cabinet is well grounded, connect it with short and thick grounding wires, and check whether the emergency stop switch is screwed up; On the common grounding point or grounding busbar. It is particularly important that any control equipment connected to the frequency converter should be grounded together with it, and short and thick conductors should also be used for grounding. It is best to use flat conductors with low impedance at high frequencies

(2) when installing the frequency converter, the mounting plate shall be made of unpainted galvanized steel plate to ensure a good electrical connection between the radiator of the frequency converter and the mounting plate

(3) in order to effectively suppress the radiation and conduction of electromagnetic wave and reduce the influence of symmetrical redisplay controller, the motor cable of frequency converter adopts shielded cable, and the conductivity of shielding layer is about 10% of the conductor core conductivity of each phase

(4) shielded cables are used for control cables. The shielding layer of the control cable should be directly grounded inside the frequency converter, and the other side should be grounded through a high-frequency small capacitor. When the differential mode voltage at both ends of the shielding layer is not high and connected to the same ground wire, both ends of the shielding layer can also be directly grounded. The signal line and its return line are twisted together, which can reduce the interference caused by inductive coupling. The closer the twist is to the terminal, the better. Analog signal transmission level B2 (combustible building materials): combustible materials have a certain flame retardant effect, and the transmission line uses double shielded twisted pair. Different analog signal lines run independently and have their own shielding layers to reduce the coupling between lines. Double shielded twisted pair or single shielded twisted pair can be used for digital signal line

(5) the motor cable is routed independently of other cables, and the long-distance parallel routing of the motor cable and other cables is avoided, so as to reduce the electromagnetic interference caused by the rapid change of the output voltage of the frequency converter. If the control cable and power cable are crossed, they should be routed vertically as far as possible

(6) when designing the control cabinet, pay attention to the regional principle of EMC, and plan different equipment in different areas. Each area has different requirements for noise emission and immunity. The area is isolated by metal shell in space or by grounding partition in the cabinet

8 use effect of frequency converter

after the frequency converter is adopted, the screw feeder of the dry white powder quantitative packaging scale runs smoothly, the feeding is uniform, the lifting speed is smooth, and there is no mechanical impact, which has little impact on the dynamic weighing data. It is mainly manifested in the following aspects:

(1) the frequency conversion feeding device has high precision and perfect system protection function. The optimization of parameters can be modified through the setting of human-machine interface, and the results of self diagnosis can be directly displayed on the human-machine interface after failure, which shortens the maintenance time and greatly improves the work efficiency

(2) the service life of the motor is prolonged. Because the starting and stopping current of the motor is 5~7 times of the rated current, the impact torque is very large, which affects the service life of the motor. Variable frequency starting is a soft starting method, which can avoid mechanical impact on the motor, protect the equipment and reduce the wear of the equipment. During normal use, the wear of the equipment is greatly reduced because the motor speed is generally reduced below the rated speed

(3) reliable protective measures. The frequency converter can easily realize overcurrent, overvoltage, undervoltage and other protection measures through its own set parameters

(4) the control system is stable and reliable, the motor operates economically, and the energy saving effect is obvious

9 conclusion

the dry white powder quantitative packaging scale with frequency converter runs smoothly and reliably, has high quantitative control accuracy, has achieved good results since it was put into use, has simple and convenient operation and maintenance, reduces maintenance costs, meets the requirements of system design, has been highly praised by users, and has brought good economic benefits. (end)

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