Sep 03, 2020
Overview of the development of frequency conversion technology
With the further development of the manufacturing process of high-power rectifier devices. The inverters produced by merchants generally use brand new IGBT (insulated gate bipolar) tubes. One of its outstanding advantages is that it has a higher switching frequency, that is, a short on-off time. The high switching frequency improves the motor's ability to carry a large load capacity and reduces the electrical loss. Inverters can generally be divided into AC-DC-AC and AC-AC. The controllable power range of the frequency converter is from 0.2~1500kW, even to 1700kW. From this point of view, all technological links required for speed regulation on the cement production line can be met.
From the perspective of application, the frequency converter used in cement production enterprises has four outstanding advantages:
(1) It can meet the technological requirements of speed regulation. The speed regulation range of the frequency conversion governor is above 10:1, while the speed regulation range of 10:1 during the cement production process can fully meet the requirements.
(2) The global realization of automatic control, because the inverter itself is controlled by a 16 (or 32) microprocessor, with RS485 (or 422), A/D input. The D/A output interface creates sufficient conditions for automatic control (networking with the host computer).
(3) Obtain considerable energy saving effect. For example, it is especially obvious in the application of larger power (above 15kW) fans and pumps, which can save electricity by more than 1/4.
(4) Reduce the labor intensity of workers. Due to the improved overall reliability of the adjustment system, the failure rate is low, and the maintenance-free period is longer, which can reduce the workload of related maintenance personnel.
2. How to choose frequency converter
Like other products, inverters also have more brands and types. Of course, we need to understand the price and product quality on how to choose this issue, and we should also understand the technical characteristics of the inverter. According to the different control methods, it can be divided into two major categories: general-purpose and engineering-type. In addition to price differences, general-purpose inverters are quite different from engineering-type inverters in technical characteristics. General-purpose inverters generally adopt U/f curve control schemes. When the motor is running at high speed, the air gap flux φ is basically constant. It can meet the operating characteristics of the equipment with constant power (or constant torque). However, at low speeds, due to the decrease of the air gap magnetic flux, the maximum torque also decreases, and it is difficult to meet the constant power requirements. General-purpose inverters generally adopt a given closed-loop control mode, and the dynamic response speed is relatively slow. The engineering type frequency converter is equipped with automatic compensation and automatic limitation links through detection, and can maintain good torque characteristics when the equipment is running at low speed, and can realize true closed-loop control.
There are usually three control setting methods for engineering inverters:
(1) Open loop control mode, same as general-purpose type;
(2) With slip compensation, that is, when the load drops suddenly, the setting can be automatically compensated;
(3) Installed/cabinet-mounted type, power 45~200kW, need additional circuit and integral fixed shell, the volume is relatively large, and the occupied space is relatively large;
(4) Cabinet type, control power 45~1500kW, in addition to the characteristics of installed/cabinet-mounted type, it takes up more space than that. When selecting, you can choose the appropriate type according to the controlled motor power and site installation conditions. From the perspective of the voltage grade of the inverter, there are 1AC 230V; there are also 3AC 208~230V, 380~460V, 500~575V, 660~690V grades. The correct choice should be made according to the requirements. In terms of frequency conversion range and accuracy requirements, the frequency converter has FC (frequency control), speed adjustment range 1:25; VC (vector control), speed adjustment range 1:100~1:1000; SC (servo control), speed adjustment Range 1:4000~1:10000;
The FC method is generally used in cement production lines to meet the production requirements. On the premise of taking into account the above four requirements, the correct choice that fully meets the requirements can be made according to the production site conditions.
3. Several problems in the on-site adjustment and operation of the frequency converter in the production line of the cement plant
A. Technical measures for on-site installation of frequency converters During the installation process, technical guarantee is the first priority. Special attention is needed to concentrate (or decentralize) technical preparations according to the number of frequency converters in a certain speed control link. In the speed control links of limestone crushing and feeding, various fans, water pumps, rotary kiln, kiln tail raw material feeding, kiln head coal feeding, raw meal mill, cement mill separator, etc., due to the relative dispersion of the single inverter devices, they interact with each other. The interference between the two is small and does not affect normal use. However, in some production links, such as raw and clinker batching systems, multiple inverters are required to be installed in a certain control panel in the same electric control room. Since the inverter itself is a strong interference source, high-order harmonics are generated during normal operation, and the interference between each other and the main control equipment (upper computer system) is inevitable, and the control system signal transmission is mostly low-to-DC (0 ~5V) method, relevant anti-interference measures must be adopted to ensure the normal operation of the system. According to the author's experience, the production site conditions are good. In addition to shielding the interference source strictly in accordance with the requirements, the control signal transmission adopts the current (4-20mA) method, which can effectively reduce the impact of interference on the system and meet the production needs. If the conditions of the production site are poor, except that the shield must be used as the connection line for the sampling and control signal of the inverter, and the signal line shielding network must be separately grounded, which can effectively suppress the interference. After the above measures are implemented, strong interference is still inevitable. If it affects the normal use of the system, consider replacing the inverter.
B. Inverter installation location and distance issues In the early stage of inverter application, most of the installation locations are on-site, and the production machinery is controlled locally: if centralized control is required, use a remote controller (the control line distance can reach more than 1km) . But in cement production, on-site installation is not advisable. Because the cement production line is dusty, most of the speed control machinery is installed outdoors or under the warehouse, and the environment is worse than other industries. The operators are generally concentrated in the electric control room, and the frequency converter is a more precise instrument and equipment. Unmanned and poor environment is unfavorable to the equipment, so in the cement production line, the inverter should be installed in the electric control room. According to the inverter product equipment, the production site is often unmanned and the environment is poor, which is not good for the equipment. Therefore, in the cement production line, the inverter should be installed in the electric control room. According to the inverter product manual, the length of the cable connecting the inverter to the motor must not exceed 50m. If a shielded cable is used, the maximum length must not exceed 25m. This must consider the distance between the inverter and the controlled motor. To solve this problem, the first method is to connect an outgoing reactor with a suitable current value in series on a line exceeding 50m (some inverters have already added this reactor in series, and the connection cable between the motor and the motor can be up to 300m long, but The price is higher, and it is wasteful if it is used under 50m). In actual use in our factory, the line distance between the kiln tail raw meal double-tube feeder and the kiln head electric control room is up to 180m, the controlled motor is 18.5kW, and the 50A outlet reactor (specially made) is connected in series and used normally. The other method is to add a power level of the frequency converter in the mining industry. On a line exceeding 50m, a 2.2kW motor is equipped with a 3kW inverter, and a 3kW motor is equipped with a 3.7kW inverter. This selection method is particularly suitable for multiple centralized group control. The installation location is small and crowded, and requires specifications (such as computer batching system) and other occasions.
C. Parameter setting and debugging of the frequency converter The setting and debugging of the frequency converter parameters is a fundamental guarantee for the normal operation of the equipment. Many problems that appear in the scene are often parameter setting problems, and have nothing to do with the equipment itself, so it is very important to set the parameters reasonably and correctly.
(1) Setting of basic parameters. The basic parameters include the rated power, voltage, current, speed, frequency, etc. on the motor nameplate. (2) Set other parameters according to the requirements of the production process. Inverters generally have dozens or hundreds of configurable parameters, most of the factory settings can be used in actual applications, and only a few special settings can meet production requirements. If it is required to start with a heavy load, the starting boost parameters must be set. An additional voltage boost can be set to increase the starting current within the range of 0~250% of the rated current of the motor to achieve a smooth start of the equipment.