Abstract : This paper introduces the application of frequency converters (VDCs) in cement production, using a production example. Keywords : Frequency converter; speed regulation; energy saving Introduction With the advancement of cement technology and the increasing automation and control of cement production, higher demands are placed on the precise control of equipment. Simultaneously, the rapid development of electronic technology has reduced the price of high-power semiconductor components, providing favorable conditions for the application of frequency converters in cement production. 1 Working Principle of Frequency Converters Simply put, a frequency converter is a semiconductor device that controls the speed of a motor by changing the frequency and voltage of the power supply. It consists of several parts, including a rectifier, filter, drive circuit, protection circuit, and controller. When in use, the frequency converter is connected in series between the power input terminals (R, S, T) and the motor output terminals (U, V, W) (see Figure 1). 2 Application Scope of Frequency Converters Frequency converters can be widely used in cement production equipment. Broadly speaking, frequency converters can be installed in any electrical drive component using an AC three-phase asynchronous motor. In our factory's 1,200 t/d new dry process production line, frequency converters are used in limestone plate feeders, stacker-reclaimer traveling trolleys and spreading belts, raw material batching speed-regulating belt scales, raw material mill classifiers, rotary kilns, cement mill batching speed-regulating belt scales, cement mill classifiers, dryers, packaging machine rotating frames, etc. Currently, various fans do not use frequency converters, and their air volume is controlled by adjusting the opening of valves, which causes a great waste of electricity; circulating water pumps also do not use frequency converters, resulting in excessive water pressure when water consumption is low, which poses a safety hazard to the equipment. 3 Energy-saving advantages of frequency converters (1) When controlling the U/f characteristic of frequency converters, it is necessary to use energy-saving instructions for water pumps and fan loads. The principle is that when the load characteristic is square torque, the frequency converter should select square torque external characteristic, so that its output voltage will be lower at a certain set frequency, and thus save more electricity. (2) Reduce the output voltage according to the size of the load. The main measures include: ① Manual setting: When the load is basically constant, such as belt conveyors and screw conveyors; ② Automatic setting: A frequency converter with this function should be selected; ③ According to the principle of minimum loss, efficiency and power factor. When copper loss and iron loss are equal, the motor loss is the minimum and the efficiency and power factor are the highest. A frequency converter with this function should be selected; ④ Utilizing the U-shaped characteristic of the motor, the frequency converter automatically optimizes to achieve the minimum current at the target speed, which has a significant energy-saving effect on fans and water pumps. 4. Precise speed regulation of frequency converters Compared with slip-ring motors, frequency converters have precise speed adjustment functions and a wider speed range, from 0 to 400 Hz, while slip-ring motors range from 0 to the rated speed of the driven motor; the input and feedback of slip-ring motors are analog signals, and their speed display often uses pointer-type voltmeters. Since feedback adjustment and tachometer calibration are set on the control panel, human factors can easily cause inaccurate speed indication. For example, the cement mill classifier in our vertical kiln production line once had its speed indicator and feedback potentiometer manually adjusted. Normally, the speed indication is 800 r/min, but even if it is adjusted to 1000 r/min, the fineness index is still not qualified. The mill operator dared not adjust it any higher for equipment safety reasons and had to stop the machine. After on-site testing by technicians, it was found that the indication of 1200 r/min was only equivalent to the normal 800 r/min, causing unnecessary production losses. The frequency converter can be regarded as a microcomputer. All operations can be set on the panel. It also has terminals for connection to the host computer. It can intuitively reflect the current, speed, frequency and other conditions of the equipment operation. The displayed values ​​are all digital quantities. The speed can be accurate to the single digit, which provides an effective guarantee for rotary kilns that require precise speed adjustment. 5. Issues to be noted when using frequency converters (1) The performance of frequency converters is not ideal when operating in the low frequency range. The main reason is that the starting torque is small when starting at low frequency, which makes it difficult or even impossible to start the system. This situation has occurred many times on the rotary kiln in our factory. The solution is to use slow drive to pull it before starting, and then switch to frequency conversion start after it starts rotating. (2) The rectifier circuit AC/DC or inverter circuit DC/AC of the frequency converter will generate harmonics, which will have a certain impact on the power grid and motor. Lower harmonics have a greater impact on the motor load and will cause torque pulsation; while higher harmonics will increase the leakage current of the output cable of the frequency converter, resulting in insufficient motor output. This can be controlled in two ways: First, select a frequency converter with good quality indicators, requiring the voltage component THD≤5% and the current component THD≤3%. A rectifier circuit with more than 12 pulses is required to achieve this indicator; Second, select harmonic suppression devices, such as AC reactors, DC reactors, output reactors, input and output filters, magnetic ring type zero-sequence allowable reactors and signal filters, etc. (3) Load matching problem. For constant torque loads, the starting torque and braking torque of the inverter are required to be relatively large, generally 150% of the rated torque; for constant power loads, it refers to a certain speed range. Constant torque speed regulation is used below a certain speed point, and constant power speed regulation is used above that speed point. (4) Inverter heating problem. The heating of the inverter is caused by its internal losses, of which the main circuit accounts for 98% and the control circuit accounts for 2%. The inverter must be installed in a well-ventilated environment. If the ambient temperature is high, an air conditioner should be installed to cool it down. Generally, the ambient temperature is required to be between -10 and 50℃. (5) Insensitive to phase loss protection. This is a problem found in actual use. Although the inverter is sensitive to overcurrent, locked rotor, undervoltage and other protections, it is easy to burn out the motor when the power supply or motor loses a phase, especially the low-power motor. Our factory once burned out the motor on the pre-filled water booster pump due to the power supply phase loss, and the inverter only activated after the motor burned out. (6) When measuring the insulation of the inverter, a 500 V megohmmeter should be used. When testing only the frequency converter, all connections to the converter terminals must be disconnected. After cleaning the components, all main circuit terminals should be short-circuited with wires and tested to ground with a megohmmeter; a result >5 Mn indicates normal operation. 7. Conclusion Frequency converters are increasingly widely used in cement production, playing a positive role in energy conservation and consumption reduction for enterprises. Although frequency converters are generally stable and reliable in operation, the lack of internal circuit diagrams provided by manufacturers makes repair difficult in case of damage. This necessitates that electrical technicians improve their professional skills and enhance their ability to maintain and use frequency converters. [b][align=center]For more details, please click: Application of Frequency Converters in Cement Production[/align][/b]