At present, the competition in the cement industry is very fierce, but the key is still the competition of manufacturing costs. Electric motor power consumption accounts for 30% of the cost, so it is extremely important to do a good job in reducing the power consumption and increasing the efficiency of electric motors. Therefore, we need to carry out detailed work in every aspect, such as speed regulation method, electric motor selection, and starting device. At the same time, we should vigorously apply new technologies and achievements to promote energy conservation and consumption reduction in enterprises. I. Energy saving by variable frequency speed regulation 1. Energy saving by variable frequency speed regulation on fans and water pumps Most cement plants' equipment, especially some high-power equipment, are not fully loaded for most of the time during the production process. During the production process, different air (water) volumes are met by mechanical adjustment methods such as adjusting the opening angle of the baffle plate or valve. The disadvantages of this operation method are: (1) The speed of the motor and fan or water pump is high, the load intensity is heavy, and the power waste is serious; (2) The automation level of equipment operation is quite low, and it is almost entirely dependent on manual adjustment. The adjustment accuracy is poor and the control is inaccurate; (3) The electrical control adopts direct or reduced voltage starting. The current impact on the power grid is large during startup, the required power supply (power grid) capacity is large, and the power factor is low. (4) The mechanical impact is large during startup, resulting in a short service life of the equipment; (5) The noise is high and the dust pollution is serious. In cement plants, there are mainly raw material mill exhaust fans, kiln tail exhaust gas treatment fans, Roots blowers, cement mill exhaust fans, coal mill fans, grate cooler fans, air classifiers, circulating water pumps, feed water pumps, etc. Since the basic principle of variable frequency speed regulation technology is based on the relationship that the motor speed is proportional to the input frequency of the working power supply: n = 60 f (1-s) / p, (where n, f, s, and p represent the speed, input frequency, motor slip rate, and number of motor pole pairs, respectively); the purpose of changing the motor speed is achieved by changing the frequency of the motor's working power supply. According to the basic laws of fluid mechanics, fans and pumps are all square torque loads. Their speed n has the following relationship with the flow rate Q, pressure H, and shaft power P: Q∝n, H∝n2, P∝n3; that is, the flow rate is proportional to the speed, the pressure is proportional to the square of the speed, and the shaft power is proportional to the cube of the speed. The diagram below shows the pressure H-flow Q curve characteristic: n1 represents the characteristics of the motor running at its rated speed; n2 represents the characteristics of the motor running at reduced speed (n2); R1 represents the resistance characteristics when the resistance of the fan/pump pipeline is at its minimum; R2 represents the resistance characteristics when the resistance of the fan/pump pipeline increases to a certain value. When the fan/pump operates on the pipeline characteristic curve R1, the operating point is A, with flow rate and pressure of Q1 and H1 respectively. At this point, the power required by the fan/pump is proportional to the product of H1 and Q1, i.e., proportional to the area of ​​AH1OQ1. Due to process requirements, the flow rate needs to be reduced to Q2. In practice, by increasing the pipeline resistance, the operating point of the fan/pump shifts to point B on R2, increasing the pressure to H2. At this point, the power required by the fan/pump is proportional to the product of H2 and Q2, i.e., proportional to the area of ​​BH2OQ2. Obviously, the power required by the fan/pump has increased. While this adjustment method is simple, it consumes a lot of power and is not conducive to energy saving, trading high operating costs for a simple control method. If variable frequency speed control is used, the fan speed decreases from n1 to n2, the operating point moves from point A to point C, the flow rate remains Q2, and the pressure decreases from H1 to H3. At this point, the power required by the fan after variable frequency speed control is proportional to the product of H3 and Q2, that is, proportional to the area of ​​CH3OQ2. As can be seen from the figure, the power reduction is significant. That is, when the fan/pump speed decreases by 10%, the motor power consumption decreases by 27.1%. Therefore, the energy-saving effect of using variable frequency speed control for fans and pumps is very significant. 2. Replacing Traditional Speed ​​Control with Variable Frequency Speed ​​Control Traditional speed control methods such as thyristor cascade speed control, DC speed control, electromagnetic slip speed control, hydraulic coupler speed control, and asynchronous motor step-by-step speed control have disadvantages such as low transmission efficiency and difficulty in maintenance. Variable frequency speed control, on the other hand, has a simple structure, is stable and reliable, has high speed control accuracy, large starting torque, and a wide speed range. Therefore , using variable frequency speed control can improve the transmission efficiency of machinery and save about 20% of energy. 3. Application of Variable Frequency in Air Compressors Air compressors with constant pressure supply use a variable frequency drive (VFD) and pressure control to form a closed-loop control system, reducing pressure fluctuations by 1.5%, lowering noise and vibration. This ensures long-term stable operation of the equipment, thereby reducing maintenance workload and extending equipment lifespan. With a VFD, the air compressor can start at any pressure, breaking the previous rule against starting under pressure, and the starting current is also significantly reduced. In examples using VFDs, most compressors achieve energy savings of approximately 20%. In summary, the use of frequency converter control has the following advantages: (1) Using frequency converter to control the motor speed eliminates the need for baffle adjustment, reduces the equipment failure rate, and significantly saves electricity; (2) Using frequency converter to control the motor enables soft starting of the motor, extends the service life of the equipment, and avoids impact on the power grid; (3) The motor operates at a speed lower than the rated speed, reducing the impact of noise on the environment; (4) It has automatic protection functions such as overload, overvoltage, overcurrent, undervoltage, and power phase loss; (5) It improves product quality and output. Practice has proven that frequency conversion has significant energy-saving effects and is an ideal speed control method. It not only improves equipment efficiency and meets the requirements of production processes, but also greatly reduces equipment maintenance and repair costs. In addition, when frequency conversion speed regulation is used, the DC reactor in the frequency converter can effectively improve the power factor, which can also save capacity for the power grid. The direct and indirect economic benefits are very obvious. II. Power Factor Compensation of Motors Squirrel-cage motors usually use the method of local compensation by parallel capacitors. Wound-rotor motors can employ phase shifter compensation. Phase shifter compensation comes in two types: rotary and stationary. Due to structural defects in rotary phase shifters, they are gradually being replaced by stationary phase shifters. III. Appropriate Selection of Motor Type The Y-series motor is a new series of products with a nationally unified design, and is currently one of the most advanced three-phase asynchronous motors in China. It was widely used throughout the country in the mid-1980s. Its advantages include high efficiency, energy saving, and good starting performance. Currently, many older cement enterprises in China still use a large number of JO2 series motors. Compared to the JO2 series, the Y-series motors have a 0.413% higher efficiency. Therefore, replacing older motors with Y-series motors is imperative. In addition to meeting the drive function, the selection of motor type should also consider economic operating performance. For applications with an annual operating time greater than 3000 hours and a load rate greater than 50%, the high-efficiency YX series three-phase asynchronous motor should be selected. Compared to the Y-series, its efficiency is on average 3% higher, and losses are reduced by 20% to 30%. Although the price is higher than the Y-series motors, the economic benefits are significant in the long run. Synchronous motors can improve the power factor of a company's power grid and reduce power line losses, but their control systems are complex and expensive. Reasonable selection of motor rated capacity is crucial. The state has stipulated the following three operating zones for three-phase asynchronous motors: a load rate between 70% and 100% is the economic operating zone; a load rate between 40% and 70% is the general operating zone; and a load rate below 40% is the uneconomical operating zone. If the motor capacity is too large, although it can ensure the normal operation of the equipment, it not only increases investment but also results in low efficiency and a waste of electricity. Therefore, considering both meeting the operational needs of cement plant equipment and maximizing its efficiency, a load rate of 60% to 100% is generally ideal for cement companies. For delta-connected motors with a load rate less than 40%, a star connection can be used to improve their efficiency. Synchronous motors can improve the power factor of a company's power grid and reduce power line losses, but their control systems are complex and expensive. With the improvement of asynchronous motor manufacturing technology, they are rarely used in new equipment. IV. Motor Starting and Operating Modes Low-voltage squirrel-cage motors (large and medium-sized): If full-voltage direct starting is used, this requires a sufficiently large power system capacity. However, in actual operation, the power system load rate is very low, affecting power supply efficiency. Furthermore, direct starting easily burns out switches and motors, affecting the operation of other equipment in the power grid. Often, to minimize the number of motor starts, it is preferable to let the motor idle without stopping, resulting in significant waste. These types of motors can be started using a motor soft starter. A motor soft starter uses a high-power thyristor module as the switching element in the main circuit, controlling its conduction angle to achieve a soft voltage ramp-up. It has many advantages, such as minimal impact on the power grid, low vibration to the mechanical transmission system (gears and shaft connectors), and smooth and stable starting torque. The starting current is adjustable between 2.5 and 3.5 times the rated current, and the starting time is also adjustable. High-voltage squirrel-cage motors: Traditional starting methods often use reactors, autotransformers, etc., but these starting devices cannot adequately meet the starting requirements, making it difficult to obtain ideal starting parameters. Currently available thermal resistance soft starters can better meet the starting requirements. Thermostatic resistors are made of resistive materials with a negative temperature coefficient. Connected in series in the motor stator circuit, when the motor starts and the starting current flows through the resistor, its temperature rises, and its resistance decreases accordingly. This gradually increases the motor terminal voltage and starting torque, achieving a smooth start. Based on the motor parameters and the required starting torque of the load, an appropriate starting resistance value can be easily configured to obtain optimal starting parameters—that is, obtaining a sufficiently large starting torque with a relatively small starting current. Large wound-rotor motors Previously, frequency-sensitive rheostats were mostly used for starting, but their failure rate was too high. Currently, a more mature method is to use a liquid rheostat starter. It utilizes the liquid resistance between two electrodes, and through a mechanical transmission device, the distance between the plates gradually approaches until contact, achieving a stepless decrease in resistance in the rotor circuit until it reaches zero, realizing a smooth, shock-free start for the motor. Its characteristics include low starting current, no impact on the power grid, large heat capacity, continuous starting capability for 5-10 times, convenient maintenance, and reliable operation. Currently, all motors of this type in our factory use liquid rheostat starters. Medium and Small Wound-Roller Motors Previously, starting systems primarily used frequency-sensitive resistors and oil-immersed resistors. Because these starting systems, consisting of components like slip rings, carbon brushes, short-circuit rings, relays, AC contactors, and frequency-sensitive or oil-immersed rheostats, were installed in dusty production environments, they suffered from high failure rates and maintenance requirements, frequently affecting equipment operation. Brushless and ringless starters effectively solve these problems. They offer smooth starting, do not alter operating characteristics, and are unaffected by dust. Their primary starting current is limited to 3.0–4.0 Ie, suitable for high and low voltage wound-rotor motors ranging from 11 to 600 kW. This starter utilizes the principle of a frequency-sensitive rheostat, leveraging the frequency-sensing characteristics of ferromagnetic materials. It is installed in the location where the slip rings were originally installed on the motor shaft, rotating synchronously with the rotor, eliminating the need for an auxiliary starting device. V. Pelletizing Water Supply System The raw material pelletizing process is one of the key processes affecting the sintering quality of cement clinker, with the water-to-material ratio directly influencing the pelletizing quality. After applying a frequency converter, the speed of the pelletizing pre-water pump can be steplessly adjusted by tracking the raw material supply, thereby achieving fully automated closed-loop control. This ensures stable material-water matching, excellent pelletizing effect, and significantly improves cement sintering quality. This system upgrade primarily aims to improve automation and manufacturing process level; energy saving is secondary due to the lower power consumption. VI. Raw Material Homogenization Feeding System After the system was upgraded with a frequency converter, all feeding motors at the feeding ports are synchronously and inorganically speed-regulated using frequency converters, ensuring proportional feeding and improving homogenization. This is also from a manufacturing process perspective. VII. Cement Classification System The cement classification system works by adjusting the speed of the classifier and classifying fan according to the grade of the cement being produced, thereby selecting cement products of different fineness. Traditional air classifiers require adjusting the number and angle of the fan blades on the blower shaft, and comparative tests are needed to achieve the required particle fineness. Newer air classifier systems consist of a classifier and a classifier fan. The classifier's speed is controlled by a slip-ring motor, while the airflow of the classifier fan is adjusted by changing the angle of the baffle plate. Both systems suffer from complex operation, poor adjustment precision, and significant energy waste. Slip-ring machines, in particular, are not only energy-intensive but also suffer from high expansion rates due to the dusty environment of cement manufacturing, making maintenance difficult. After frequency conversion modification, both the old and new systems can select particles of the required fineness simply by adjusting the motor to a specific speed. This saves energy while achieving continuous and automated production, improving labor efficiency and reducing labor intensity, resulting in significant overall benefits. VIII. Vertical Kiln Unloading System To achieve balance in feeding, air supply, and unloading during cement sintering, vertical kilns commonly use slip-ring motors (electromagnetic speed-regulating motors) as the power source for the disc-type unloading device. However, these motors not only fail to meet the protection requirements of cement production environments, but also consume approximately 20% more power than conventional motors at the same output speed, with the difference increasing even further when the speed is reduced. Therefore, replacing slip-ring motors with variable frequency drives (VFDs) solves these shortcomings, offering significantly superior speed regulation performance. This results in energy savings and reduced maintenance costs, leading to widespread adoption across various industries. Using VFDs to control speed-adjustable motors saves substantial amounts of energy and provides a soft-start function, reducing the motor's starting and running currents. This lowers the load on the entire power and mechanical systems during startup and operation, extending the lifespan of mechanical components. Furthermore, the VFD retrofit of slip-ring motors improves their protection level, reducing motor failure rates caused by harsh environments. IX. Unexpected Benefits The decrease in current during the operation and startup of the frequency converter provides necessary assurance for the startup of other equipment, thus increasing the factory's power capacity. This is particularly beneficial in situations with unstable grid voltage and insufficient power capacity. For example, after a comprehensive upgrade like that of Tianma Cement Co., Ltd., 200KVA of transformer capacity can be saved. When installing new equipment, the substation capacity can be temporarily not increased, saving a significant amount of investment. Of course, after the frequency converter upgrade, production process management should be strengthened, and the motor speed should be reasonably adjusted under permissible production conditions to achieve ideal energy-saving results. This needs continuous improvement in future work. 1. Application in Vertical Kiln Roots Blowers Of the electrical energy consumed in the calcination of clinker in a vertical kiln, the Roots blower typically accounts for about 60%. With adjustments in electricity prices, the proportion of electricity costs in cement production costs is increasing. Therefore, reducing the energy consumption of blowers has become an important aspect of improving the economic efficiency of enterprises. For Roots blowers, the air volume can be adjusted by steplessly changing the power supply frequency of the blower using a frequency converter. Chongqing Diwei Cement Co., Ltd. installed a frequency converter on the 132KW Roots blower of its No. 1 kiln, achieving an energy saving rate of up to 62.2%. The power consumption per ton of clinker decreased from 15.22 kWh before installation to 5.55 kWh after installation. Henan Jiaozuo Cement Plant installed a frequency converter on the 55KW Roots blower in the raw material fluidization system of its 10,000/ton cement clinker rotary kiln production line, achieving an energy saving rate of up to 73.2%. The average daily power consumption decreased from 606 kWh before installation to 162 kWh after installation, a daily saving of 444 kWh. 2. Application in Centrifugal Fans Some cement plants use high-pressure centrifugal fans for air supply. The air volume of these cement kilns is adjusted by regulating the opening degree of the dampers. Frequency conversion speed regulation retrofitting of centrifugal fans also has significant energy-saving potential. This is because the flow rate of a centrifugal blower is directly proportional to its rotational speed, the pressure is directly proportional to the square of the rotational speed, and the power is directly proportional to the cube of the rotational speed. Therefore, when adjusting the air volume or flow rate, reducing the air volume or flow rate by 20% will reduce power consumption by 50%. However, it must be noted that the rotational speed and pressure have a square relationship; when the rotational speed decreases by 20%, the pressure will decrease by 60%. Therefore, the pressure range required by the process cannot be ignored like with Roots blowers, where the relationship between rotational speed and air pressure is not considered. 3. Application in Vertical Kiln Unloading Machines If a slip-ring speed-regulating motor is used in a vertical kiln unloading machine, its speed is usually controlled between 300 and 1000 rpm (the unloading speed is controlled according to the kiln conditions). Replacing the slip-ring motor with a variable frequency speed control method has shown, based on application results from multiple manufacturers, an average energy saving of about 40%. This is because slip-ring speed regulation is an energy-intensive and inefficient speed regulation method. From the following formulas, we can see that: Output power of the main motor shaft of the slip differential motor: P0 = KM0N0 (P0 represents output power, M0 represents load speed, N0 represents motor speed, and K is a constant) Output power of the slip differential head: P1 = KM0N1 (P1 represents output power, N1 represents slip differential head speed) Power loss of the slip differential head: P = P0 - P1 = KM0 (N0 - N1) Therefore, the lower the speed of the slip differential motor, the greater the energy waste. Since the unloading machine typically operates at around 400 rpm, using variable frequency speed control would result in a 50-60% energy saving effect. 5. Application in Pre-hydrated Pelletizing Systems Currently, pre-hydrated pelletizing technology is widely used in vertical kiln cement plants. It has achieved significant results in improving pelletizing quality, improving calcination operating conditions, and increasing the output and quality of vertical kiln clinker. Combined with a microcomputer dual-loop regulator, it can automatically track and adjust the water-to-material ratio, achieving constant pressure water supply. Timely adjustment greatly reduces the labor intensity of workers and improves pellet quality, ensuring the pre-watering system truly functions as a pre-wet pelletizer, creating conditions for high-quality, high-yield clinker production in vertical kilns. 6. Application in Air Classifiers The cyclone air classifier of Chongqing Diwei Cement Co., Ltd. was originally designed to be driven by a JZT392-4 type 75KW electromagnetic speed-regulating asynchronous motor (slip differential motor). Its advantages are a simple speed regulation system, low price, and a certain speed range. However, it also has many disadvantages: high motor noise, large vibration, high energy consumption, large reactive power loss, and a particularly high bearing failure rate. The slip differential controller is installed next to the dusty motor, resulting in repeated failures such as starting under load, inability to adjust speed, and sudden stalling. This leads to a large amount of on-site maintenance and affects the safe operation of the entire system. To address the aforementioned issues, and considering the characteristic that the classifier in the raw material workshop operates at a speed not exceeding 600 r/min, the electrical components of the classifier were upgraded using variable frequency speed control technology. Actual measurements showed that before the upgrade, at an operating speed of 594 r/min, the input voltage was 385V, the input current was 72A, and the power factor was 0.82, resulting in an input power of 40 kW. After the upgrade, at an operating speed of 594 r/min, the input voltage was 387V, the input current was 18A (the thermal relay was also adjusted accordingly), and the power factor was 0.92 (a direct reactor was added to the frequency converter), resulting in an input power of 11 kW. In the year following the upgrade, no malfunctions occurred, ensuring the safe operation of the system, significantly reducing maintenance workload and repair costs, and demonstrating remarkable energy-saving effects. The applications of frequency converters in cement plants are not limited to these. For example, they can be used for rotary kiln ball mills, unloading rings, disc feeders, double-tube auger skirts, speed-regulating belt feeders for plate feeders, coal augers, grate coolers, and all other equipment that requires AC speed regulation.