[Abstract]: This paper introduces the application of AC drives on ball grinding machines, which are widely used in the cement and ceramic industries, and conducts energy-saving analysis and calculations. The results show that there is still a certain energy-saving potential in using AC drives on ball grinding machines. [Keywords ]: Ball grinding machine, AC drive, hydraulic coupler, energy-saving mode I. Overview The application of AC drives in the cement and ceramic industries is already very common. Production machinery such as rotary kilns, kiln head and kiln tail fans, conveyor belts, etc., which originally used electromagnetic speed regulation or other speed regulation methods, have been successively transformed into AC variable frequency speed regulation. The purpose of the transformation is to facilitate process speed regulation during production, improve product output and quality, achieve automation, and save energy. However, the use of AC drives on ball grinding machines is basically still a blank. The reason is that the rotational speed of the ball mill cylinder is constant, and even if it needs to be varied, the range of variation is not large. While reducing the rotational speed might save energy, it could increase the milling time, making the energy-saving effect insignificant. In reality, the ball mill production process is relatively simple. For example, a ceramic factory's ball mill motor is 90kW, with 16-18 tons of material added to the cylinder. It runs at 50Hz for 8 hours, and once the required fineness is achieved, the material is discharged. Then, more material is added, and the process is repeated. Generally, this is empirical process data; the process parameters will vary depending on the manufacturer and the raw materials used in the ball mill. In the ceramic industry in a certain region, the commonly used electrical drive method for ball mills is a three-phase AC squirrel-cage asynchronous motor—hydraulic coupling—gear reducer—pulley reducer. Here, the ball mill cylinder is used as the pulley of the reducer. With a 90kW motor, if there is no hydraulic coupling in the transmission link when the ball mill is started under heavy load, even using an autotransformer or star-delta starter will cause a significant impact on the power grid and frequently result in start-up failures. To buffer the impact during startup, a hydraulic coupling is added to the transmission system, allowing the ball mill to start smoothly under any conditions. II. Hydraulic Coupling and Variable Frequency Speed ​​Control for Energy Saving The hydraulic coupling transmits energy to the motor by controlling the change in the angular momentum of the working oil within the working chamber. The motor drives its driving impeller through the input shaft of the hydraulic coupling, accelerating the working oil. The accelerated working oil then drives the driven turbine of the hydraulic coupling, transferring energy to the output shaft and the load. Hydraulic couplings are divided into speed-regulating and torque-limiting types; the former is used for speed regulation of electrical drives, and the latter for motor starting. The hydraulic coupling in the system acts as a buffer during motor startup, with a theoretical efficiency of 95%. The efficiency of the variable frequency drive is 96%. By removing the hydraulic coupling from the transmission system and using an AC speed-regulating variable frequency drive to drive the ball mill motor, a 90kW motor can be started smoothly at rated current, and theoretically, the efficiency can be improved by 1%. In reality, the efficiency of a hydraulic coupling is related to the amount of oil injected into the coupling chamber. During operation, the hydraulic coupling experiences a certain temperature rise, and factors such as incomplete sealing and leakage can occur. Therefore, its efficiency is generally less than the theoretical value of 95%, and a reduction of 2% to 3% is common. The ball mill motor speed is 1440 r/min, and after being reduced in speed by the hydraulic coupling and reducer, the barrel's operating speed is 16 r/min. The formula for calculating the efficiency of a ball mill is: Where: nT—is the speed of the output shaft of the hydraulic coupling, nB—is the speed of the input shaft of the hydraulic coupling on the motor side. Its efficiency is 0.95. In the transmission system, the hydraulic coupling is removed, and a flexible coupling is used for direct connection. At this time, the speed of the feed cylinder will exceed 16 r/min. To maintain the original process conditions, the motor should be decelerated, and the output frequency of the frequency converter drops to 50 × 0.95 = 47.5 Hz. Since the ball mill is a constant torque load, the motor operating frequency decreases, and its current remains basically unchanged or decreases slightly, but the output voltage decreases proportionally. According to the three-phase motor power calculation formula: There is a local compensation capacitor on site. After using the frequency converter, the power factor remains basically unchanged, therefore, 5% power should be saved. The sum of the first two terms is approximately 8% energy saving. III. Motor Voltage Adjustment and Energy Saving The starting current of the ball mill is relatively large. After starting, the operating current is 80-110A, which is equivalent to light load operation for a 90kW motor. At this time, the motor efficiency is also relatively low. Automatic energy-optimal control, utilizing the voltage regulation function of the frequency converter, can maintain the motor's efficiency at its highest level under conditions unrelated to the load. Asynchronous motors suffer from losses including copper loss, core loss (often called iron loss), mechanical friction loss, and stray losses. For mass-produced motors, mechanical friction loss is constant during operation; copper loss is proportional to the square of the motor current and varies with the load; iron loss is approximately proportional to the square of the motor terminal voltage; and stray losses are also proportional to the square of the motor current and vary with the load. For a 90kW motor, the percentages of each type of loss in the total losses are: copper loss 20%–30%, iron loss 20%, mechanical friction loss 40%–50%, and stray losses 15%–20%. For the same load, an increase in motor terminal voltage leads to a decrease in current; therefore, when the terminal voltage increases, copper loss and stray losses decrease, while iron loss increases. Thus, there exists a point in the motor's terminal voltage where the total motor loss is minimized, as shown in Figure 1. The load conditions of a 90kW ball mill in a ceramics factory are as follows: the ball mill motor is 90kW, the cylinder speed is 16r/min, each working cycle is 3.75s, and the current varies between 80 and 110A, equivalent to 46% to 62.5% of the rated current, as shown in Figure 2. [align=center] Figure 2: Ball Mill Load Diagram[/align] The energy-saving effect is analyzed below: 1. Without energy-saving control: Power consumption at high load P10 = 90 * 0.625 / 0.884 = 63.63 (KW) 0.884 represents the motor efficiency at 62.5% load. Power consumption at low load P11 = 90 * 0.46 / 0.65 = 63.69 (KW) 1. When the motor efficiency is 0.65 at 46% load, the average power per cycle is P1 = (P10 * 0.75 + P11 * 3) / 3.75 = 63.68 (KW). 2. When the inverter is performing energy-saving adjustment: Power consumption at higher load is P20 = 90 * 0.625 / 0.886 = 63.49 (KW). Power consumption at lower load is P21 = 90 * 0.46 / 0.70 = 59.14 (KW). Power consumption at lower load is P21 = (P20 * 0.75 + P21 * 3) / 3.75 = 60.01 (KW). Power consumption per cycle is P2 = (P20 * 0.75 + P21 * 3) / 3.75 = 60.01 (KW). Based on 320 days per year and 20 hours per day, the annual electricity saving is (63.49-60.01)*6400=22272 (kW.h). This translates to an energy saving rate of (63.49-60.1)/63.49=0.055=5.5%. As seen from the data above, when using a frequency converter for energy-saving adjustment, for the two load conditions, the efficiency of the 62.5% load increases from 0.884 to 0.886, a mere 0.002; the efficiency of the 46% load increases from 0.65 to 0.70, also only by 0.05, but the energy saving rate is still 3%. IV. Conclusion Currently, energy saving in ball mills is considered from only two aspects: removing the hydraulic coupling and using a frequency converter results in an 8% energy saving rate, and utilizing the frequency converter's energy-saving mode can achieve a 3% energy saving. The two combined result in an 11% energy saving. Therefore, it is evident that frequency conversion retrofitting of ball mills can achieve the goal of energy saving. Currently, some researchers are modifying the ball milling process. Specifically, after feeding the material, they appropriately increase the inverter's output frequency for a period to accelerate the milling process, then reduce the output frequency. After running for a while, they continue to reduce the output frequency. This approach can save milling time while maintaining milling quality. This is another energy-saving strategy, but it still needs practical testing.