In my country's ceramic industry, ball mills are used in large quantities and are indispensable and important production equipment for material crushing.
I. Overview
Ball mills generally have large power and low working efficiency, thus becoming the largest power-consuming equipment in the ceramic industry. The energy-saving transformation of the ball mill is of great economic and social significance for building a conservation-minded society and reducing environmental pollution in my country.
2. Working principle
At present, the driving methods commonly used in ball mills are: three-phase AC motor-hydraulic coupler-gear reducer-belt reducer. Here, the barrel of the ball mill is used as the pulley of the reducer. When the ball mill starts under heavy load, the motor starting current can reach more than ten times the rated current. In order to alleviate the mechanical shock at the start, a hydraulic coupling is added to the transmission link. The hydraulic coupling transmits the driving energy of the motor by controlling the change of the momentum moment of the working oil in the working chamber. The electric motor drags its active working wheel through the input shaft of the hydraulic coupler to accelerate the working oil, and the accelerated working oil then drives the driven working turbine of the hydraulic coupler to rotate, transmitting the driving energy to its output shaft and load. The hydraulic coupler in the transmission system acts as a buffer when starting, allowing the motor to start at light load to reduce the starting current. After the motor is started, it is slowly loaded to achieve the smooth start of the ball mill and reduce the machinery at start. Shock. During the work, the speed of the hydraulic coupling is used to search for the best working speed of the ball mill and improve the efficiency of the ball mill. Because the ball mill is a constant torque load, when the speed is adjusted by the hydraulic coupling, the speed regulation efficiency is equal to the speed ratio, and a large part of the energy is wasted in the hydraulic coupling.
The ball mill generally runs at a fixed speed, and the speed of the barrel is determined by the pulley or gear reduction mechanism (also with a hydraulic coupling). The speed of the ball mill directly affects the movement of the steel balls and materials and the grinding process of the materials. At different speeds, the movement of the steel balls and materials in the barrel is shown in the following figure:
(A) When the rotation speed is low, the steel ball and the material rise with the inner wall of the cylinder. When the inclination angle of the steel ball and the material is equal to or greater than the natural inclination angle, they slide down along the inclined surface and cannot form a sufficient drop. The steel ball grinds the material The effect is very small and the efficiency of the ball mill is very low.
(B) When the rotation speed of the cylinder is at a certain speed between the two, the steel ball is brought to a certain height and falls along the parabola, which has the largest impact on the material at the bottom of the cylinder and the highest grinding efficiency. The rotation speed at this time This is called the optimal working speed.
(C) If the rotation speed of the cylinder is very high, due to the centrifugal force, the material and the steel ball no longer leave the wall of the cylinder, but rotate together with it. At this time, the steel ball has no impact on the material, and the grinding efficiency is lower. . The minimum speed in this state is called the critical speed.
The actual operation shows that the optimal working speed is related to the diameter of the steel ball and its load, the shape of the armor, the friction coefficient between the steel ball and the armor, and other factors. Generally, the optimal working speed is usually 0.7 to 0.8 of the critical speed. It can be seen that the speed still has a certain adjustment range, but the adjustment range is not large. The error between the actual speed of the ball mill and the best working speed is less than 10%, so if the ball mill uses frequency conversion to search for the best working speed, the energy saving effect of improving the grinding efficiency is also within 10%.
3. System configuration
In addition to the use of frequency conversion speed regulation to search for the best working speed and improve the energy saving effect of the grinding efficiency of the ball mill, because the ball mill is designed, the maximum output torque of the motor is considered, but in the actual production process, it often fails to reach To the maximum output torque, the motor is in light load (not full load) working state, and its power factor and efficiency are low. At this time, through the energy-saving operation function of the inverter, by reducing the output voltage of the inverter, automatically adjusting the output torque that matches the actual production needs, to achieve the purpose of improving the power factor and efficiency of the motor and saving energy.
In addition, because the inverter is generally an “AC-DC-AC” voltage source inverter, there is a large filter capacitor in the middle DC link, which can greatly improve the grid-side power factor at the input end of the inverter, reduce the transmission current, and thus reduce the energy consumption. Transmission loss in the line not only has energy-saving effect, but also can reduce the capacity of the power supply system and reduce equipment investment. Or under the same grid capacity, it is allowed to install more ball mill equipment
The wiring diagram is as follows:
Four, system advantages
After installing the frequency conversion energy-saving control system, the ball mill achieved the following effects:
① Because the impact at the start is eliminated, the service life of mechanical parts such as crankshaft, gearbox gears, belts, etc. is extended, and the maintenance cost is reduced.
② After the ball mill uses variable frequency speed regulation, the starting current can be more than 5 times smaller than the original (for example, the original starting current of the 132kW motor is above 1000A, and the starting current of the frequency conversion energy-saving control system is between 180A and 200A), realizing a real soft start. Also received the effect of energy saving. Since the starting current is greatly reduced, it will not cause impact on the power grid and the voltage of the power grid will drop, eliminating the tripping or failure of other electrical equipment caused by the start of the ball mill. Under the same power grid capacity, the number of installed units can be increased.
The energy saving effect of the ball mill frequency conversion speed regulation energy-saving system can generally reach about 12% to 15%.
Take a well-known ceramic factory in Xiqiao Town, Foshan City as an example. The factory has installed 37 high-power inverters from Alpha. Take one of the 40-ton ball mills as an example, the motor power is 132 kW, and the original ball mill’s monthly electricity consumption is: 115 kW×18 hours×30 days=62100 kW.h (degrees). The frequency conversion energy saving for the ball mill is installed. After the control system, the power saving rate can reach 14.6%, and the monthly electricity cost can be saved: 62100×14.6%×0.65 yuan/degree=5893 yuan. If the investment of each inverter is 55,000 yuan, the investment recovery period is 55000÷5893=9.3 months.
If the average power saving rate is 12%, the monthly power saving benefit is 62100×12%×0.65=4843 yuan, the investment of each inverter is calculated as 55,000 yuan, and the investment recovery period is 55000÷4843=11.3 months, that is The full investment can be recovered in less than a year. The service life of the inverter is generally 10 to 15 years, and the pure return period is one year later, that is, the annual power saving income is about 60,000 yuan.