The air-swept coal mill is the key equipment for material grinding. The transmission device is the driving part of the mill, and its characteristics directly affect the stability of the mill operation and the energy consumption of the mill. The traditional drive mode of an air-swept coal mill is traditional asynchronous motor + reducer + slow drive, which leads to problems such as a long mechanical transmission chain, low efficiency, complex mechanism, and heavy operation and maintenance workload in the system. As the country puts forward higher requirements for energy saving and emission reduction, improving the operating efficiency of the mill drive system has become the key to energy saving. Therefore, improving the operating efficiency and operating power factor of the motor itself, reducing reactive power consumption and line loss, thereby reducing the energy consumption of the mill, is a direction for mill energy saving. Using a variable frequency permanent magnet direct drive motor to reduce energy loss in the process of transmitting torque is an important means to realize energy saving of dry ball mill.
1 Working principle of air-swept coal mill transmission system and grinding system
As a low-speed, heavy-load, and constant-speed equipment, the air-swept coal mill needs to overcome the wear loss of the transmission system and the effective torque generated by the grinding body offset on the transmission shaft. The crushing impact force of the grinding body on the material in the mill makes the mill work accompanied by vibration and impact, and the continuous characteristics of the dry grinding work require the mill drive system to have high reliability and low failure rate, and the operating rate needs to be as high as above 95.
1.1 Working principle of the transmission system
The traditional air-swept coal mill is mainly composed of a mill body, a gear transmission part, a final reducer, a slow drive device, and an asynchronous motor. The structure is shown in Figure 1. When the air-swept coal mill is running, the asynchronous motor transmits the power to the pinion of the gear transmission part through the main reducer, and through the meshing of the large and small gears, the large ring mounted on the rotary part of the mill drives the mill to realize the mill. When the mill is installed or overhauled, the asynchronous motor stops the power supply, and the motor in the slow drive device transmits the power to the large and small gears of the gear transmission part through the final reducer, and the large gear ring mounted on the rotary part of the mill. The ring drives the mill to realize the slow rotation of the mill.
Figure 1 Schematic diagram of the transmission system of a traditional air-swept coal mill
1. Mill body 2. Gear transmission part 3. Main reducer 4. Slow drive device 5. Asynchronous motor
Due to the complex composition of the air-swept coal mill transmission system, including asynchronous motors, two-stage or three-stage reducers, and slow-speed drive devices, as well as low-speed couplings, high-speed couplings, helical gear clutches, etc., the overall transmission efficiency is low. Any problem in one link will cause the mill to fail to operate normally.
1.2 Working principle of grinding system
The structure of the grinding system of the air-swept coal mill is shown in Figure 2. Raw materials (accompanied by hot air at about 280 ℃) are sent into the feeding device of the mill by the feeding equipment and are quickly introduced into the drying chamber of the turning part of the mill through the feeding chute and the spiral. There is a lifting plate inside, containing a certain amount of Moisture materials that are dried by intense heat exchange here, and then enter the grinding chamber through the middle compartment, and together with the steel ball grinding medium, they are continuously impacted, crushed, or powdered under the action of the liner with lifting and grinding and finally grinding into coal powder, and then the special induced draft fan is taken out of the mill through the discharge device to enter the next process.
Fig. 2 Schematic diagram of the grinding system structure of the air-swept coal mill
1. Transmission device 2. Discharging device 3. Main bearing 4. Gear transmission part 5. Rotary part 6. Sliding shoe bearing at the feeding end 7. Feeding device
2 Principles of asynchronous motors and permanent magnet motors
2.1 Asynchronous motor
The excitation magnetic field of the asynchronous motor is provided by the stator winding, and the excitation current is used to generate the air gap magnetic field, so that the motor rotor winding forms an induced current, thereby generating an induced magnetic field, driving the rotor to rotate, and the rotor speed is inconsistent with the rotating magnetic field speed. Since the excitation magnetic field is provided by the stator winding, it is necessary to use short-distance distributed windings to ensure the sine degree of the excitation magnetic field, as shown in Figure 3, it is difficult to make multi-pole low-speed. Therefore, the induction motor + mechanical deceleration method is often used to achieve low-speed operation.
Figure 3 Distributed winding of the asynchronous motor
The asynchronous motor is only used as an electromechanical energy conversion device in the mechanical equipment transmission system to provide the power required by the system, which makes the system have problems such as a lengthy mechanical transmission chain, low efficiency, complex mechanism, and heavy operation and maintenance workload. In order to meet the driving requirements of the maximum load of mechanical equipment, asynchronous motors with relatively large rated power are often selected, and asynchronous motors need a part of the power to generate a magnetic field to maintain the normal operation of the motor, resulting in the load rate of most motors at 60% to 80%. As shown in Figure 4, the efficiency and power factor are low and the power consumption is high.
Fig.4 Relationship curve of asynchronous motor efficiency, power factor, and load rate2.2 Permanent magnet direct drive motor
2.2 Permanent magnet direct drive motor
Permanent magnet motors use permanent magnet poles instead of current-excited poles. The magnetic steel is set in the rotor, which can generate a magnetic field without inputting current. During operation, the rotor and the rotating magnetic field rotate synchronously with the help of the rotating magnetic field generated by the permanent magnets. Permanent magnet direct drive motors do not require reactive excitation current, can significantly improve efficiency and power factor, reduce stator current and stator loss, and have no rotor resistance loss during stable operation, reducing total loss and reducing fan and corresponding wind friction loss. Compared with asynchronous motors, permanent magnet motors have a simpler rotor structure, no excitation windings, less rotor copper loss, and their power factor can be increased by 10% to 15% compared with asynchronous motors of the same specification, as shown in Figures 5.
Fig.5 Relationship curve of permanent magnet synchronous motor efficiency, power factor, and load rate
Although the replacement of asynchronous motors with permanent magnet motors improves the energy efficiency level of the motors, it still fails to solve the problem of lengthy transmission chains of large mechanical equipment due to constant speed replacement. The excitation magnetic field of the rare earth permanent magnet motor is provided by the rotor, and the design purpose of the multi-pole low-speed permanent magnet motor can be realized by using the concentrated winding of the true fractional slot. Through the electromechanical integration design of the motor and the mechanical structure, all or part of the intermediate links of power transmission are canceled. Made into a permanent magnet direct drive solution, which can directly meet the demand of the load, thus eliminating the need for a reducer device.
3 Frequency conversion permanent magnet direct drive motor
3.1 Working principle of frequency conversion speed regulation
Frequency conversion speed regulation refers to changing the frequency and voltage of the motor in proportion to achieve the purpose of speed regulation of the main motor of the mill. It has many advantages such as a wide speed regulation range, high-speed regulation precision, and high operating efficiency. The frequency converter is the main device for frequency conversion and speed regulation of the motor. It is composed of a rectifier, an intermediate DC link, a power inverter, and a controller. It converts the constant voltage and constant frequency AC provided by the grid into a new voltage and frequency. , and connected to the stator winding of the motor to realize the step-less speed regulation of the AC motor.
3.2 Influence of variable frequency permanent magnet direct drive motor on a slow drive
The slow-speed driving device of the air-swept coal mill is composed of a brake motor, a slow-drive reducer, and a clutch, and is mainly used for the installation and maintenance of the mill. When installing and replacing mill components and overhauling the inside of the mill, the mill cylinder can be driven to run at a low speed of 0.1-0.2 r/min. The mill using the frequency conversion permanent magnet direct drive motor can run stably at the low frequency of the frequency converter, which meets the needs of mill installation and maintenance. In addition, the inching control of the mill can also be realized, and the position of the mill cylinder can be precisely positioned by inputting the arc of the mill rotation. The variable frequency permanent magnet direct drive motor can completely replace the slow drive device, reduce equipment investment, and reduce the mechanical impact of large and small gears dragged by the slow drive.
3.3 Brake system
In order to avoid potential safety hazards, it is necessary to add a braking system to the drive system of the variable frequency permanent magnet direct drive motor. Its main function is to provide braking to the full-load mill during mill maintenance or liner replacement to ensure personnel safety; When the mill lubrication system, frequency converter, motor, etc. fail or the production line fails, it can provide complete braking for the full-load mill running at high speed to avoid damage to the mill sliding shoe bearing or main bearing.
3.4 Advantages of frequency conversion permanent magnet direct drive motor
The traditional asynchronous motor + frequency converter cannot remove the reducer and its auxiliary oil station, the later maintenance work has not been reduced, and the problem of efficiency transmission loss of the drive system has not been solved; the original asynchronous motor is a non-variable frequency motor, and long-term variable frequency operation is likely to cause stator lines As the temperature of the rod rises, the insulation of the wire rod ages faster. The permanent magnet direct drive motor can run below the rated frequency, has the characteristics of constant torque and large starting torque, can be better matched with a frequency converter, and is suitable for the working conditions of ball mills. Therefore, as shown in Figure 7, replace the asynchronous motor, main reducer, main reducer oil station, auxiliary transmission reducer, reducer motor, and water resistance cabinet/starter with low-speed high-torque permanent magnet direct drive motor + frequency converter Cabinets, feeders, and other equipment can optimize the structure, reduce procurement costs, reduce equipment failure points, improve transmission efficiency, and reduce power consumption. At the same time, it can be directly designed as a dust explosion-proof structure, and the explosion-proof design meets the explosion-proof electrical requirements of the coal powder preparation environment, with a simple structure and strong stability.
Fig. 7 Schematic diagram of the structure of the mill adopting frequency conversion permanent magnet direct drive motor
1. Mill body 2. Gear transmission part 3. Frequency conversion permanent magnet direct drive motor
4 Apply Effects
The φ3.80 m×7.75 m+3.5 m air-swept coal mill configured in a cement production line has a transmission system equipped with a traditional asynchronous motor, final reducer, slow drive device, brake, high-speed coupling, low-speed coupling, clutch, etc., use water resistance to start. Its main motor parameters are listed in Table 1. Steel balls are used as the grinding medium, the filling rate is 23.5%, the loading capacity is 86 t, the output is 37.4 t/h, the operating current is 125.6 A, the voltage is 6000 V, and the power consumption of the mill is 29.6 kW·h/t.
Table 1 Parameters of asynchronous motor
Another cement production line is equipped with a φ3.80 m×7.75 m+3.5 m air-swept coal mill with a rotational speed of 16.7 r/min and a designed production capacity of >42 t/h. mm, 95% passed, and the comprehensive moisture content of the grinding material is 9% to 14%.
The air-swept coal mill is equipped with a variable-frequency permanent-magnet direct-drive motor, which adopts a low-speed high-torque permanent-magnet direct-drive motor + frequency converter mode, and the structure of the transmission device is simple. The motor adopts a dust explosion-proof structure. The pinion shaft of the mill is connected to the shaft head of the variable frequency permanent magnet direct drive motor through a diaphragm coupling. At the same time, a braking system is added to achieve safety control, as shown in Figure 8.
Figure 8 Air-swept coal mill for cement production line
The air-swept coal mill uses steel balls as the grinding medium, the filling rate is 23.5%, the loading capacity is 86 t, the output is 45.2 t/h, the finished product particle size is 80 µm and the sieve residue is ≤3%. The parameters of the frequency conversion permanent magnet direct drive motor are listed in Table 2. In actual operation, the operating frequency is 44 Hz, the operating voltage is 9360 V, the operating current is 76.7 A, and the power consumption of the mill is 25.3 kW h/t.
Table 2 Parameters of variable frequency permanent magnet direct drive motor
The comparison of the transmission system of the air-swept coal mill of the above two cement production lines is listed in Table 3. Compared with the traditional asynchronous motor, the power factor and system efficiency of the variable frequency permanent magnet direct drive motor have been significantly improved, and the power consumption of the single machine has also been reduced to a certain extent. At the same time, the configuration of the mill transmission system is simpler, replacing the traditional complex structure, reducing the volume of the transmission system, the floor area of the plant, and the quality of equipment, reducing raw material consumption and procurement costs, and reducing failure points and system maintenance costs, Improve the efficiency and reliability of the transmission.
Table 3 Comparison of two driving efficiencies
The air-swept coal mill using the frequency conversion permanent magnet direct drive motor is in good condition in the field, with stable performance, and has been continuously and safely running, and all technical indicators have reached the design requirements.
5 Conclusion
The frequency conversion permanent magnet direct drive motor integrates the permanent magnet motor and the mechanical load, changes the transmission mode of the mechanical equipment, eliminates various transmission links, and greatly simplifies the transmission chain of the mechanical equipment;
(1) The variable frequency permanent magnet direct drive motor makes the transmission system of the mill simpler, reduces the purchase cost, reduces the volume of the transmission system, the floor area of the workshop, and the quality of the equipment, reduces the consumption of raw materials and improves the transmission efficiency. It saves the power consumption of the single machine of the mill, and can be directly made into a dust explosion-proof structure with strong stability;
(2) The variable frequency permanent magnet direct drive motor can meet the installation and maintenance needs of the mill, and realize the inching control of the mill, which can completely replace the slow drive device;
(3) The mill adopting frequency conversion permanent magnet direct drive motor can save equipment such as reducer and its affiliated oil station, which greatly reduces equipment failure points and machine maintenance workload;
(4) It is necessary to add a braking system to the drive system of the variable frequency permanent magnet direct drive motor to improve the safety and reliability of the mill.
(5) The application of a variable frequency permanent magnet direct drive motor in the grinding machine is very important for energy saving and consumption reduction of the grinding operation. It has the advantages of reducing the grinding cost, improving the grinding efficiency, and reducing the energy consumption of the grinding machine. This application provides a new idea for the design of the mill, can accumulate data and experience in the design and application of coal mills, and form a unified standard, which has a key demonstration effect for the wide application of variable frequency permanent magnet direct drive motors in dry mills in the future.
