Study on Energy Saving of Certain Constant Torque Loads by Frequency Conversion Speed ​​Regulation

1 Overview

The application of frequency converters in cement, power generation, electrolytic aluminum, ceramics and other industries is very common. Production machinery such as the main drive in the rotary kiln, various fans, conveyor belts, etc., which originally did not have speed regulation or used electromagnetic speed regulation and other speed regulation methods, have been transformed into variable frequency AC speed regulation. The purpose of the transformation is to make the process speed regulation in the production process more convenient, thereby improving the output and quality of the product, and realizing automation and energy saving.

However, the use of variable frequency speed regulation for ball mills, the main equipment in the grinding process, is basically blank. The reason is that the speed of the barrel of the ball mill is basically constant when it is working. Even if it needs to change, the range of change is not large. If the speed is reduced to save energy, the time of ball milling may increase. Therefore, it is difficult to say whether the use of variable frequency speed regulation has the effect of energy saving. In fact, the ball milling production process is relatively simple. For example, if 16~18 tons of material is added to the barrel of the ball mill in the ceramic factory and it runs for 8 hours at a working speed of 16~18 r/min, the fineness of the material can meet the process requirements. After the material is discharged, it is added again and the above process is repeated. Generally speaking, due to the different manufacturers and the raw materials of the ball mill, the performance parameters of the ball mill are also different. For example, the MTZ3570 ball mill used for coal grinding in power plants has an effective inner diameter of 3500 mm, a barrel length of 7000 mm, a working speed of 17.3 r/min, and a motor power of 1120 kW (6 kV). The electrical transmission mode of the ball mill is three-phase AC squirrel cage asynchronous motor-hydraulic coupling-gear reducer-pulley reducer, or three-phase AC squirrel cage asynchronous motor-gear reducer-pulley reducer. Generally, the barrel of the ball mill is used as the pulley of the reducer. When the ball mill is started under heavy load, if there is no hydraulic coupling in the transmission link, even if an autocoupler or star-delta starter is used, it will cause a large impact on the power grid and often cause difficulty in starting. Therefore, adding a hydraulic coupling in the transmission link will buffer the impact during starting, and the ball mill can be started smoothly in any state.

2 Critical speed and optimal working speed of ball mill

The speed of the ball mill directly affects the movement of the steel balls and materials and the grinding process of the materials. The movement of the steel balls and materials in the cylinder at different speeds is shown in Figure 1.

If the speed is relatively 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, the steel ball slides down the inclined surface, as shown in Figure 1 (a). It cannot form a sufficient drop, and the grinding effect of the steel ball on the material is very small. This situation is very inefficient. If the speed of the cylinder is very high, due to the centrifugal force, the material and the steel ball no longer separate from the cylinder wall, but rotate with it, as shown in Figure 1 (c). The lowest speed that produces this state is called the critical speed nlj. At the critical speed, the steel ball has no impact effect, the material is only slightly ground, and the efficiency is also very low. When the speed of the cylinder is between the above two, the steel ball is brought to a certain height and falls along a parabola, as shown in Figure 1 (b). At this time, the steel ball has a strong impact on the material at the bottom of the cylinder, and the efficiency is the highest. The working speed with the highest efficiency is called the optimal working speed nzj.

nzj=0.765nlj(r/min) (circle) Actual operation shows that the optimal working speed is related to factors such as the diameter of the steel ball and its loading capacity, the shape of the armor, and the friction coefficient between the steel ball and the armor. Generally, the optimal working speed is usually nzj=(0.74-0.8)nlj. It can be seen that the speed still has a certain adjustable range, but the adjustable range is not large. In fact, as mentioned above, the ball mill in the ceramic factory has a barrel speed range of 16~18 r/min, the motor speed is 1440 r/min, and the barrel speed after deceleration by the gear reducer and the pulley is within the above-mentioned allowable speed range. If the reduction mechanism is configured so that the barrel runs at exactly 18 r/min, there is an 11% speed regulation range; if the reduction mechanism is configured so that the barrel runs at 16 r/min, the speed regulation range is zero. In fact, it is impossible for a mechanical reducer to be very precise, so the speed of the barrel is between 16 and 18 r/min. Therefore, the adjustable speed range is 0 to 11%.
The ball mill is a constant torque load machine, and the shaft power PZ output by the motor is

It can be seen from formula (3) that the electric power consumed by the ball mill is proportional to the first power of the motor speed. Therefore, using a frequency converter to adjust the motor speed below the base frequency can save energy, but the extent of energy saving is related to the speed regulation range.

3. Efficiency of electric motor

The motor of a ball mill is 90 kW. During normal operation, the motor current is 80~110 A, and the load rate is 62%. Since the load rate is not high, the efficiency of the motor should also be reduced. During normal operation, the power of the motor will not exceed 55 kW. Considering that the ball mill is started under heavy load, it is necessary to increase the capacity of the motor appropriately to ensure smooth starting. On the surface, it seems that there is a large space for energy saving. In fact, in order not to affect the production efficiency of the ball mill, if the variable frequency speed regulation is adopted, the output frequency of the inverter is still 50 Hz. Even if the speed regulation is required, it is impossible to make a large range of adjustments. The frequency can only be reduced in a small range, so the energy saving is limited. The current inverter generally has the function of "energy-saving operation". For example, the parameter F82 of the Senlan SB40S series inverter is automatic energy-saving operation. The "energy-saving operation" of the inverter is essentially a voltage regulation and power saving function. The inverter automatically adjusts its output voltage under the condition that the operating frequency remains unchanged, so that the efficiency of the motor is improved. To know how much efficiency can be improved, it is necessary to analyze the efficiency of the motor.

3.1 Motor efficiency

The efficiency and power factor of small and medium-sized asynchronous motors are functions of the load rate. The efficiency indicates the utilization rate of the active power when the motor is running, which is the ratio of the output power to the input power, that is,

It can be seen from formula (4) that for a certain load, when the output power of the motor is constant, the efficiency of the motor is related to the total loss. The greater the total loss, the lower the efficiency; conversely, the higher the efficiency. The total loss of the motor consists of two parts: fixed loss and variable loss. The fixed loss does not change with the load of the motor and can be approximately expressed by the no-load input power of the motor. The variable loss is proportional to the square of the motor load rate.

3.2 Motor loss analysis

The total loss of an asynchronous motor during operation is generally divided into four categories: basic copper loss, basic core loss, mechanical friction loss, and stray loss.

3) Mechanical friction loss Pf W Mechanical friction loss includes ventilation system loss PV and bearing friction loss PT. For a standard motor, the mechanical friction loss is a constant.

4) Stray loss PS Generally, the eddy current loss generated by the leakage magnetic field in the metal components and the loss caused by the high-order harmonic magnetic field in the air gap in the stator and rotor cores and conductors are collectively referred to as stray loss. This type of loss is proportional to the square of the current and changes with the load.

For a 15 kW motor, the proportion of the above-mentioned losses in the total loss is: basic copper loss PCU accounts for 30%~50%; basic iron core loss PFe accounts for 20%; mechanical friction loss PfW accounts for 20%~35%; stray loss PS accounts for 10%~15%. Among these four types of losses when the motor is running, the mechanical friction loss is basically unchanged, the basic copper loss and stray loss are proportional to the square of the current, that is, the load must be inversely proportional to the square of the terminal voltage; the basic iron core loss PFe is proportional to the square of the terminal voltage. From this, an optimal point with low motor loss can be found, such as point U0 in Figure 1. At this point, the motor loss is the smallest. During energy-saving operation, the inverter automatically adjusts its output voltage to this optimal point to achieve the highest efficiency.

4. Voltage regulation and power saving of electric motors

As mentioned above, the efficiency of the motor under light load can be improved by adjusting the supply voltage. The voltage regulation coefficient calculation formula in the supervision guide for the implementation of the mandatory national standard GB12497 "Economic Operation of Three-Phase Asynchronous Motors" is:

It can be seen that the energy saved by the variable frequency energy-saving operation of the ball mill is limited. This is because the efficiency of the motor itself is already very high, with a rated efficiency of 93.5%. Adjusting the voltage can only reduce copper loss, iron loss and stray loss, which is just a reduction. Moreover, the mechanical friction loss PfW, which accounts for a large proportion of the total loss, is basically unchanged. Therefore, the improvement of motor efficiency is very limited. For example, in this example, the active power is saved by 0.27 kW, and the power saving rate is extremely low. Moreover, after the installation of the frequency converter, the efficiency of the frequency converter is not 100%, which not only does not save energy but consumes energy. For equipment that still requires 50 Hz operation after the frequency conversion transformation, whether it is a load with constant torque characteristics or a load with square torque characteristics of fans and water pumps, it is meaningless to consider energy saving only. The purpose of the transformation should be to improve the performance of the equipment and enhance the automation level of the equipment, which not only improves productivity but also ensures product quality. Such working conditions are common in actual production. For example, the speed of the barrel of the ball mill in cement plants and tile plants is determined by calculation or experimental data, generally 16~18 r/min. The speed deviation will affect the efficiency of the ball mill to varying degrees. Therefore, even if such mechanical equipment is equipped with a frequency converter, there will be no energy saving if the operating speed is the same as the original speed. The same is true for the spinning frame in the textile industry. The transmission motor of the spinning frame is generally 15 kW and the working current is about 15 A. In order to ensure work efficiency, the same speed is still required after frequency conversion speed regulation. The frequency converter will not save much electricity when "energy-saving operation" at a frequency of 50 Hz. Therefore, the purpose of frequency conversion speed regulation transformation of such equipment should be to improve the level of automation, so as to facilitate network control and facilitate speed control after raw materials or processes are changed.

5. Energy saving by hydraulic coupling and variable frequency speed regulation

The hydraulic coupling transfers the energy of the motor by controlling the change of the momentum of the working oil in the working chamber. The motor drags its active working wheel through the input shaft of the hydraulic coupling to accelerate the working oil. The accelerated working oil then drives the driven working turbine of the hydraulic coupling to transfer the energy to the output shaft and the load. The hydraulic coupling is divided into speed regulating type and torque limiting type. The former is used for speed regulation of electrical transmission, and the latter is used for starting the motor. The hydraulic coupling in the system acts as a buffer when the motor starts. Its theoretical efficiency is 95%, and the efficiency of the frequency converter is 96%. That is to say, if the hydraulic coupling is removed from the transmission link and the ball mill motor is driven by an AC speed regulating frequency converter, the ball mill can be started smoothly at the rated current or slightly higher than the rated current, and the efficiency can be increased by 1% in theory.

In fact, the efficiency of the hydraulic coupling is related to the amount of oil injected into the hydraulic coupling cavity. During operation, the hydraulic coupling has a certain temperature rise, and there are factors such as poor sealing and leakage. Its efficiency is generally less than the theoretical value of 95%, so a reduction of 2% to 3% is common.

If the motor speed of the ball mill is 1440 r/min, after being decelerated by the hydraulic coupler and the reducer, the barrel speed is 16 r/min, then the efficiency calculation formula of the hydraulic coupler is

Its efficiency is the ratio of the speed of the output shaft to the speed of the input shaft, which is equal to 0.95. In the transmission link, the hydraulic coupling is removed and the elastic coupling is used for direct connection. At this time, the speed of the barrel will exceed the original speed. In order to maintain the original process conditions, the motor should be decelerated, and the output frequency of the inverter is reduced to 50 times 0.95 = 47.5 Hz. Since the ball mill is a constant torque load, the three-phase motor power calculation formula is

It can be seen that when the motor's operating frequency decreases, its current remains basically unchanged or decreases slightly, but the output voltage decreases roughly in proportion. There are local compensation capacitors on site, and the power factor of the motor is relatively high. After using the inverter, the local compensation capacitors are removed. Since the power factor of the inverter itself is relatively high, the power factor of the system does not change much. In this way, the power saving rate should be around 5%, plus the previous 1%, a total of 6% energy saving.