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AC drive system
- Leading electric forklift technology revolution
Yang Xiaojun, Beijing Jiaotong University
The forklift, known as the “God of Handling”, plays a huge role in the logistics field. In recent years, the average annual growth rate of China's forklift sales has exceeded 30%, far exceeding the growth rate of the national economy. China has become one of the most active countries in the global forklift market and has attracted worldwide attention.
Although the internal combustion forklift still dominates the domestic market, with the increasing awareness of environmental protection, more reliable and cleaner electric forklifts are favored by more and more enterprises. The AC drive system with AC motor as the core is praised by industry experts as the revolutionary technology of the 21st century electric forklift because of its high production efficiency and low maintenance cost. The world's top ten forklift giants competed to launch a better-performing AC-driven electric forklift to enrich their products, meet user needs and win market share. Leading forklift companies in China are also beginning to focus on the research and development of technology applications, and the new AC-driven electric forklifts are the winning weights for participating in domestic and global market competition. As the share of electric forklifts in the forklift market continues to rise, AC drive systems will lead the new round of electric forklift technology revolution, giving users higher productivity, lower cost of use and a more comfortable driving experience.
First, the demand for electric forklifts has increased year by year, and foreign-invested enterprises occupy most of the market share. According to the power method, forklifts can be divided into internal combustion forklifts and electric forklifts. The electric forklift uses the battery as the power source. When it is used, it has no exhaust emission, low noise, convenient operation and dexterity, and stable operation. It is precisely because of the above characteristics of electric forklifts that the demand for electric forklifts has increased continuously at home and abroad, and the proportion of electric forklifts in the sales of forklifts has steadily increased (see chart below). Especially in ports, warehouses and tobacco, food, textile and other industries, electric forklifts are gradually replacing internal combustion forklifts.
Figure 1 Japan forklift market sales situation (Taiwan)
Figure 2 Comparison of sales of Japanese internal combustion forklifts and electric forklifts
Figure 3 Comparison of sales of European internal combustion forklifts and electric forklifts (%)
Since the introduction of the first forklift in the 1950s, it was not until the 1970s that the forklift industry began to take shape. From the perspective of forklift manufacturers, at present, China's domestic forklift trucks have formed an enterprise group with Anhui Forklift Group Co., Ltd., Hangzhou Forklift Co., Ltd. and Dalian Forklift Co., Ltd. as the backbone. In 2004, the three companies sold a total of 34,689 internal combustion forklifts. The whole industry (including independent and joint ventures) has a total output of 74,893 units of 74%; electric vehicles have also formed with Anhui Forklift Group Co., Ltd., Hangzhou Forklift Co., Ltd., Baoji Shuangli Forklift Group Co., Ltd., Fushun Forklift Manufacturing Co., Ltd. and Hengyang Battery Cars. Factory-focused key enterprises.
According to statistics, in 2004, China sold a total of 59,676 mobile industrial vehicles, an increase of 31.2% over the previous year's 45,482 units. According to the driving method, the sales volume of internal combustion forklifts was 46,893 units, an increase of 31.7% compared with 35,616 units in the previous year; the sales volume of electric forklifts was 12,783 units, an increase of 30% compared with the previous year's 9,866 units. The picture below shows the composition of the sales of domestic forklift trucks and electric forklifts.
Figure 4 Comparison of sales of domestic forklift trucks and electric forklifts
From the perspective of the industrial cycle, the forklift industry in developed countries is at a mature stage, while the domestic forklift industry and forklift products are still in the development stage. From the perspective of market demand, the scope and quantity of domestic forklift products have not been fully expanded. In the next few years, with the rapid development of the world manufacturing industry to China and the rapid development of the domestic logistics industry, the growth potential of China's forklift market is huge. The annual output is expected to exceed 80,000 units in the next three years. At the same time, the forklift industry will further improve its technical level and gradually integrate with the world. . In the world car market, the demand for electric forklifts is far greater than that of internal combustion forklifts, which is closely related to the global environmental protection factors. With the promulgation of China's environmental protection policy and the increasing awareness of environmental protection among the whole people, as well as the large number of foreign-invested enterprises, China will also adapt to the global development direction, and the demand for electric forklift market will rise rapidly.
However, it should not be overlooked that the production of electric forklifts in China is still in its infancy, especially the backwardness of the three electric (electric control, electric motor, battery) plagued the development of electric forklifts. The technical level and product quality of domestic manufacturers are relatively advanced compared with foreign countries. There is still a gap in the number of industrial vehicle manufacturers. Independent and joint ventures still have an absolute advantage in the field of electric vehicles in China, showing great advantages in terms of product advancement and reliability. In 2004, independent and joint ventures sold a total of 7,183 electric vehicles in China, accounting for more than half of the entire industry. At the same time, from the import data of forklifts released by the General Administration of Customs in 2004, it can be seen that the forklifts are still mainstream in the high-end market. In 2004, the proportion of domestic imported forklifts in the high-end electric forklifts was 62.49%, an increase in the number in 2003. 21.05%, indicating that the current demand for high-end electric forklifts in China has increased significantly, and the existing forklift enterprises cannot fully meet the market needs. Not only that, with the vigorous promotion of the domestic logistics industry, large containers will become the mainstream of transportation, and the handling and stacking of containers will require a large number of non-polluting and high-lift electric forklifts. These are opportunities that forklift companies cannot give up. Therefore, if China's forklift enterprises want to gain an advantage in the future market competition, they should speed up the research and development of electric forklifts, improve the quality of products by adopting world-leading technology, and win more customers with first-class products.
Second, the advantage of the forklift with AC drive system is obvious <br> In recent years, with the advancement of AC induction motor frequency conversion technology, and the high speed of high-power semiconductor devices and microprocessors, the induction motor AC drive system and DC motor Compared with the drive system, it has the advantages of high efficiency, small size, small mass, simple structure, maintenance-free, easy cooling and long life. The system has a wide speed range and can realize low-speed constant torque and high-speed constant-power operation, which satisfies the rotational speed characteristics required for the actual running of the electric vehicle. It can be said that it is the rapid advancement of semiconductor technology that has given birth to the technological revolution of AC motors, which greatly enhances the control capability of AC motors; and as the price of electronic components continues to fall, the cost of AC motor controllers is reduced, thus being the AC drive system. The large-scale promotion and application laid the foundation and created the conditions.
For the forklift with AC drive system, the overall performance is significantly improved, the fault and component replacement rate is significantly reduced, and the reliability is greatly enhanced. The forklift has higher productivity per unit time and lower operating and maintenance costs, which will bring significant benefits to users. Therefore, once the AC drive system is launched, it has not only gained the trust of the forklift manufacturer, but also has become more and more popular among users, and has become the trend and direction of the development of electric forklift technology in recent years. Its advantages are mainly reflected in the following aspects:
The AC motor is maintenance-free for life.
The AC motor eliminates the need for a reversing contactor (forward and reverse commutation), saving parts. More importantly, the AC motor has no carbon brushes and commutators. Not only is the motor lighter and smaller, the speed is increased, and the trouble of periodically detecting and replacing the carbon brushes is completely eliminated. The biggest benefit is that the forklift motor does not need maintenance for almost a lifetime, which greatly enhances the reliability and stability of the forklift. At the same time, it does not need to consider the reserved motor maintenance space in the design of the forklift, and even seal the motor. Make the forklift structure design more compact.
Caption: Toyota forklift maintenance costs with AC drive system are greatly reduced (DC stands for DC)
Regenerative braking is used to reduce mechanical wear.
Regenerative braking is a non-contact brake that is much simpler than conventional braking systems. Regardless of whether the driver brakes on the brake pedal or shifts the direction of travel brake, the motor will be in the generator state, and its electromagnetic torque will become the braking torque. This means that the brake pads are worn to a minimum. The mechanical wear is greatly reduced, which reduces the maintenance cost of the forklift and makes the running cost lower.
At the same time, AC motors are more efficient in driving and braking. Regeneration energy is generated when braking or reversing. The stronger the brakes, the more energy is regenerated. The accumulators of the AC drive system are automatically activated during braking or commutation, returning energy to the battery, extending battery life and longer life. Although some DC-driven forklifts also have regenerative braking, they must be activated during strong braking, which means that part of the regenerative energy is converted into heat when braking. In almost all cases, the AC motor produces energy regeneration and continues to act until the truck is completely stationary, which is clearly more efficient than DC motors.
2 . Increased production efficiency.
The maximum speed of the AC motor is much higher than that of the DC motor, and the power is more powerful. Moreover, the AC motor can return the obtained regenerative energy to the battery, which prolongs the battery life and can be used to improve the overall performance of the forklift. As a result, the forklift starts faster during driving, and the acceleration/deceleration performance is greatly improved, shortening the time and walking distance to the highest speed. Studies have shown that in most practical applications, the distance from one start to stop of the forklift truck is rarely longer than 20m. The excellent acceleration performance of the AC motor makes it possible to achieve full speed operation of the forklift within such a short distance, and the work efficiency of the forklift is significantly improved.
3. Easy to program and greatly enhanced.
With the rapid development of semiconductor technology, the frequency conversion speed control technology has made a breakthrough development, which can control the operation of the AC motor in real time, greatly improve the control ability of the AC motor, and obtain the same speed regulation performance as the DC motor. The AC drive adopts speed torque control, the sensitivity of the control is improved, and the operation efficiency of the forklift can be improved. The accelerator pedal is used to release the brake function (that is, the throttle is slightly loose when the forklift is walking, the regenerative braking will work), and the reversing direction is The smooth transition can be achieved when reversing, which improves the stability and reliability of the forklift. At the same time, the CAN bus is adopted, and the distributed nodes are not limited by the installation position; the modular structure, the system is easy to expand, and the functional features can be seamlessly added or modified; the bus interface standardization is realized, the system integration is simpler, and the unit design is more flexible. .
4. The forklift is more comfortable to operate.
The AC drive system plays a different role in improving the comfort of the forklift driver. Since the AC motor is smaller and lighter than the DC motor, this makes the design of the forklift relatively more flexible. For example, with this flexibility, Toyota Forklifts has adopted a unique low center of gravity design to develop a 7FB series electric forklift with comfortable driving and superior performance.
The following table compares the difference (maintenance cost, efficiency comparison and other) of the same type of forklift equipped with two different drive systems, AC motor and DC motor, with the Jungheinrich electric counterbalance forklift as an example.
AC motor (AC) and DC motor (DC)
Maintenance cost (take 5 years as an example)
Parts and service costs | DC | AC motor |
Replace the carbon brush on the drive motor | 3 x = EUR 950 | no |
| Maintenance cost of the drive motor | EUR 1,125 | no |
Commutator on the drive motor | 1 x = EUR 300 | no |
Replace the carbon brush on the lift motor | 3 x = EUR 980 | no |
Raise the commutator on the motor | 1 x = EUR 100 | no |
Replacement contactor | 2 x = EUR 920 | no |
Total cost | EUR 4,375 | no |
Efficiency comparison (take Jungheinrich electric balance forklift as an example)
| Driving speed (full load) | 17 km/h | 15 km/h | + 13% |
| Lifting speed (no load) | 0.56 m/s | 0.48 m/s | + 16% |
| Lifting speed (full load) | 0.40 m/s | 0.36 m/s | + 11% |
| Acceleration (no load) | 3.5 s | 3.8 s | - 7% |
| Acceleration (full load) | 4.0 s | 4.5 s | - 11% |
| Maximum climbing performance (no load) | 35% | 28% | + 25% |
| Maximum climbing performance (full load) | twenty four% | 17% | + 41% |
| Energy recharge efficiency during braking | 30% | 15% | + 100% |
| Hourly transport (L: 100m, H: 3m) | 52 Pallet | 39 Pallet | + 33% |
Other comparisons (take Jungheinrich electric counterbalance forklift as an example)
EFG-Vac 20 (AC) | EFG-V 20 (DC) | |
Internal environment | No dust generated by carbon brush wear, the internal environment is clean, and the life of the whole electrical part is extended. | Carbon brush wear is prone to dust, and excessive wear will directly burn the motor. |
motor | The motor is a closed structure, dustproof and moisture proof. Heat is generated in the outer casing during operation, which is fast in heat dissipation and facilitates continuous operation for a long time. | The motor is a non-hermetic design. Heat is generated inside the motor and the heat dissipation is slow, which is not conducive to continuous operation at high temperatures for a long time. |
Energy recharge | The energy recharge efficiency of the brakes is nearly 30%, which greatly improves the battery continuous use time and prolongs the battery life. | The energy is recharged by about 15% when braking. |
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