In recent years, switched reluctance motors have gradually entered the market. Due to their advantages over other motors, they have become a major direction for future market development and have been successfully applied in various fields such as electric vehicle drives, general industry, home appliances, and textile machinery. So what exactly are the advantages of switched reluctance motors? Let's find out!
The switched reluctance motor speed control system is a mechatronics product based on modern power electronics and microcomputer control technology. It consists of a switched reluctance motor and a microcomputer intelligent controller, characterized by high efficiency, excellent energy saving, wide speed range, no inrush starting current, large starting torque, and flexible control. In 1998, my country incorporated the development of motor speed control and energy-saving technologies and power electronics into the "Energy Conservation Law of the People's Republic of China." The National Development and Reform Commission's "Motor Energy Conservation Plan" explicitly proposed to improve motor efficiency by 15-20%, achieving energy savings of 100 billion kWh/year. Therefore, this type of motor is widely used in applications requiring heavy-load starting, frequent starts, and forward/reverse rotation, such as in transport vehicle drives, gantry planers, and forging equipment.
In recent years, with the increasing popularity of energy-saving concepts in motors, switched reluctance motors, due to their following characteristics, are being applied in various applications. Characteristics of switched reluctance motor speed control systems:
I. High efficiency and excellent energy saving. Tests have shown that its overall efficiency is at least 3% higher than that of AC asynchronous motor frequency conversion speed control systems, and at least 10% higher at low speeds. Compared with DC speed control, cascade speed control, and electromagnetic speed control, its energy-saving effect is more significant.
Second, it has a large starting torque, making it suitable for heavy-load starting and applications with significant load changes and frequent starts. Tests have shown that when its starting torque reaches 150% of the rated torque, the starting current is only 30% of the rated current, a significant advantage.
Third, it has a wide speed range. Switched reluctance motors can operate at low speeds for extended periods. Due to their high efficiency, the temperature rise at low speeds is lower than under rated operating conditions, solving the problem of motor overheating when the variable frequency speed control motor is running at low speeds. The maximum speed can also be flexibly set according to actual needs.
Fourth, it can frequently start and stop in both forward and reverse directions, has good system controllability and braking performance, can achieve regenerative braking, and has a significant energy-saving effect.
Fifth, the starting current is small, avoiding impact on the power grid. Switched reluctance motors have soft-start characteristics and do not have the phenomenon of ordinary AC motors having a starting current greater than 5-7 times the rated current.
VI. High power factor, no need to add reactive power compensation device. Tests have shown that the power factor of the switched reluctance motor system is greater than 0.98 under both no-load and full-load conditions.
VII. The motor has a simple and robust structure, simple manufacturing process, low cost, and reliable operation, and can be used in various harsh high temperature and strong vibration environments.
8. It can still operate under phase loss and overload conditions. When a phase of the power supply is lost, or any phase of the motor or controller fails, the motor output power will decrease, but it can still run. When the system exceeds 120% of the load, the speed will only decrease, and the motor and control cabinet will not be burned out.
9. The power converter in the controller is connected in series with the motor winding, which eliminates the shoot-through fault that may occur in the power converter of the variable frequency speed control system, greatly improving reliability.
Current motors all have both advantages and disadvantages; no single type of motor can replace all other types. Switched reluctance motors also have the following disadvantages:
1. Torque Ripple: Switched reluctance motors operate in pulse power supply mode, resulting in large instantaneous torque ripple. At very low speeds, the stepping state is obvious, and due to its inherent nonlinearity, torque control is difficult. Although there are many studies on SRD direct torque control technology, it is still difficult to achieve precise torque control. In precision control applications such as servo motors, SRD does not have an advantage over other types of motors.
2. Noise: In switched reluctance motors, the phase windings conduct alternately, and the radial force causes stator deformation, which is more pronounced during commutation. This results in high motor noise. Reducing vibration and noise is an important research topic. Many researchers are studying improvements from the aspects of electromagnetic parameter design, speed regulation methods, and motor structure. However, compared with other motors, the noise level is still relatively high.
3. Position Detector: The position detector diminishes the performance of a simple switched reluctance motor and reduces system reliability. Therefore, exploring practical detector-free detection schemes is essential. Current research, both domestically and internationally, largely focuses on using transient inductance information from the stator windings to achieve detector-free solutions, but further in-depth research is needed before practical application.
The domestic application market for switched reluctance motors mainly includes:
(1) Forging Machinery: Currently, this is one of the most successful applications of switched reluctance motors. This time, it fully meets the above five conditions. It has advantages such as overload capability, forward and reverse rotation, wide speed range, and energy saving, completely ignoring the disadvantages of switched reluctance motors: noise, vibration, and torque ripple. In the switched reluctance motor industry, there is a saying that forging has won the world. Shandong Kehui Company has been in a leading position in the country in recent years, which is inseparable from its accurate market development and positioning in the early stage.
(2) Textile Machinery: First, we should thank foreigners for their application. Picanol, a major importer of textile machinery in China, was the first to adopt switched reluctance motors, which naturally led domestic loom manufacturers to accept them. Since 2009, switched reluctance motors have gradually gained a foothold in rapier looms and towel looms.
Textile applications meet conditions 1, 3, and 5 above: energy saving, quick start-up (the motor can reach rated speed from zero speed through 3 rotations), and no vibration or noise requirements. Foreign looms use four-phase 8/6-pole switched reluctance motors, while my country initially used three-phase 12/8-pole motors. Currently, my country has also introduced four-phase motors, which may gradually eliminate the three-phase 12/8 structure in the textile industry. Oddly enough, Beijing Zhongfang Company originated from the Textile Research Institute. However, Beijing Zhongfang's switched reluctance motors have never been widely adopted in textile machinery. Instead, companies in Shandong, Shenzhen, and Hangzhou have achieved better adoption in the textile industry.
Figure 3 Textile Machinery
(3) Oilfields and Coal Mines: Energy and environmental protection have always been key areas of national strategic planning. The use of switched reluctance motors in applications such as oil pumps and coal mine conveyor belts has yielded significant results. In recent years, Shengli Oilfield, Zhongyuan Oilfield, and Changqing Oilfield have all undertaken renovation projects, generally meeting the renovation requirements. However, the oilfield market is dominated by state-owned enterprises, making large-scale promotion difficult at present. As for coal mines, success or failure is uncertain for Beijing Zhongfang. These years are the off-season for coal, making it difficult to expand.
Figure 4 Oilfield pumping unit
(4) Small household appliances: This area mainly focuses on the application of high-speed switched reluctance motors. Vacuum cleaners, cooking machines, blenders, and washing machines abroad widely use switched reluctance motors, resulting in significantly improved performance and a large quantity used across a wide range. However, in China, only a few news reports mention prototype development, and large-scale applications have not yet been seen. Perhaps cost is the main bottleneck limiting their widespread adoption in these areas.
Figure 5 Application of small household appliances
(5) Electric Vehicles: The application of switched reluctance motors (SRMs) in electric vehicles can be a painful topic. Both domestically and internationally, the earliest SRM prototypes were developed specifically for electric vehicles. They were drawn to the advantages of SRMs, such as high starting torque and good acceleration performance, making them ideal for electric vehicles. Huazhong University of Science and Technology and Beijing Zhongfang Company jointly installed SRMs in 20 Wuhan hybrid buses and conducted long-term operational tests, achieving good results. However, due to noise and power density issues, the electric vehicle market has been largely dominated by permanent magnet motors in recent years, which is also related to the low price of permanent magnet motors in China. In the long run, permanent magnets are a non-renewable resource and cannot meet the huge market demand for electric vehicles. SRM practitioners should continue to explore this area further and wait for the technology to mature.
Figure 6. SRD-driven mining forklift
(6) Other Applications: In recent years, the number of companies producing switched reluctance motors has gradually increased, and these companies have also ventured overseas to showcase their capabilities. They have experimented in various applications. In the simplest energy-saving retrofit applications, such as fans and water pumps, as well as ball mills, sliding doors, shearing machines, starters/generators, and injection molding machines, they have achieved some success, but the scale of application in these applications remains relatively small. Whether this can be promoted across the entire industry in the future remains to be seen.
in conclusion:
Switched reluctance motors are a relatively new speed control system developed over the past 30 years. Their advantages and disadvantages are also quite apparent. It is unrealistic for individual manufacturers to attempt to replace conventional asynchronous motors (or implement innovation programs) with switched reluctance motors in order to attract investment. How to combine the characteristics of field applications, fully utilize the advantages of switched reluctance motors, and avoid or reduce the drawbacks caused by their disadvantages is a direction that researchers should continuously strive towards!
