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Permanent magnet motors utilize permanent magnets to create their magnetic field, eliminating the need for an excitation coil or current. Characterized by high efficiency and a straightforward design, these motors are recognized for their energy-saving qualities. Indeed, the development of permanent magnet motors has been an extensive journey, reflecting significant advancements over time.
Development process
TThe evolution of permanent magnet motors is intricately linked with the advancements in permanent magnet materials. China holds the distinction of being the first nation to recognize and practically apply the magnetic properties of such materials. Over 2,000 years ago, these properties were harnessed to create compasses, which had a profound impact on navigation and military strategies, among other areas. This innovation is celebrated as one of ancient China’s four great inventions.
The world’s first electric motor appeared in the 1820s. In fact, this motor is a permanent magnet motor. Its magnetic field is generated by permanent magnets. But in fact, this technology disappeared for a while and was not widely used until recent years. Because the permanent magnet material used at that time was natural magnetite. The magnetic energy density of this magnetite was particularly low. Using it to make motors makes the motors particularly large. Due to their low practicality, permanent magnet motors were slowly replaced by electric excitation motors. Its development also stalled for a while. But some scientists believed that there is still a need to study permanent magnet motors, so while others switch fields, a small number of people remain deeply involved in this field.
As various types of motors continued to advance rapidly, and current magnetizers were invented, researchers conducted extensive investigations into the mechanism, composition, and manufacturing technology of permanent magnet materials. This led to the discovery of a range of permanent magnet materials, including carbon steel, tungsten steel, and cobalt steel. Notably, the magnetic properties of AlNiCo permanent magnets, which emerged in the 1930s, and ferrite permanent magnets, which appeared in the 1950s, were significantly enhanced. As a result, permanent magnet excitation has become widely utilized in various micro and small motors. The power output of permanent magnet motors ranges from a few milliwatts to tens of kilowatts, and they are extensively employed in military, industrial, agricultural production, and daily life. Hence, the production of permanent magnet motors has experienced a dramatic increase. This period also saw significant progress in the design theory, calculation methods, magnetization, and manufacturing technology of permanent magnet motors. A set of analytical and research methodologies, such as the permanent magnet working diagram method, was established during this era.
However, the low coercivity of AlNiCo permanent magnets and the low remanence density of ferrite permanent magnets limited their application range in motors. Until the 1960s and 1980s, rare earth cobalt permanent magnets and neodymium iron boron permanent magnets (both collectively referred to as rare earth permanent magnets) came out one after another. Their high remanence density, high coercive force, high magnetic energy product and excellent magnetic properties of linear demagnetization curve are particularly suitable for manufacturing motors, bringing the development of permanent magnet motors into a new historical period.
Characteristics of permanent magnet motors
Compared with traditional electric excitation motors, the permanent magnet motors include :
1.Simple structure and reliable operation
2.Small size and light weight
3.Low loss and high efficiency
4.Flexible and versatile motor shapes and sizes
Therefore, the application range of permanent magnet motors is extremely wide, covering almost all fields of aerospace such as national defense, industrial and agricultural production and daily life. The following are the main characteristics of several typical permanent magnet motors and their main applications.
Compared with traditional generators, rare earth permanent magnet generators do not require slip rings and brush devices. The permanent magnet synchronous generator has a simple structure and reduces the failure rate. The use of rare earth permanent magnets can also increase the air gap magnetic density, the motor speed to the optimal value, and improve the power to mass ratio. Almost all modern aviation and aerospace generators use rare earth permanent magnet generators. Its typical products are 150 kVA 14-pole 12 000 r/min ~ 21 000 r/min and 100 kVA 60 000 r/min rare earth cobalt permanent magnet synchronous generators manufactured by General Electric Company of the United States.
Permanent magnet generators are also used as auxiliary exciters for large turbine generators. In the 1980s, China successfully developed the world’s largest capacity 40 kVA to 160 kVA rare earth permanent magnet auxiliary exciters, equipped with 200 MW to 600 MW turbine generators. After that, the reliability of power station operation has been greatly improved. At present, small generators driven by internal combustion engines for independent power sources, permanent magnet generators for vehicles, and small permanent magnet wind turbines driven directly by wind wheels are gradually being promoted.
Applications in various fields
1. Energy-saving rare earth permanent magnet motors are mainly targeted at consumption, such as textile, chemical fiber, petroleum, mining and other fields. Rare earth permanent magnet synchronous motors are used in coal mines to help transport machinery and drive various pumps and fans.
2.The rare earth permanent magnet motor AC servo system is an advanced mechatronic machinery, consisting of an electronic, high-performance speed control system. The rare earth permanent magnet motor represents a promising high-tech industry with significant potential for growth.
3.Another new area is the use of various rare earth permanent magnet DC micromotors to support new variable frequency speed control systems for air conditioners and refrigerators. Rare earth permanent magnet brushless DC motors are instruments with different powers, and the demand for such motors is also great.
Rare earth permanent magnet materials offer significant advantages in aerospace applications and play a crucial role in the advancement of the aerospace industry. While rare earth permanent magnet motors are already utilized in certain aerospace applications, such as generator voltage regulation and short-circuit protection, scientists unanimously agree that these motors represent a vital direction for the development of next-generation aerospace engines.
Technical difficulties faced by permanent magnet motors
1.High price of permanent magnet materials
The cost of permanent magnet materials often accounts for more than 50% of the overall material cost. Permanent magnet materials require rare earth resources. In most countries, rare earths are extremely scarce resources with high prices and low quantities. Most of the world’s rare earth materials are exported from China.
2.Demagnetization phenomenon
Permanent magnet motors are susceptible to irreversible demagnetization when exposed to harsh conditions such as high temperatures and frequent vibrations. Factors contributing to demagnetization include the motor’s high operating temperature, elevated ambient temperature, and heat accumulation. Once demagnetized, the motor’s performance significantly diminishes, rendering it potentially unusable. To mitigate magnetic degradation during operation, one approach involves addressing the issue at its root by developing new high-heat-resistant, high-magnetic NdFeB permanent magnets. Another strategy is to enhance anti-demagnetization technology. For instance, integrating load detection, reducing maximum load, implementing improved heat dissipation measures, and minimizing frequent starts can all help address this issue.
3.Control Technology
Because of the “permanent magnet” phenomenon in the permanent magnet synchronous motor, it is extremely difficult to adjust its magnetic field externally. For the current permanent magnet synchronous motor, the control idea is not to perform magnetic field control, but only to perform armature control. The permanent magnet synchronous motor is controlled using a combination of electronic devices and microcomputer control. Achieve refined management in position, speed and torque control.
In addition to the challenges discussed earlier, Permanent magnet synchronous motors encounter several technical obstacles that necessitate further attention and innovation. These challenges include vulnerability to power outages, limitations in achieving high speeds, and difficulties in initiating motor operation. Addressing these issues is crucial for unlocking the full potential of PMSMs and maximizing their utility across various applications.
One of the primary technical challenges facing PMSMs is their vulnerability to power outages. Unlike traditional induction motors, which can continue running even in the absence of power, PMSMs rely on an external power source to maintain magnetic field excitation. In the event of a power outage, PMSMs may cease operation, leading to disruptions in critical processes and systems.
To mitigate the impact of power outages on PMSM operation, researchers are exploring innovative solutions such as energy storage systems and backup power sources. Integrating battery storage or capacitors into PMSM systems can provide temporary power supply during outages, enabling continuous operation and minimizing downtime. Additionally, advancements in power electronics and control algorithms can enhance the resilience of PMSMs to power fluctuations and interruptions.
Another technical challenge associated with PMSMs is their inherent limitations in achieving high speeds. While PMSMs offer advantages such as high torque density and efficiency, they may struggle to operate at ultra-high speeds due to factors such as rotor inertia and centrifugal forces. This limitation poses constraints on applications that require rapid acceleration and deceleration or high-speed operation.
To address this challenge, researchers are investigating novel rotor designs, advanced materials, and innovative cooling techniques to improve the speed capabilities of PMSMs. By optimizing rotor construction and reducing rotational inertia, engineers can enhance the responsiveness and performance of PMSMs at elevated speeds. Furthermore, advancements in magnetic materials and thermal management systems can help mitigate overheating and mechanical stresses associated with high-speed operation.
Initiating motor operation poses another technical challenge for PMSMs, particularly in applications where precise control and synchronization are required. Unlike induction motors, which can self-start when connected to a power source, PMSMs require external control signals to initiate rotation. This dependency on external control signals can introduce complexities in motor startup procedures and increase system complexity.
To overcome this challenge, researchers are exploring innovative control strategies and sensorless motor startup techniques for PMSMs. By implementing advanced algorithms and sensor technologies, engineers can develop robust and reliable startup procedures that minimize reliance on external control signals. Additionally, advancements in motor design and construction can enhance the efficiency and effectiveness of motor startup sequences, streamlining operations and improving overall system performance.
Despite the technical challenges facing PMSMs, we maintain a strong belief in the continued advancement of permanent magnet motor technology. As researchers and engineers continue to push the boundaries of innovation, we anticipate significant breakthroughs in addressing key technical obstacles and unlocking new opportunities for PMSMs in various industries.
By overcoming challenges such as vulnerability to power outages, limitations in achieving high speeds, and difficulties in initiating motor operation, PMSMs can further enhance convenience and efficiency in human production and daily life. Through collaborative efforts and ongoing research, we are confident that PMSMs will play an increasingly prominent role in powering the future of technology and industry.
