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Permanent magnet motors rely on permanent magnets to generate their magnetic field. They do not require any excitation coil or current. These motors are known for their efficiency and straightforward design. They are considered energy-saving motors. Permanent magnet motors have undergone a long development process that reflects great improvements over the years.
Development process
The permanent magnet motor’s evolution is closely tagged to the development of a permanent magnet material. The country of China became the very first country that recognize, as well as apply the magnetic properties of this kind of material practically. Over 2,000 years ago, the putting of properties into compass applications, having great significance, including navigation and military strategy among others, was accomplished. This is regarded to be one of the four great inventions of ancient China.
The first electric motor in the world appeared in the 1820s. This motor is a permanent magnet motor. The magnetic field of this kind of motor is generated by permanent magnets. However, this technology disappeared for some time and only developed extensively in recent years. Because the permanent magnet material used at that time was natural magnetite, its magnetic energy density was especially 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. However, some scientists believe 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.
While all kinds of motors were developing rapidly, and at the same time as current magnetizers were invented, researchers studied the mechanism, composition, and manufacturing technology of permanent magnet materials extensively. As a result, a series of permanent magnetic materials were discovered such as carbon steel, tungsten steel, and cobalt steel. Especially, the magnetic properties of the AlNiCo permanent magnet invented in the 1930s and the ferrite permanent magnet invented in the 1950s have been greatly improved, so the permanent magnet excitation method has been widely adopted for various micro and small motors. Permanent magnet motors are widely used in military, industrial, agricultural production, and daily life, with power output ranging from a few milliwatts to tens of kilowatts. Therefore, the production of permanent magnet motors has increased dramatically. During this period, the design theory, calculation methods, magnetization, and manufacturing technology of permanent magnet motors have also been greatly improved. During this period, a series of analytical and research methods were developed, including the permanent magnet working diagram method.
However, due to the low coercivity of AlNiCo permanent magnets and the low remanence density of ferrite permanent magnets, their application range in motors was very limited. Until the 1960s and 1980s, a series of rare earth permanent magnetic materials came out one after another, including rare earth cobalt permanent magnets and neodymium iron boron permanent magnets. They have high remanence density and coercive force, high magnetic energy product, and excellent magnetic property of linear demagnetization curve, especially suitable for manufacturing motors, promoting permanent magnet motors into a new historical period.
Characteristics of permanent magnet motors
Compared with the traditional electric excitation motors, the permanent magnet motors include:
The scope of the application is very extensive. It can be said that in all areas, the aerospace departments include national defense, industrial and agricultural production, and the lives of the people. The following are the principal characteristics of several typical permanent magnet motors and their primary applications.
Compared to conventional generators, the rare-earth permanent magnet generator does not have slip rings and a brush device. The permanent magnet synchronous generator structure is simple and failure rates are reduced. By employing rare earth permanent magnets, the air gap magnetic density can be increased as well as the motor speed towards the optimal value to improve the power-to-mass ratio. Rare earth permanent magnet generators are almost employed on all modern aviation and aerospace generators. Their 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. The largest capacity 40 kVA to 160 kVA rare earth permanent magnet auxiliary exciter in the world was successfully developed for 200 MW ~ 600 MW turbine generators in the 1980s. Since then, 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
3. The other 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. The 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 have great advantages in aerospace and are of great significance to the development of the aerospace industry. Rare earth permanent magnet motors have been used in some fields of aerospace, such as generator voltage regulation and short circuit protection, but scientists in the world uniformly believe that the rare earth permanent magnet motor is one of the essential directions for the development of the next generation of aerospace engines
Technical difficulties faced by permanent magnet motors
1.High price of permanent magnet materials
The cost of the permanent magnet material often occupies over 50% of all material costs. Permanent magnetic material needs rare earth resources. In most countries rare earth is considered to be an extremely lean mineral resource with high prices and quantity low. Most products of the world’s rare earth materials are exported from China.
2. Demagnetization phenomenon
Permanent magnet motors are always in danger of irreversible demagnetization under the unfavorable conditions of high temperatures and frequent mechanical vibrations. The contributing factors to demagnetization are the high operating temperature of the motor, the rise in ambient temperature, and the accumulation of heat. Once this happens, performance reduces drastically, and the motor becomes practically useless. To decrease the magnetic degradation during the working process, one is to research and develop a series of high-temperature-resistant and high-magnetic NdFeB permanent magnet materials to solve the problem from its roots; the other is to promote anti-demagnetization technology. For instance, it can be realized by load detection, reduction of maximum load, enhancement of heat dissipation measures, and reduction of frequent starts.
3. Control Technology
Due to the “permanent magnet” phenomenon in the permanent magnet synchronous motor, it is very difficult to adjust its magnetic field externally. For the permanent magnet synchronous motor applications at present, the idea of control is not to carry out magnetic field control, but only armature control. The permanent magnet synchronous motor is controlled by electronic devices in coordination with microcomputer control. Accomplish refined management in position, speed, and torque control.
Besides the issues discussed above, a few more critical technical difficulties arise in Permanent magnet synchronous motors that need further attention and innovation. Such difficulties include susceptibility to power outages, inability to reach very high speeds, and problematic motor start-up. Addressing such issues is important for unlocking the full potential of PMSMs and maximizing their utility across various applications.
The susceptibility to power outages is one of the major technical problems that PMSMs face. While induction motors conventionally continue their operation with no power supplied to them, PMSMs always require an external power source for magnetic field excitation. In case of a power outage, PMSMs may just stop working, thus disrupting critical processes and systems.
Some of the methods that can be used to reduce the impact of power outages on the functioning of PMSMs are energy storage systems and backup powers. Integrating a battery or capacitor with the PMSM system will enable it to sustain the power supply for some time in case of an outage, ensuring further running and reducing the standstill period. Furthermore, hardening in power electronics and control algorithms enhances the robustness of PMSMs against power fluctuations and interruptions.
Another technical challenge associated with PMSMs is their inherent limitation in high speeds. While PMSMs have some favorable features, such as high torque density and efficiency, they may not be capable of operating at ultra-high speeds due to factors like rotor inertia and centrifugal forces. This limitation imposes constraints on applications requiring rapid acceleration and deceleration or high-speed operation.
To address this challenge, innovative rotor designs, advanced materials, and innovative cooling techniques are under consideration to improve the speed capabilities of PMSMs. With optimized rotor construction, a reduction in rotational inertia allows engineers to further increase responsiveness and performance at elevated speeds. Besides, further development of magnetic materials and thermal management systems can be used to reduce overheating and mechanical stresses at high-speed operation.
Another technical challenge in PMSM is related to its startup process. At startup, for applications requiring precision in control and synchronization, PMSM has another technical challenge. Unlike the induction motor, which will self-start once connected to a power source, PMSM needs some external control signals from outside to start the rotation. This will increase the system complexity, especially during the startup of the motor operation process.
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.
We are indeed convinced that, notwithstanding these technical challenges, improvement in permanent magnet motor technology is going to prevail. Considering the pace at which innovative ideas are being proposed and tested by researchers and engineers, remarkable achievements can be expected in overcoming certain main technical challenges and opening perspectives toward new applications of PMSMs in different industries.
If the problems of vulnerability to power outages, limitations in reaching high speeds, and problematic motor operation startup have been solved, PMSMs would make human life and production even more comfortable and efficient. We easily predict, through these collaborations and continuous research that PMSM motors play a big role in powering technology and industries shortly.
