In today's rapidly advancing technological landscape, high-speed motors are becoming the core power source in multiple high-end fields. From new energy vehicles to aerospace, from precision manufacturing to energy equipment, behind these cutting-edge applications lies the support of a key technology—high-speed motor stator technology.
When we talk about high-speed motors, high rotational speed and high power often come to mind. In fact, motors with speeds exceeding 10,000 r/min are classified as high-speed motors. Their ability to become the core of numerous industrial sectors is entirely due to their characteristics of small size and high power density.
As the "heart" of a high-speed motor, the stator's technological superiority directly determines the entire motor's performance, efficiency, and reliability.
01 Challenges of High-Speed Motors
High-speed motors are not simply ordinary motors running faster. As speed soars, a series of unprecedented challenges arise.
High-Frequency Loss is the first and foremost challenge. The frequency of the stator winding current and the magnetic flux in the iron core increases sharply with the rising speed, generating significant high-frequency additional losses in the motor windings, stator core, and rotor.
Skin effect and proximity effect are usually negligible at low frequencies but become extremely significant at high frequencies.
The Heat Dissipation Problem is equally challenging. High-speed motors are much smaller than conventional-speed motors of equivalent power, leading to high power density and loss density, making heat dissipation difficult.
Without special cooling measures, excessive motor temperature rise can occur, shortening the winding lifespan.
For permanent magnet motors, excessive rotor temperature rise can also lead to irreversible demagnetization of the permanent magnets.
Challenges in Manufacturing Processes should not be underestimated. Improper handling of the stator bore's cylindricity and coaxiality can cause magnetic field force imbalance during rotor operation, generating vibration acceleration based on air gap variations.
02 Stator Winding Technology
Stator windings are a key factor in improving motor efficiency, lifespan, volume, and cost. To meet the challenges of transportation electrification, selecting the appropriate winding technology and proper design is crucial.
There are currently three main winding technologies: Pull-In Windings, Hairpin Windings, and Formed Litz Wire.
Pull-In Windings consist of round wires inserted into slots, with each wire insulated and multiple wires placed side-by-side. The fill factor for this type of winding can reach 40% to 45%.
Hairpin Windings, also known as bar windings, are composed of individually insulated solid copper bars. Pre-formed U-shaped bars are inserted into the motor slots, and the open ends of the copper bars are bent and connected by welding. The fill factor for HPW can exceed 50%.
Formed Litz Wire consists of bundles of strands twisted, compressed, and parallel-connected to form bars. The individually insulated strands are continuously transposed along the motor's axial direction. The achievable fill factor for FLW is comparable to that of HPW.
Among the three winding types, Hairpin Windings and Formed Litz Wire have higher fill factors, meaning more compact designs and higher power density.
03 Innovative Stator Structures
Facing the special challenges of high-speed environments, researchers have developed various innovative stator structures.
The Stator Permanent Magnet Motor is a breakthrough design. It breaks the constraints of traditional vehicle traction motors by placing the permanent magnets in the stator instead of the rotor.
This design offers many advantages: the rotor has neither permanent magnet material nor armature windings, making it simple and robust, especially suitable for high-speed operation; both the permanent magnets and armature windings are located in the stator, facilitating easier cooling; magnetization of the permanent magnets can be achieved by reasonably changing the connection method of the armature windings.
The Slotless Stator Structure is another innovative solution. In high-speed permanent magnet motors, the alternating magnetic field frequency is very high, leading to significant stator iron loss, severe heating, and cogging torque causes torque ripple.
Adopting a slotless stator structure can effectively reduce iron loss and completely eliminate the effects of cogging torque.
Some studies combine the high permeability, high resistivity, and low-cost characteristics of soft magnetic ferrite, using it as the stator core for high-speed permanent magnet brushless DC motors, while also employing a slotless stator structure.
The Toroidal Winding Structure is a novel design proposed by Shenyang University of Technology in their research on high-speed permanent magnet motors.
In the toroidal winding, the lower layer sides of the coils are placed in 6 slots of the stator core, while the upper layer sides are distributed in 24 slots on the outer edge of the stator yoke. This not only increases the ventilation and heat dissipation area on the stator surface but also allows cooling airflow to directly cool the stator windings.
04 Cooling Technology and Thermal Design
The cooling system of a high-speed motor is key to its reliable operation. A well-designed cooling system can effectively reduce the temperature rise of the stator and rotor, which is crucial for the long-term stable operation of high-power high-speed motors.
Stator Cooling Technologies are diverse. For enclosed water jacket structures, the temperature at the winding ends is relatively high during internal self-cooling.
Engineers have found through practice that adjusting the water flow direction, adopting a method where water enters from the middle and exits from both sides, can effectively improve heat dissipation.
For large high-speed motors, a rotor fan can be added, and the internal airflow organization can be designed to segment the stator in the middle, serving as an air intake channel from the outer middle part of the casing, with gas exhausting from both ends. The rest of the casing features a water-cooled structure with water entering from the middle and exiting from both sides.
Enhanced Heat Dissipation Treatment for Winding Ends is also a technology specific to high-speed motors. Unlike traditional motors, high-speed motors adopt methods such as eliminating conventional slot wedges to improve cooling conditions.
Spray cooling technology is also applied to dissipate heat from the winding heads. This direct cooling method effectively removes heat generated by the windings, ensuring stable motor operation in high-temperature environments.
05 Future Development Trends
With continuous technological progress, high-speed motor stator technology is developing towards higher efficiency, greater reliability, and more intelligence.
Application of New Materials will be key. The use of materials like soft magnetic ferrite with high permeability and high resistivity, along with high-performance insulating materials, will further improve the stator's working efficiency and reliability.
Integrated Design is another major trend. The design of high-speed motors is a comprehensive, iterative process involving multiple physical fields: electromagnetic fields, rotor strength, rotor dynamics, fluid fields, and temperature fields.
In the future, through multi-physics coupling simulation and optimization, stator technology will be more tightly integrated with other motor systems.
Innovation in Manufacturing Processes will also drive stator technology forward. With the development of 3D printing and precision machining technologies, more complex and optimized stator structures will become possible, further pushing the performance limits of high-speed motors.
Currently, research in the field of high-speed motors in China is continuously deepening. Several universities and research institutions, such as Zhejiang University, Shenyang University of Technology, and Harbin University of Science and Technology, have made significant progress in this field.
From industrial equipment to daily life, innovations in high-speed motor stator technology are quietly changing our world.
In the future, with the application of new materials and new processes, high-speed motor stator technology will continue to break through, providing stronger and more efficient, powerful momentum for human technological progress.
