A high-speed motor is simply a motor with a very high rotational speed. However, there’s no strict definition of what constitutes a high-speed motor. Generally, it’s said that a motor with a speed exceeding 10,000 rpm is considered high - speed. But this is just a general statement; there’s no fundamental difference between speeds greater than 10,000 rpm and less than 10,000 rpm . It’s more scientifically accurate to distinguish high-speed and low-speed motors based on rotor surface linear velocity, as many limitations in motor design are closely related to linear velocity. Therefore, the high-speed motors we’re referring to here are those whose high-speed requirements are difficult to meet using standard motor design and manufacturing techniques, requiring special consideration.
Those who have studied motor design know that motor speed is closely related to motor size and weight; for the same power, a higher speed results in a smaller size and weight. With the rapid development of motor and power electronics technologies, high-speed motors are gaining increasing attention. Many high-speed devices aim to eliminate gearboxes and use high-speed motors directly (the so-called “ direct drive “ ) to simplify systems, reduce costs, and improve efficiency. From the motor’s perspective, increasing speed also reduces size and weight to lower costs. Therefore, the demand for high-speed motors has been growing in recent years, and many manufacturers are touting their products. However, when it comes to actually placing orders, it’s difficult to find mature, readily available products. Why are there so few mature products for something that both manufacturers and users are pursuing? As mentioned earlier, it’s not that manufacturers don’t want to make them, but rather that developing high-speed motors is extremely difficult. What makes it so difficult? Today, I’ll explain why high-speed motors are so challenging.
1. Bearing Limitations. Rotating electric machines all rely on bearings. Besides their load-bearing capacity, traditional bearings face a significant limitation: their dn value (which represents the linear velocity of the bearing section). The dn value is the product of the bearing section’s diameter and its rotational speed. Generally, the dn value of a bearing cannot exceed 2 * 10^6 . If the motor speed is too high and the power is relatively large, this value will inevitably limit its performance, and ordinary bearings will not meet the requirements. Solving this problem requires both technological advancements in the bearing industry and the adoption of alternative approaches, such as air-suspended bearings and magnetic levitation bearings.
2. Rotor Structure Limitations. When the rotor rotates at high speed, its components are inevitably subjected to strong centrifugal forces, as well as radial and tangential electromagnetic forces from the air gap magnetic field. When the rotational speed reaches a certain level, the structural strength becomes constrained, particularly for components and structures such as the rotor winding ends, magnet fastening structures, slot roots, slot wedges, magnetic pole fastening structures, rotor cast aluminum structures or copper bar welding, and the commutator. Traditional design and fastening methods are no longer sufficient, necessitating specialized designs and processes. New technologies addressing this include high-strength stainless steel sleeves, embedded magnet design technology, high-strength fiberglass clamping technology, high-strength carbon fiber clamping technology, new welding technologies, high-strength silicon steel material technology, and solid rotor technology.
3. Rotor Dynamics Limitations. The speed of a typical motor is far below the first-order critical speed of the rotor structure; this is called a “ rigid rotor, “ and there’s no need to consider second-order or higher vibration modes and deformation. However, high-speed motors may exceed the first-order or even second-order critical speeds; these are called “ flexible rotors . “ Such rotors rotate like noodles, requiring precise dynamic analysis and calculations, and appropriate suppression measures. The necessary technologies include accurate simulation calculations of rotor dynamics, balancing techniques for flexible rotors, rotor stiffness balancing techniques, and shaft steel material technology.
4. Limitations in Heat Generation and Cooling. The small size and weight of high-speed motors are significant advantages, but this reduction in size inevitably leads to a smaller heat dissipation area. Since motor efficiency remains relatively consistent, this means that motors of the same power have similar losses (i.e., heat generation), but vastly different heat dissipation areas. Therefore, cooling and heat dissipation become extremely critical issues for high-speed motors. Furthermore, from the perspective of motor heat generation, the extremely high speeds and frequencies, coupled with iron losses ( which are proportional to the 1.3 to 1.5 powers of frequency), result in significant increases in iron losses. Additionally, the extremely high speeds drastically worsen windage and stray losses. I have personally witnessed cases where insufficient rotor surface smoothness caused the rotor surface to discolor due to air friction alone during high-speed operation. With heat dissipation worsening on one hand and overheating continuing on the other, the need for rotor clamps or stainless steel sleeves for fastening further exacerbates the problem. Stainless steel sleeves are excellent conductors of heat, increasing stray losses, while clamping materials, whether fiberglass or carbon fiber, are poor conductors of heat. The combination of these factors makes temperature rise in high-speed motors a very challenging issue. Solutions include precise electromagnetic and thermal calculation techniques, advanced cooling technologies (such as evaporative cooling) and cooling structures, and advanced materials technologies.
5. Vibration and Noise Limitations. Vibration and noise are crucial technical indicators for motors. High-speed motors, due to their extremely high rotational speeds, exhibit very high excitation frequencies. Furthermore, their slender shape, light weight, low damping, and diverse vibration modes make them prone to vibrations at various frequencies. Vibration is a source of noise; high vibration inevitably leads to high structural noise. Moreover, the extremely high rotational speeds also easily generate significant airborne noise. During testing, it is common to encounter situations where vibration and noise levels become so high that further testing is impossible before the specified rotational speed is reached. Therefore, vibration and noise reduction are essential and particularly challenging issues that must be addressed for high-speed motors, requiring comprehensive consideration in electromagnetic and structural design.
6. Limitations in Structural Dimensions. Theoretically, the smaller size of high-speed motors is their biggest advantage. However, compared to low-speed motors, motors of the same power and voltage will have similar or identical currents, meaning the required lead wire cross-section will not decrease. In relatively low-speed motors, due to their larger size, there is ample space to arrange the lead wires, and cable routing is often not a problem. However, high-speed motors have limited internal space, especially at the winding ends, often lacking the space to accommodate larger cross-section cables. Therefore, cable routing in high-speed motors becomes a significant issue that must be specifically considered. This may even necessitate enlarging the overall size, greatly diminishing the advantage of the high-speed motor’s small size.
7. Control Limitations. High-speed motors typically require frequency converters for control. Due to their extremely high speeds, the fundamental frequency is very high. However, the switching frequency of power electronic devices is finite, and excessively high switching frequencies can cause severe overheating of the devices and reduce the efficiency of the frequency converter. At a given switching frequency, modulating a higher fundamental frequency inevitably leads to a lower modulation ratio, increased harmonics, and a deteriorated waveform, further worsening the motor’s heating and efficiency, thus limiting further increases in speed. Frequency converters for high-speed motors usually require specialized design.
8. Manufacturing limitations. High-speed motors require high dimensional accuracy, geometric tolerances, and surface roughness, necessitating precision machining equipment and processes. Furthermore, many special designs of high-speed motors often rely on specialized technologies, such as special welding processes (ion beam welding, electron beam welding, friction stir welding, etc.), carbon fiber clamping, precision machining of thin-walled parts, and high-speed dynamic balancing.
9. Material limitations. As mentioned earlier, high-speed motors involve the application of many new materials, such as high-strength, low-loss magnetic materials, high-temperature rare-earth permanent magnet materials, high-strength glass fiber and carbon fiber materials, and high- dv/dt insulating materials, etc.
10. Electromagnetic Compatibility (EMC) Limitations. High-speed motors already have a high fundamental frequency, and the high-order harmonics are also significant, resulting in substantial electromagnetic emission energy. Not only is conducted emission high, but high-frequency radiation also becomes a crucial factor to consider, otherwise it will cause electromagnetic interference to surrounding electrical equipment . Furthermore, due to the extremely high speed of high-speed motors, precise control requires not only highly resolution sensors but also even higher control signal frequencies, demanding higher dynamic performance from the controller. This inevitably leads to higher controller sensitivity and reduced anti-interference capabilities. Therefore, the EMC performance of high-speed motors is also a problem that cannot be ignored.
The above reasons explain why, despite the popularity of high-speed motors, developing truly mature products is extremely difficult. The factors listed above represent only the main problems encountered with high-speed motors.
Post time: Jan-20-2026