Key Technologies of High-Speed Motors

With the increasing demand for smaller size and higher power, the speed of motors has been continuously increasing. From the initial speed of a few thousand rpm to tens of thousands, and even hundreds of thousands of rpm, higher speeds lead to an increase in power density and material utilization. Therefore, high speed is a strong trend. For example, the maximum speed of the first generation of Toyota's Prius was only 6000 r/min, while the fourth generation reached 17000 r/min. In this article, we will take a higher perspective to look at the applications of high-speed motors and the key technologies behind them.

Main technical issues of high-speed motors

The application prospects of high-speed and ultra-high-speed motors are broad, but they also bring extreme challenges to motors. After summarizing these problems, we found six major categories: heat dissipation, selection, rotor structure, vibration and noise, high-efficiency design, and bearings.

Heat Dissipation

The loss of the motor increases geometrically with the speed, and the heat generated by the high loss rapidly increases the temperature of the motor. To maintain high-speed operation, it is necessary to design a good heat dissipation cooling method. Common cooling methods of high-speed motors include:

1. "Internal forced air cooling" - strong wind can directly blow into the motor to take away the heat on the coil and iron core. This method is commonly used in vacuum pumps, blowers, and aircraft motors that already have strong wind available.
2. "Internal oil cooling" - in applications where the motor must be sealed for protection, or where there is no strong wind, the most commonly used method is internal oil cooling. For example, AVL's high-speed motor uses a combination of stator slot oil cooling. Some motors also use combinations of winding spray oil cooling + stator oil cooling + rotor oil cooling.

For high-speed motors, achieving high power density, heat dissipation, and cooling are important issues that must be addressed.

Motor Selection

Permanent magnet motor or induction motor? Or other types of motors such as switched reluctance? The selection of high-speed motors has always been a question without a standard answer. Generally, from the perspective of power density and efficiency, permanent magnet motors have an advantage, while from the perspective of reliability, induction motors and switched reluctance motors are better. However, the vibration and noise are relatively large for switched reluctance motors, so their application is limited.

In ultra-high-speed applications, induction motors are more common, while in high-speed applications, both induction motors and permanent magnet motors are used. As long as this principle is followed, we can choose the motor type according to our needs.

Rotor Structure

The rotor structure of high-speed motors must overcome centrifugal force. In the "high-speed" range, metal sleeves and rotor structures are commonly used, while in the "ultra-high-speed" range, carbon fiber winding or solid rotor structures are used.

Most permanent magnet high-speed motors use rotor sleeve structures. This type of design is also very demanding, requiring protection of the permanent magnet and prevention of sleeve failure. To avoid stress concentration, if the magnetic steel is not filled in the entire circular circumference, stress concentration will occur on both the sleeve and the magnetic steel. This is why high-speed permanent magnet motors all use complete circular magnetic steel. If a complete circular ring cannot be made, filler material is used to fill the circumference.

Vibration and Noise

Vibration and noise are major obstacles for high-speed motors. Compared to ordinary motors, high-speed motors have dynamic problems caused by the rotor, such as critical speed problems and shaft deflection vibration problems. They also have problems with high-frequency electromagnetic force generated noise, as the electromagnetic force frequency of high-speed motors is higher and the distribution range is wider, making it easy to excite the resonance of the stator system.

To avoid critical speed vibration, the design of high-speed motor rotors is crucial and requires strict modal analysis and testing. During design, the length-diameter ratio should be used as an optimization variable: if the rotor is too thick and short, the critical speed limit will be increased, and resonance will be less likely to occur. However, the difficulty of overcoming centrifugal force will increase. On the other hand, if the rotor is too thin and long, the centrifugal force problem will be improved, but the critical speed will be lowered, and the probability of resonance will increase. The electromagnetic power will also decrease accordingly. Therefore, the design of the rotor needs to be repeatedly balanced, which is a key issue in the design of high-speed motors.

High-Efficiency

The loss of the motor increases geometrically with the speed, causing the efficiency of the motor to rapidly decrease. To achieve high efficiency, it is necessary to properly manage various types of losses. For example, iron loss accounts for a large proportion. To reduce eddy current loss, ultra-thin silicon steel sheets with a thickness of 0.10 mm or 0.08 mm are commonly used. Although ultra-thin sheets can reduce eddy current loss, they cannot improve magnetic reluctance loss. Therefore, the iron loss of ultra-thin sheets is mainly due to magnetic reluctance loss, while that of ordinary sheets is mainly due to eddy current loss. To improve magnetic reluctance loss, three solutions can be considered:

1. Optimize the magnetic circuit design to improve the sinusoidality of the magnetic field and reduce harmonic iron loss.
2. Reduce magnetic load and increase thermal load to reduce fundamental wave iron loss.
3. From the perspective of material selection, choose silicon steel sheets with lower magnetic reluctance loss.

In addition to iron loss, high-speed motors also need to pay special attention to AC loss, which is caused by high-frequency alternating magnetic fields that penetrate into the magnetic steel, metal sleeves, and stator windings. One common method to address AC loss in magnetic steel is to divide it into multiple segments, which can be done radially or axially. Segmentation reduces the area of eddy current loops, reducing AC loss. The simulation below shows the effect of segmentation on eddy current fields. The more segmented grains, the lower the AC loss. In addition to segmentation, there are many other solutions to address AC loss, but they will not be expanded upon here due to space limitations.

In high-speed motors, the highest frequency magnetic field component is introduced by the PWM carrier of the inverter. Due to the working principle of pulse width modulation, high-frequency current harmonics are inevitably generated, which in turn produce high-frequency magnetic fields. These high-frequency magnetic fields penetrate into the magnetic steel and rotor surface, causing high-frequency loss. Some high-speed motors use multi-level drive structures to improve PWM edge frequency harmonics.

Bearings

The selection of bearings for high-speed motors is a key issue. There are four main types to choose from: magnetic levitation, air bearings, sliding mechanical bearings, and rolling mechanical bearings. Magnetic levitation bearings are used in larger power applications, while air bearings are used in smaller power and size applications. Mechanical bearings often require lubrication, which is limited in many oil-free applications.

Other Considerations

There are many other key issues and technologies for high-speed motors. Addressing these issues simultaneously is more challenging than for ordinary motors. A multi-physical field coupling design approach that considers force, magnetism, heat, and NVH is required. This presents new challenges and opportunities.

Overall, high-speed motors have broad application prospects and high technical challenges. Although some technologies may seem far away, from a developmental perspective, we can see the progression of "low-high speed - medium-high speed - ultra-high speed - ultra-ultra-high speed." Compared to ten years ago, motors with speeds of one or two hundred thousand rpm are now common. Therefore, high-speed technology is a "long-term" trend that will gradually change the layout of the industry. Therefore, high-speed technology is worth investing in for both seeking new opportunities and improving the competitiveness of existing products.

Conclusion

In conclusion, high-speed motors have become an increasingly important technology in modern industries due to their advantages in power density, efficiency, and size. However, designing high-speed motors presents many technical challenges, including heat dissipation, selection, rotor structure, vibration and noise, high-efficiency design, and bearings. To address these challenges, engineers must use a multi-physical field coupling design approach that considers force, magnetism, heat, and NVH. While some high-speed motor technologies may seem far away, the trend towards higher speeds is a long-term one that will gradually change the layout of the industry. Therefore, investing in high-speed motor technology is a wise choice for both seeking new opportunities and improving the competitiveness of existing products. Overall, the development of high-speed motors requires a combination of advanced technology and innovative design, and has broad application prospects in fields such as aerospace, electric vehicles, and renewable energy.