Key Elements in High-Speed Motor Development

With the vigorous development of the global new energy vehicle market, the speed of driving motors has shown astonishing growth. From 18,000 rpm several years ago to comfortably exceeding 20,000 rpm today, this represents not only a numerical breakthrough but also rigorous tests of motor design and manufacturing technologies. This article discusses several aspects of high-speed motor development.

01. Selection of Rotor Pole Pair Number

In high-speed motors, iron loss has become an unavoidable critical factor, especially in high-speed ranges. There is a close relationship between the number of motor poles and iron loss because as motor speed increases, the frequency of magnetic flux changes in the core also increases, leading to a significant increase in iron loss.

For example, in a motor operating at 20,000 rpm, a 6-pole motor reaches a working frequency of 1000 Hz, while an 8-pole motor increases this to 1333 Hz. According to the calculation formula for iron loss mentioned above, the increase in operating frequency directly leads to increased iron loss.

In the design trend of high-speed motors, we can see a gradual decrease in the use of 8/48 pole-slot combinations and an increase in the use of 6/54 pole-slot combinations.

The reason for this shift lies in the aforementioned considerations of iron loss. To reduce iron loss during high-speed operation, designers tend to choose the 6/54 pole-slot combination to achieve better electromagnetic performance and higher efficiency.

02. Selection of Cooling System

For high-speed permanent magnet motors, temperature significantly affects their performance. Since the operating point of permanent magnets drifts with temperature, excessively high temperatures may even risk demagnetization of the magnets. Moreover, the high power density of electric motors in new energy vehicles limits the cooling surface area, making cooling system design crucial to ensuring stable motor performance.

When considering cooling methods, I suggest using an oil cooling system for motors with speeds exceeding 18,000 rpm. This is because heating issues of the rotor become particularly prominent when speeds exceed 16,000 rpm. In a water-cooled motor, the stator is primarily cooled, whereas under high speeds, dissipating rotor heat effectively through water cooling becomes challenging.

Regarding temperature monitoring, current motor designs typically embed temperature sensors inside the stator. In water-cooled motors, due to stable flow channel structures, the temperature distribution of stator windings is relatively uniform and well-controlled. However, in oil-cooled motors, the greater design flexibility of flow channels results in more noticeable temperature differences between windings compared to water-cooled motors. Therefore, when selecting the sensor location, it is crucial to consider areas with higher winding temperature rises to minimize the temperature difference between monitored temperature and the highest winding point, accurately reflecting the motor's actual thermal state.

03. Technological Challenges of High-Speed Bearings

The rotor support system is a core component in the development of high-speed motors, with bearing technology selection being particularly critical. Currently, deep groove ball bearings are commonly used in motor bearings.

In high-speed environments, ball bearings face serious challenges such as overheating and the risk of running. This is because as speed increases, friction and heat generation inside the bearings also increase sharply, leading to decreased bearing performance or even failure. Therefore, lubrication of high-speed bearings is crucial.

After motor speeds exceed 18,000 rpm, another important reason for recommending oil cooling is bearing lubrication. In water-cooled motors, self-lubricating ball bearings are typically used for bearings. However, during high-speed operation, these bearings face challenges such as grease leakage and large temperature differences between inner and outer rings.

In contrast, open-type ball bearings used in oil cooling systems can effectively cool the inner and outer rings of the bearings, avoiding grease leakage issues and having a lower rolling friction coefficient. However, attention must be paid to the design of lubrication oil paths to ensure adequate bearing cooling. In the shoulder hole, the protruding structure is embedded to ensure the cooling oil flow speed is relatively uniform before and after the shoulder.