The Importance of Rotor Design in High-Speed Brushless Motors

High-speed brushless motor rotor typically operate at speeds ranging from 20,000 to 100,000 RPM. The design of high-speed motors significantly differs from that of conventional low-speed, low-frequency motors. A dynamic analysis of the rotor and bearing systems is crucial for the operational reliability of high-speed motors.

The design of the rotor is key in high-speed brushless motor design, with main considerations including: the selection of rotor diameter and length, choice of permanent magnet materials, and protection methods employed (as the permanent magnets cannot withstand the immense centrifugal forces encountered at high speeds and must be shielded with high-strength materials). This involves analysis of rotor strength and rigidity and the design of bearings (since high-speed brushless motors cannot use standard bearings and must instead use non-contact types like air or magnetic bearings).

For high-speed brushless motors, the design of the permanent magnet rotor must consider both electromagnetic and mechanical aspects. This means the permanent magnet rotor must provide a sufficiently strong rotational magnetic field for the stator windings, while also being capable of withstanding the enormous centrifugal forces generated by high-speed rotation.

High-speed brushless motors generally have fewer poles, typically using 2 or 4 poles. A 2-pole motor facilitates the use of a solid structure for the permanent magnets to ensure mechanical and electromagnetic symmetry. Furthermore, the magnetic field of the stator core and the winding current and frequency of a 2-pole motor are only half that of a 4-pole motor, which helps reduce the iron and copper losses in the motor stator. However, a major drawback of 2-pole motors is that the stator windings are longer and require a larger stator core area.

The choice of permanent magnet materials in high-speed brushless motors significantly influences the motor's size and performance. When selecting permanent magnet materials, considerations include:

1. To enhance the motor's power density and efficiency, materials with high residual flux density, coercivity, and magnetic energy product should be selected.

2. The demagnetization curve of the permanent magnet material should linearly change within the allowable operating temperature range. To ensure the operating temperature of the permanent magnet rotor does not exceed the demagnetization temperature of the magnets, high-temperature resistant permanent magnet materials should be used.

Given the immense centrifugal forces that high-speed brushless motor permanent magnet rotors must withstand, the mechanical properties of the magnet materials are also critical. Considering the technical requirements and material costs, sintered neodymium iron boron, a type of powder metallurgy permanent magnet material, is commonly used. Protection methods for these magnets include adding a high-strength, non-magnetic protective casing outside the magnet, fitted tightly over it. Another protection method involves using carbon fiber banding to secure the magnets.