A high-speed motor rotor is a critical part of a high-speed motor, typically embodying a rotating shaft. It operates by harnessing the electrical power generated by the motor to impart rotational motion to mechanical devices. A defining characteristic of high-speed motor rotors is their high rotational speed, often exceeding 10,000 revolutions per minute (rpm).
In the structural design of high-speed motor rotors, significant consideration must be given to factors such as centrifugal force and impact force that arise from high-speed operation. This necessitates the optimization of axial lightweighting, dynamic balancing performance, and wear resistance. Several common structural types of high-speed motor rotors exist, including sleeve-type, disc-type, magnetic suspension-type, and coplanar-type. The choice of structural type should be based on practical needs.
High-speed motors, featuring small size, high power density, direct connection with high-speed loads, elimination of traditional mechanical speed-increasing devices, reduced system noise, and improved system transmission efficiency, have broad application prospects in various fields such as high-speed grinding machines, air circulation refrigeration systems, energy storage flywheels, fuel cells, high-speed centrifugal compressors for natural gas transportation, and distributed power generation systems used as aircraft or shipboard power supply equipment. They have become one of the research hotspots in the international electrical engineering field.
The main characteristics of high-speed motors include high rotor speed, high stator winding current and magnetic flux frequency in the iron core, and high power density and loss density. These characteristics necessitate key technologies and design methods unique to high-speed motors, distinguishing them from conventional-speed motors. High-speed motor rotors typically rotate at speeds above 10,000 rpm. During high-speed rotation, conventional laminated rotors struggle to withstand immense centrifugal forces, necessitating the adoption of special high-strength laminated or solid rotor structures. For permanent magnet motors, rotor strength issues are even more prominent since sintered permanent magnet materials cannot withstand the tensile stress generated by high-speed rotor rotation, necessitating protective measures for the permanent magnets.
Moreover, the high-speed friction between the rotor and the air gap results in friction losses on the rotor surface that are much greater than those in conventional-speed motors, posing significant challenges for rotor cooling. To ensure sufficient rotor strength, high-speed motor rotors are often slender, increasing the likelihood of approaching critical rotational speeds compared to conventional-speed motors. To avoid bending resonance, it is crucial to accurately predict the critical rotational speed of the rotor system.
Additionally, conventional motor bearings cannot reliably operate at high speeds, necessitating the adoption of high-speed bearing systems. The high-frequency alternating current in the winding and the magnetic flux in the stator iron core of high-speed motors generate significant high-frequency additional losses in the motor winding, stator iron core, and rotor. Skin effect and proximity effect on winding losses can usually be ignored when the stator current frequency is low, but in high-frequency situations, the stator winding exhibits significant skin effect and proximity effect, increasing winding additional losses.
The high magnetic flux frequency in the stator iron core of high-speed motors cannot neglect the influence of skin effect, and conventional calculation methods can lead to significant errors. To accurately calculate the stator iron core loss of high-speed motors, it is necessary to explore iron loss calculation models under high-frequency conditions. Spatial harmonics caused by stator slotting and non-sinusoidal winding distribution, as well as current time harmonics generated by PWM power supply, all produce significant eddy current losses in the rotor. The small rotor volume and poor cooling conditions pose great difficulties for rotor cooling. Therefore, accurate calculation of rotor eddy current losses and exploration of effective measures to reduce them are crucial for the reliable operation of high-speed motors.
Furthermore, high-frequency voltages or currents pose challenges to the controller design of high-power high-speed motors. High-speed motors are much smaller than conventional-speed motors of equivalent power, featuring high power density and loss density, as well as difficult cooling. Without special cooling measures, the motor temperature can rise excessively, shortening the winding life. Especially for permanent magnet motors, excessive rotor temperature can lead to irreversible demagnetization of permanent magnets.
High-speed motors generally refer to motors with rotational speeds exceeding 10,000 rpm or difficulty values (the product of rotational speed and the square root of power) exceeding 1×10^5. Among the various types of motors currently available, those successfully achieving high speeds primarily include induction motors, interior permanent magnet motors, switched reluctance motors, and a few exterior permanent magnet motors and claw pole motors. The rotor structures of high-speed induction motors are relatively simple, with low rotational inertia and the ability to operate for extended periods under high temperature and high-speed conditions, making them widely used in high-speed applications.
In summary, high-speed motor rotors are pivotal components that enable the high-speed operation of motors, characterized by their high rotational speeds, special structural designs, and challenges in cooling and bearing systems. With technological advancements and industrial upgrades, high-speed motors are increasingly being applied in fields such as electric vehicles, aerospace, industrial robots, and clean energy, driving the development of high-performance materials and technologies. The widespread use of carbon fiber rotors, for example, significantly enhances motor efficiency and durability, marking a new era of high-speed motor technology.
