Permanent Magnet Synchronous Motors: Design And Application Of Rotor Structure For High-Speed Operation in High-Temperature Environments

In high-end fields such as aerospace, petroleum exploration, high-end equipment manufacturing, and new energy vehicles, high-speed permanent magnet synchronous motors (PMSMs) in high-temperature environments have become core components of drive systems due to their core advantages of high efficiency, energy saving, high power density, and fast response speed. As a key carrier for motor energy conversion, the rotor structure design directly determines the operational stability, power output, and service life of the motor under the dual harsh working conditions of high temperature (usually referring to an operating temperature ≥ 150℃) and high speed (rotational speed ≥ 10000r/min). Combining industry technical practices and the latest research results, this article details the core design points, mainstream types, material selection, and optimization strategies of the rotor structure of high-speed PMSMs in high-temperature environments, providing professional reference for industry practitioners and facilitating the upgrading of related technologies and product implementation.

The superposition of high-temperature and high-speed dual working conditions puts forward three core strict requirements for the rotor structure: first, high temperature resistance and anti-demagnetization. High temperature will lead to the attenuation of the magnetic properties of permanent magnets, and even irreversible demagnetization, directly affecting the motor's power output; second, anti-centrifugal and anti-shedding. The huge centrifugal force generated by high-speed rotation is likely to cause safety hazards such as rotor structure deformation and permanent magnet shedding; third, low loss and temperature rise control. The rotor eddy current loss will increase with the increase of rotational speed, further increasing the temperature, forming a vicious cycle of "high temperature - loss - demagnetization". Therefore, the rotor structure design needs to balance the three core requirements of high temperature resistance, anti-centrifugal force, and low loss.

At present, the rotor structures of high-speed permanent magnet synchronous motors PMSMs suitable for high-temperature environments are mainly divided into three categories: internal-mounted, surface-mounted, and composite. Each type of structure has its own focus on design logic and material selection according to the applicable scenarios, taking into account practicality and reliability, and covering the needs of different high-temperature and high-speed working conditions.

The internal-mounted rotor structure is the most widely used type in high-temperature and high-speed scenarios. Its core advantage is that the permanent magnets are embedded inside the rotor core, avoiding direct exposure of the permanent magnets to high-temperature environments, and at the same time, relying on the core structure to improve the mechanical strength of the rotor and resist high-speed centrifugal force. This structure can be further divided into embedded and internal magnetic barrier structures. The embedded structure fixes the permanent magnets through the slots of the rotor core and reinforces them with high-temperature resistant adhesives to prevent the permanent magnets from shifting during high-speed rotation; the internal magnetic barrier structure blocks the eddy current path by setting multiple layers of magnetic barriers, reduces rotor eddy current loss, lowers the temperature rise amplitude, and at the same time improves the magnetic shielding effect to protect the permanent magnets from external magnetic field interference.

The internal-mounted rotor structure is suitable for scenarios with a rotational speed of 10000-30000r/min and an operating temperature of 150-250℃, such as motors supporting aero-engines and high-temperature pump motors. In terms of material selection, the rotor core mostly adopts low-loss non-oriented silicon steel sheets of 0.2mm or less, such as Baosteel B20AT1200, which can effectively reduce core loss; permanent magnets prefer high-temperature grade samarium-cobalt (SmCo) magnetic steel or high-temperature grade neodymium-iron-boron (NdFeB) magnetic steel. Among them, SmCo magnetic steel has a Curie temperature of 700-800℃, which can effectively avoid high-temperature demagnetization and meet the needs of medium and high-temperature scenarios.

The surface-mounted rotor structure adopts a design where permanent magnets are directly pasted on the rotor surface, which has the advantages of simple structure, high magnetic coupling efficiency, and high power density, and is suitable for high-speed and high-power density high-temperature scenarios. To meet the challenges of high temperature and high speed, this structure needs to focus on solving the problems of permanent magnet fixation and anti-demagnetization: permanent magnets are selected from SmCo magnetic steel with excellent high-temperature resistance or new samarium-iron-nitrogen (SmFeN) magnets. Among them, SmFeN magnets have a Curie temperature as high as 470℃, can still maintain strong magnetism above 200℃, and can achieve stable operation at high temperatures without heavy rare earths; at the same time, a high-strength protective sleeve is wrapped on the surface of the permanent magnets, mainly using carbon fiber composite sleeves, whose tensile strength is 3-5 times that of traditional metals, and it is non-magnetic and non-conductive, which can effectively resist high-speed centrifugal force, avoid eddy current loss, and reduce temperature rise.

In addition, the surface-mounted rotor structure can further reduce eddy current loss and optimize heat dissipation effect by opening circumferential shallow grooves on the sleeve and adding copper shielding rings between the permanent magnets and the sleeve. It is suitable for scenarios with a rotational speed of 20000-40000r/min and an operating temperature of 180-300℃, such as new energy vehicle drive motors and high-speed compressor supporting motors.

The composite rotor structure is an optimized design combining the advantages of internal-mounted and surface-mounted structures. Its core is that part of the permanent magnets are embedded in the rotor core and part are pasted on the rotor surface, taking into account mechanical strength and magnetic performance. The rotor core of this structure is made of high-temperature resistant silicon steel sheets or soft magnetic composite (SMC) laminated. SMC material has isotropic magnetic permeability, which can effectively suppress high-frequency eddy current loss and is suitable for high-speed and high-frequency scenarios; the embedded permanent magnets are used to improve the motor's overload capacity, and the surface-mounted permanent magnets are used to improve power density, and the eddy current loss is reduced by optimizing the magnetic circuit design.

To further improve the high-temperature resistance, the composite rotor structure is usually equipped with built-in cooling channels, and oil cooling technology is used to introduce cooling liquid into the rotor to simultaneously cool the magnetic steel and bearings, effectively controlling the rotor temperature, which is suitable for an operating temperature of 200-300℃.