I. The "Survival Test" under High-Speed Rotation
Magnetic levitation motors, with their advantages of contactless operation, high efficiency, and extremely high rotational speeds, are increasingly being adopted in high-end equipment such as industrial blowers, compressors, and energy storage flywheels. However, when the rotation speed reaches tens of thousands of revolutions per minute or even higher, the permanent magnets on the rotor are subjected to a severe "survival test."
Where does the problem lie?
Magnetic levitation motors commonly use sintered NdFeB as the permanent magnet material. Although NdFeB offers excellent magnetic properties – including very high magnetic energy product and coercivity – it has a critical weakness: its compressive strength is far greater than its tensile strength. Sintered NdFeB, produced via powder metallurgy, typically has a tensile strength of no more than 80 MPa. At high speeds, the centrifugal force generates significant tensile stress inside the permanent magnet – under operating conditions of 18,000 rpm, the centrifugal stress in NdFeB can exceed 160 MPa, nearly double its own strength limit.
This is like a rope made of a brittle material: it withstands compression without issue, but breaks easily under tension. When the motor rotates at high speed, the permanent magnets are subjected to tensile forces as they are "thrown outward." Once the limit is exceeded, the magnet steel will crack, shatter, or even cause the rotor to burst.
II. Carbon Fiber Sleeve: A "Tight Armor" for the Magnet Steel
How can we protect the fragile permanent magnets from cracking under centrifugal force? The most effective solution available today is to add a carbon fiber sleeve over the permanent magnets.
Carbon fiber has a tensile strength of over 5000 MPa, far exceeding the strength limit of NdFeB. More importantly, compared with traditional metal sleeves such as titanium alloy, the carbon fiber sleeve offers three major advantages:
Lightweight and high strength – The specific strength (strength-to-density ratio) of carbon fiber is much higher than that of metals, so a thinner and lighter material can provide sufficient protective strength.
No eddy current loss – Carbon fiber is a poor conductor, so it does not generate high-frequency eddy current losses like metal sleeves, thus avoiding additional power loss and heating issues.
Low thermal expansion – Carbon fiber has a low coefficient of thermal expansion, ensuring good dimensional stability under high temperature operating conditions.
III. The Core Secret to Anti-Cracking: Pre stress
Does adding a carbon fiber sleeve mean everything is solved? Not quite.
The key point is that both the sleeve and the permanent magnets undergo radial expansion due to centrifugal force during high speed rotation. If the sleeve is simply "fitted" over the magnets, a gap will appear between them – because the radial deformation of the sleeve is often greater than that of the magnets. Once a gap forms, the sleeve loses its constraint on the magnets, and the magnet steel will still crack.
The solution is to apply a continuous "pre stress" to the permanent magnets.
By creating an interference fit between the sleeve and the magnets (i.e., the sleeve’s inner diameter is slightly smaller than the magnets’ outer diameter), the sleeve acts like a "tight suit" that tightly wraps around the magnets, applying an inward radial compressive stress. When the rotor rotates at high speed, this pre stress effectively counteracts the tensile stress caused by centrifugal force.
Research shows that when the interference reaches more than 0.10 mm, the maximum centrifugal stress in the permanent magnets can be reduced from over 160 MPa to below 70 MPa, well below their strength limit. Under extreme conditions (e.g., 200 °C high temperature plus overspeed rotation), although the hoop stress in the carbon fiber sleeve may rise to above 1000 MPa, there is still sufficient safety margin relative to the carbon fiber material’s strength limit of 1400 MPa.
IV. How to Achieve Pre-stress? Two Process Routes
Currently, there are two mainstream methods to achieve pre-stress in a carbon fiber sleeve:
Route 1: Interference Assembly
The carbon fiber sleeve is manufactured separately and then assembled onto the rotor by thermal or cold fitting. For example, cooling the rotor to –190 °C allows the sleeve to be slid on with very little axial force; alternatively, an axial press fitting method with a pressing force of up to 25 kN can be used.
However, this method has drawbacks: carbon fiber is brittle and has poor toughness, making it prone to damage and cracks during interference assembly. Moreover, the assembly process is complex and interference control is difficult.
Route 2: High-Tension Winding (the better solution)
Carbon fiber is wound directly onto the rotor surface, and during the winding process, high tension is applied to the fiber tows, making each layer of fiber tightly wrap around the permanent magnet surface.
The subtlety of this method is that the winding process itself is the pre stress application process. By controlling the fibre tension, the desired prestress field can be imposed on the sleeve, replacing the traditional mechanical interference method.
V. SDM’s Carbon Fiber Winding Process
In the field of magnetic levitation high-speed motor rotors, Hangzhou SDM Magnetics Co., Ltd. has mastered a mature carbon fiber winding process. Its technical features are mainly reflected in the following aspects:
High-tension circumferential winding technology. SDM adopts the process route of directly winding carbon fiber circumferentially onto the rotor surface. By precisely controlling the tension applied to the carbon fiber tows during winding, the fibre layers are tightly conformed to the outer surface of the permanent magnets. This process simultaneously provides the required pre-tightening force to the magnets while fabricating the sleeve, avoiding the crack risks and assembly difficulties associated with traditional interference assembly.
Precise tension schedule control. SDM's process flexibly employs different tension control modes according to various operating requirements. To meet different stress distribution needs – such as "looser inside, tighter outside" or "tighter inside, looser outside" – they can choose constant tension, constant torque, or tapered tension winding modes. By controlling the winding tension layer by layer, the residual stress in the fibre layers can be made uniformly distributed to an ideal state.
Quantitative verification of pre-tightening force. SDM has established a complete technical closed loop, from theoretical calculation to finite element simulation, and finally to experimental verification. For the pre-tightening force generated by the high-tension wound carbon fibre sleeve on the permanent magnets, the average error between experimental test results and analytical calculations is 8.56%, and the average error relative to finite element simulation is 7.88% – this level of accuracy fully guarantees the reliability of the pre-stress design.
Integrated full-process capability. From carbon fibre material selection, structural design, and electromagnetic design to moulding assembly processes, equipment manufacturing, and inspection and testing, SDM possesses a complete technical capability. The company is headquartered in Hangzhou and has an industry trade integrated layout, enabling it to provide customers with a full-chain solution from magnets to rotor assemblies.
It is precisely with this refined carbon fibre winding process that SDM’s magnetic levitation high-speed motor rotors can effectively prevent magnet steel cracking under high-speed centrifugal conditions, ensuring safe, stable, and reliable operation of the rotor under the demanding conditions of tens of thousands of revolutions per minute.
