Magnetic bearing motors, with their advantages of contactless operation, no wear, and high efficiency, are rapidly replacing traditional motors in fields such as high-speed compressors, blowers, and flywheel energy storage. However, when rotation speeds reach tens of thousands or even over one hundred thousand revolutions per minute, rotor reliability becomes the decisive factor for product success – vibration and abnormal noise, magnet detachment, and high-speed failure are three persistent problems that have long troubled engineers in the industry. This article starts from the root causes, analyses the physical mechanisms behind these issues, and introduces the most effective current solution – carbon fiber winding technology.
1. Vibration and Abnormal Noise: The Invisible “Low-Frequency Killer”
1.1 Phenomena and Hazards
During operation, magnetic bearing motors sometimes exhibit abnormal vibration and noise that are independent of rotation speed. Unlike the unbalance vibration common in ordinary rotating machinery, this vibration is not affected by the speed level; it persists even at a stable speed. Prolonged exposure to such vibration not only accelerates fatigue damage to bearings and structural parts but also produces irritating noise, seriously affecting equipment reliability and user experience.
1.2 Root Cause Analysis
Studies show that the low‑frequency vibration of Magnetic levitation motor rotor is determined by the natural frequency of the closed‑loop control system and is excited by external noise. In other words, this is not a purely mechanical issue but a coupling phenomenon between the control system and the mechanical structure.
Specifically, the following factors can induce low-frequency vibration:
Rotor unbalance: center-of-mass offset caused by machining and assembly errors;
Bearing clearance: mismatch between the control parameters of the magnetic bearings and the rotor’s dynamic characteristics;
Intermediate links in the control system: delays and nonlinearities in signal acquisition, processing, and output.
1.3 Solutions
For low-frequency vibration, the mainstream technical approaches include:
(1) Dynamic balancing correction: use high-precision balancing equipment to correct the rotor, adding or removing counterweights to bring the unbalance within the allowable range.
(2) Control algorithm optimization: researchers have proposed vibration compensation strategies based on extended state observers. Experimental results show that under the same white noise excitation, the maximum rotor vibration with the compensator is reduced by about 21% compared with PID control alone; at 30,000 rpm, the maximum rotor vibration is reduced by 26.6%.
(3) Structural optimization: optimize the rotor structure design to improve the stiffness and damping characteristics of the rotor system.
2. Magnet Detachment: The “Centrifugal Pain” at High Speeds
2.1 Phenomena and Hazards
Magnet detachment is one of the most serious failures in permanent magnet motors. At speeds of tens of thousands of rpm, the centrifugal force on the magnets can reach thousands of times their own weight. Once a magnet detaches from the rotor surface, at best the motor performance drops sharply; at worst, it can cause rotor jamming, stator bore scoring, and other catastrophic consequences.
2.2 Root Cause Analysis
Magnet detachment and edge lifting can be attributed to five key factors:
(1) Insufficient strength: the shear strength of the adhesive is lower than the centrifugal or impact force on the magnet, so the bond cannot hold.
(2) High- and low-temperature failure: the adhesive becomes brittle at low temperatures or fails at high temperatures, drastically reducing bonding performance. Ordinary adhesives typically have an operating temperature around 120°C, while the internal temperature rise of the motor often exceeds this range.
(3) Mismatch in thermal expansion coefficients: the thermal expansion differences between the magnet (e.g., NdFeB) and the rotor material (e.g., aluminum alloy) are large, and temperature changes induce internal stress that cracks the adhesive layer.
(5) Environmental corrosion: moisture, heat, salt spray, etc., attack the adhesive layer and weaken the bond.
In addition, improper segmentation design of the magnets can worsen the problem. When a single magnet segment has too large an area in contact with the rotor, wrapping carbon fiber on the outside can easily crack the magnet; even if it does not crack during winding, it may crack after some operation.
2.3 Solutions
(1) Optimize adhesive bonding process: select high-performance structural adhesives, ensure clean bonding surfaces, and strictly control curing conditions.
(2) Magnet segmentation design: divide the magnets along the horizontal direction into smaller segments to reduce the area of each piece and lower the risk of cracking.
(3) Physical constraint reinforcement – this is the most fundamental solution: add a high-strength sleeve outside the magnets to provide physical restraint against centrifugal force. Carbon fiber winding is currently recognized as the best reinforcement method.
3. High-Speed Failure: When the Rotor “Cannot Hold On”
3.1 Phenomena and Hazards
When the motor speed approaches or exceeds the structural limit of the rotor, the rotor faces catastrophic failure. Typical manifestations include rotor deformation, permanent magnet fragmentation, sleeve rupture, and rotor drop. Once high speed failure occurs, not only is the equipment scrapped, but it may also cause serious safety accidents.
3.2 Root Cause Analysis
The fundamental cause of high-speed failure is the contradiction between centrifugal force and material strength.
Take NdFeB permanent magnets as an example. Although they have extremely high magnetic energy product and coercivity, making them the best performing permanent magnet material today, their tensile strength is low (<80 MPa), and they are temperature-sensitive with poor thermal stability. At speeds of tens of thousands of rpm, the centrifugal stress on the permanent magnets far exceeds their own strength limit, so an external sleeve is essential for protection.
The traditional solution is to use non-magnetic metal sleeves (such as Inconel 718 or titanium alloy). However, metal sleeves have a fatal drawback: eddy current losses. The higher the conductivity of the sleeve, the greater the eddy currents generated, and the more serious the eddy current loss, which causes the rotor temperature to rise sharply, further aggravating the risk of demagnetization of the permanent magnets.
3.3 Solutions
Carbon fiber composite sleeves are currently recognized as the best solution.
The advantages of carbon fiber sleeves are:
Low conductivity: they generate virtually no eddy current losses, resulting in the lowest rotor temperature rise;
High strength: the specific strength of carbon fiber is much higher than that of metals, providing stronger restraint with lighter weight;
High modulus: through optimization of resin materials and winding processes, the elastic modulus can be increased from the traditional 130-160 GPa to over 200 GPa.
4. The Ultimate Solution: Carbon Fiber Winding Technology
To simultaneously solve the three major problems of vibration noise, magnet detachment, and high-speed failure, carbon fiber winding is an indispensable core technology. Its principle is to wind high-strength carbon fiber composite material around the permanent magnets, forming a tight “armor” over the rotor that provides continuous radial constraint against the centrifugal force generated by high-speed rotation.
4.1 Two Mainstream Processes
At present, there are two main approaches for manufacturing carbon fiber rotors:
Press-fitting method: first fabricate the carbon fiber sleeve, then press it onto the rotor or use shrink-fitting. In shrink-fitting, the rotor is cooled to -190°C, and the sleeve can be installed with very little axial force. The press-fitting method is relatively mature, but it requires extremely precise control of the interference fit – too much interference may crack the magnets, while too little provides insufficient restraint.
Direct winding method: wind the carbon fiber directly onto the permanent magnet surface, then cure it. This method demands extremely strict control over winding tension, curing temperature, interlayer bonding, and other process parameters, but it can achieve more uniform pre stress and higher material utilization.
4.2 Key Technical Difficulties
(1) Pre-stress control: an appropriate initial tension must be applied during winding so that the carbon fiber exerts a continuous pre-compression on the magnets after curing. Excessive tension can crack the magnets, while insufficient tension cannot provide adequate restraint.
(2) Thermal matching: the thermal expansion coefficients of the carbon fiber composite, the permanent magnets, and the shaft material need to be precisely matched to avoid excessive internal stress due to temperature changes.
(3) Stress analysis: finite element analysis software (e.g., MSC Patran/Nastran) should be used to accurately analyze the stress and deformation of the rotor structure, determining the optimal winding layer thickness, angle, and process parameters.
Studies have shown that a Magnetic levitation motor rotor with a carbon fiber reinforcement ring can meet the strength and deformation requirements at high speeds of 72,000 rpm.
5. SDM’s Carbon Fiber Winding Process
In the field of carbon fiber winding for magnetic bearing / high-speed motor rotors, SDM is one of the few domestic companies that master the core technology.
In the field of magnetic bearing / high speed motor rotors, SDM’s carbon fiber winding process features the following outstanding characteristics:
(1) Full-chain manufacturing capability: the company possesses a one-stop full-chain manufacturing capability from magnetic materials (soft magnetic + hard magnetic) to motor stator/rotor components, and then to resolver sensor micromotor systems. This means that from magnet selection and rotor design to carbon fiber winding and final testing, everything is done in-house, ensuring extremely high quality control.
(2) Fourth-generation rare-earth permanent magnet R&D: the company continuously invests in the development of fourth-generation rare-earth permanent magnet materials, providing better magnet substrates for carbon fiber winding. The quality of the magnets themselves – including tensile strength, thermal stability, and dimensional accuracy – directly determines the final performance of the carbon fiber winding.
(3) Precision machining capability: the company uses precision machining processes such as CNC cylindrical grinding to ensure the dimensional accuracy of rotors and sleeves. Carbon fiber winding requires extremely high roundness and coaxiality of the rotor substrate; any slight machining error will be amplified at high speed.
(4) Optimized magnet segmentation design: SDM designs the magnet segments with full consideration of the characteristics of carbon fiber winding, rationally segmenting the magnets to ensure sufficient magnetic performance while avoiding the cracking risk caused by excessively large individual magnet areas – this design approach directly addresses the pain points of the winding process.
(5) Synergistic optimization of winding process and materials: through continuous research on resin materials and optimization of the winding process, the company has steadily increased the elastic modulus of the carbon fiber composite, minimizing eddy current losses while ensuring strength, thus fundamentally solving the problem of excessive temperature rise associated with metal sleeves.
Conclusion
The vibration noise, magnet detachment, and high‑speed failure of Magnetic levitation motor rotor are essentially manifestations of the contradiction between centrifugal force and materials, structure, and control systems at high rotational speeds. Carbon fiber winding technology, by providing strong, low‑loss physical restraint, has become the optimal solution to these three major challenges.
SDM, with its 16 years of experience in the magnetic materials industry, full-chain manufacturing capability, fourth-generation rare-earth magnet R&D strength, and refined carbon fiber winding process, is providing increasingly reliable rotor solutions for magnetic bearing / high-speed motors. In the future, with continued advances in carbon fiber materials and winding technologies, the speed limits and reliability of magnetic bearing motors will be pushed even further.
SDM Magnetics is one of the most integrative magnet manufacturers in China. Main products : Permanent magnet,Neodymium magnets,Motor stator and rotor, Sensor resolvert and magnetic assemblies.
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