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Efficient Sleeves for Electric Motors using AFP

Updated: Oct 3, 2023

The electrification of the automotive industry is not just about batteries and charging stations; it's also about the intricate details that make these vehicles run efficiently. One such detail, often overlooked, is the motor sleeve. As we transition to a world dominated by electric vehicles, understanding the nuances of motor design and efficiency becomes paramount. In this comprehensive resource guide, we've curated a selection of research papers, articles, and studies that shed light on the advancements in motor sleeve technology, particularly the use of carbon fiber.

We recently concluded the webinar (Get access to the recording here) and here is the list of articles we referred to in preparation for the webinar.
  • Increased Efficiency: The use of a lighter rotor, particularly one wrapped in carbon fiber, requires less energy to spin. This means electric motors can operate more efficiently, converting electrical energy into mechanical energy with reduced energy input.

  • Improved Performance: Carbon fiber-wrapped rotors can enhance the acceleration, top speed, and handling of electric vehicles. This is especially significant for sports cars and high-performance electric vehicles.

  • Longer Battery Life: Vehicles equipped with lighter rotors consume less energy for movement, leading to an extended range and prolonged battery life, which is crucial for electric vehicles.

  • Rotor Design for High-Speed Rotation: For high-speed applications, the rotor's hub bending stiffness is crucial as it affects the natural frequencies or modes of vibration. Critical frequencies within the range of operation can lead to bearing damage or catastrophic shaft failure. Thus, rotors are designed to be both stiff and lightweight for high-speed applications. Additionally, the retaining sleeve is designed to resist hoop stresses and prevent "lift-off" at high rotational speeds.

  • Composite Sleeves: Composite sleeves, especially those with carbon fiber filaments, are commonly used due to their lightweight and high-strength properties. These sleeves not only increase the rotor's natural frequencies but also minimize inertial loading, which produces stresses in the material. Composite rotors have been shown to fail in a less destructive manner compared to metallic rotors.

  • Rotor-Tip-Speed (RTS) and Figure-of-Merit (FOM): These metrics are used to evaluate and rank high-speed motors. The data indicates that the range of FOM is significant and dependent on system design requirements. The sleeving technology can achieve a high FOM, indicating its efficiency in high-speed motor designs.

  • Advantages of High-Speed Permanent Magnet Motors: High-speed permanent magnet motors offer benefits such as high power density and efficiency. However, using non-laminated steel for the rotor can lead to significant losses and potential irreversible demagnetization of the permanent magnets. Research suggests that replacing the high-conductivity non-laminated steel sleeve with low-conductivity carbon fiber composite materials can effectively reduce rotor losses.

  • Carbon Fiber Production from PAN Fiber: The primary source of carbon fiber is polyacrylonitrile (PAN) fiber. The transformation involves three main steps: pre-oxidation of PAN fiber, carbonization in an inert gas environment, and a subsequent graphitization process to achieve higher modulus fibers.

  • Rotor Manufacturing Methods: The conventional methods for manufacturing carbon fiber rotors include the press-fitting method and the tension-tension winding method. The press-fitting method involves winding fiber bundles layer by layer on a mold's surface, curing, and then pressing the permanent magnet into the sleeve. The tension-tension winding method doesn't require a mold; instead, the fiber is directly wound around the permanent magnet's surface with sufficient initial tension.

Motor Overwind

  • Motor Overwind Definition: An overwind refers to the high-tension fiber placement of thermoplastic composites around a magnet, motor, or generator, effectively creating a type of motor can. The wrapping with advanced composite materials enhances strength and performance. This process offers increased mechanical and electromagnetic performance compared to alternative materials and manufacturing methods, such as metal casings that lead to electrical losses and wet-wound thermoset composites.

  • Advantages of AFP and Thermoplastic Composites: Automated fiber placement (AFP) of thermoplastic composites provides unique benefits to motor sleeves and magnet overwinds, leading to improved motor efficiencies. Some of these advantages include high-tension fiber placement, the use of mid to high-modulus composite tape that approaches or surpasses the modulus of steel, and the ability of thermoplastic composite material to absorb minimal water, reducing volumetric swell in hot-wet environments.

  • Industrial Solutions and Composite Hybrid Pipe: Jet engine generators, subsea motors, and other industrial magnet overwinds can achieve cost savings from improved efficiencies over metal magnet casings. Additionally, hybrid pipes, which are a combination of steel and continuous-fiber composites, offer benefits like lighter tensile loads in deep water environments, added hoop strength, resistance to gases, improved fatigue properties, and reduced corrosion concerns.

  • Centrifugal Force and Magnetic Rotors: Magnetic rotors, used in applications like motors, generators, alternators, and Eddy Current Separators, spin at high speeds. A primary design consideration is ensuring that the magnets remain attached to the magnet carrier. The centrifugal force experienced by rotating magnets is crucial, and it's proportional to the rotational speed squared, the mass, and the effective diameter that the mass spins around.

  • Bunting Solutions: Bunting engineers have extensive experience in designing and building high-speed magnetic rotors. They offer various engineered solutions, including metallic sleeves, direct-wound carbon fiber sleeves, pressed-on carbon fiber sleeves, and heat-shrunk wound polyester. The focus is on carbon fiber options, with metallic sleeves and heat-shrunk wound polyester reviewed separately. Direct wound carbon fiber has a maximum pre-stress of 450Mpa, while pressed-on carbon fiber can achieve up to 1200MPa in practice.

  • Pressed-on Carbon Fibre: This method allows for a realistic maximum pre-stress of 1200MPa. The additional pre-stress enables design changes, such as increasing the magnet's mass or diameter. However, the most significant gains come from increasing the running speed. Optimal magnetic rotor designs are small, fast, and can operate at high temperatures. Using laminated magnets to reduce eddy current losses at elevated temperatures, a precision-ground magnet surface, and a pre-heat cured pressed-on carbon fiber sleeve, the motor can run at very high speeds, maximizing performance.

  • Performance of Composite Material Sleeves: The fiber-reinforced composite material sleeve has significant performance advantages, especially in the application of surface-mounted high-speed permanent magnet motors. However, designing and preparing the composite material sleeve is a complex, multi-level process. The stress design, prestress preparation, and testing of the sleeve are crucial aspects.

  • Challenges and Limitations: Current test experiments for the performance of composite material sleeves are limited. There's a lack of real-time stress, damage, and evolution testing of composite sleeves. Additionally, the performance of the sleeve under the influence of fatigue, vibration, and other complex loads hasn't been extensively studied.

  • Future Prospects: For the development of high-performance motors, composite sleeves need continuous improvement. The application of quick-curing thermosetting composite or in situ forming thermoplastic composite can enhance the effect of sleeve pre-pressing. Optimizing the design and manufacturing mode of the sleeve is essential for improving its forming efficiency. Future research should focus on the quality testing of the forming sleeve and the detection throughout its life cycle to ensure the rotor's safe and stable operation.

  • Trelleborg’s Thermoplastic Composite Rotor Sleeve: Trelleborg has developed a thermoplastic composite rotor sleeve design that is expected to benefit electric vehicle motors. This design uses automated fiber placement (AFP) with in-situ consolidation to overwind carbon fiber/thermoplastic composites directly onto metal structures. The pretensioning of the composite material ensures its stability even in applications with high rotational speeds, such as permanent magnet motors.

  • Advantages of Permanent Magnet Motors (PMMs): PMMs are inherently more efficient than induction motors. This is because induction rotors consume electric power to generate their magnetic field, while PMM rotors do not. PMMs offer high power density, torque density, and low noise generation. They can provide the targeted power and torque for an EV with a smaller and lighter weight motor, supporting the critical objectives of higher performance and greater driving range in EVs.

  • Manufacturing Process and Design Freedom: Trelleborg’s manufacturing process offers new design freedom, allowing for structurally stable thin overwinds and thick overwinds without fiber waviness or buckling. The process also enables a one-step manufacture of the composite PMM sleeves. The thermoplastic composite sleeve is fabricated from a continuous fiber-reinforced prepreg that is overwound directly onto the PMM rotor. This process can produce sleeves suitable for a wide range of PMM applications, from small diameters to large ones designed for commercial aircraft propulsion.

What's Next!

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At Addcomposites, we are dedicated to revolutionizing composite manufacturing. Our AFP systems and comprehensive support services are waiting for you to harness. So, don't wait – get started on your journey to the future of manufacturing today!


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