The Use of Thermoplastic Composites in Electric Motors

Thermoplastic composite sleeves in electric motors

Why material selection becomes a key design decision

Electric motor design is no longer just an electromagnetic engineering problem. With speeds exceeding 20,000 RPM in traction motors and high-speed industrial drives, as well as continuously increasing power densities, the materials in rotor and stator sleeves have become a critical performance factor.

The question is no longer whether composite sleeves should be used – the industry has long since settled this discussion. The real question is: Which composite material, manufactured with which process?

Metals generate eddy current losses and unnecessarily increase the rotating mass. Thermosetting composites require autoclave processes, generate non-recyclable waste, and are difficult to apply directly to permanent magnets without risking demagnetization. Continuous fiber reinforced thermoplastic composites (CFR-TP), manufactured using laser-AFP (Laser-assisted Automated Fiber Placement with in-situ consolidation), address all three limitations simultaneously – and in an industrially scalable manner.

Rotor sleeves: Magnet retention under extreme load

In motors with surface-mounted permanent magnets (SPM), the primary task of the rotor sleeve is magnet retention. At high speeds, the centrifugal forces on the magnets can reach multiples of the acceleration due to gravity. The sleeve must maintain sufficient pressure preload on the magnets across the entire operating temperature range – from cold start to continuous operation at full load.

Carbon fiber reinforced thermoplastic sleeves meet this challenge with a combination of properties that no single alternative can fully represent:

• High circumferential strength through circumferentially wound carbon fiber tapes securely holds the magnets even under extreme centrifugal loads

• Almost no electrical conductivity eliminates eddy current losses that occur with metallic retention rings – a measurable efficiency gain at high switching frequencies

• Thin-walled design (realizable with laser AFP down to the submillimeter range) minimizes the effective air gap between rotor and stator and directly improves the magnetic flux density and thus the motor efficiency

• Controlled winding tension (typically 50–70 % of the fiber tensile strength) generates the required compressive preload on the magnets already during the winding process

Crucially: Laser AFP enables the direct winding on the populated rotor stack at room temperature. This eliminates the thermal stress that permanent magnets are exposed to with autoclave-cured thermoset sleeves – a process risk that can lead to partial demagnetization and reduced motor performance.

Stator sleeves: Thermal management without compromise

In actively cooled electric motors – the dominant architecture in EV traction drives and high-performance industrial drives – the stator sleeve serves a different but equally demanding function: It seals the liquid cooling circuit from the rotor space and must remain permanently shape-stable under cyclical thermal and mechanical stress.

This combination of requirements is demanding. The sleeve must:

• Be sufficiently thermally conductive to ensure efficient heat transfer from the stator windings to the coolant

• Be electrically insulating to prevent short circuits between stator laminations and coolant or housing

• Be dimensionally stable under thermal cycles, with a low coefficient of thermal expansion (CTE) for permanently secure press connections

• Be corrosion resistant to water-glycol coolants throughout the entire motor lifespan

CFR-TP composites meet – with appropriate fiber orientation and matrix selection – all four requirements. By adjusting the layer structure, for example by combining circumferentially wound carbon fiber layers with angled layers, the CTE of the sleeve can be specifically matched to adjacent metal components and thermally induced interfacial stresses can be reduced. The thermoplastic matrix (PEEK, PPS, or PA12, depending on operating temperature) offers inherent chemical resistance – without additional surface coatings.

Why Laser-AFP is the right process for motor sleeves

Direct answer for engineers: Laser-AFP is an in-situ consolidation process where a focused laser simultaneously heats the thermoplastic tape and the substrate at the point of contact and bonds each layer immediately upon deposition. No autoclave, no post-curing, no separate consolidation step. The result is a fully consolidated, pore-free component – directly from the mandrel or directly on the rotor.

For motor sleeve applications, Laser-AFP offers several process advantages that other manufacturing routes cannot replicate:

Seven performance advantages of CFR-TP composite sleeves

1. High specific strength — Carbon/thermoplastic sleeves achieve tensile strengths of over 2,000 MPa in the fiber direction – at a fraction of the mass of steel retention rings

2. Electrical insulation — No eddy current losses; compatible with high-frequency switching in inverter-fed motors

3. Thermal stability — High-performance thermoplastic matrices (e.g., PEEK: Tg ~143 °C, continuous use up to ~250 °C) retain their properties over the entire motor operating range

4. Thin, precise wall thicknesses — Thinner sleeves reduce the effective air gap and increase power density

5. Automated, scalable manufacturing — Laser-AFP delivers consistent part quality without autoclave infrastructure

6. Direct winding on permanent magnets — No demagnetization risk; simplified assembly without press-fit complexity

7. Recyclability at end of life — Thermoplastic matrix can be remelted and reprocessed; no chemical treatment required

From prototype to series: Alformet's role in the value chain

Alformet manufactures continuous fiber reinforced thermoplastic composite pipes and profiles using laser-AFP at its location in Germany. As a contract manufacturer – not as a material supplier or design office – Alformet takes on the task of producing validated sleeve designs according to specifications, in the required volume, and with the process consistency necessary for series production.

With many years of experience in the development and contract manufacturing of CFR TP, Alformet has direct manufacturing knowledge that is not available to most thermoplastic composite manufacturers. This is particularly relevant for motor sleeve applications: process parameters such as laser power, winding speed, tape tension, and consolidation pressure directly determine the consolidation quality, porosity, and residual stress state of the finished component.

For electric motor programs transitioning from prototype validation to series industrialization, Alformet offers a seamless path from first sample to series delivery – without the handover risks between development supplier and series supplier.

Conclusion

Thermoplastic composite sleeves are a technically and commercially mature solution for magnetic retention and stator cooling in high-performance electric motors. The combination of high circumferential strength, electrical insulation, dimensional precision, and in-situ consolidation using laser AFP addresses the fundamental weaknesses of both metallic and thermosetting alternatives.

For motor engineers and program managers evaluating materials for next-generation drives, the critical question is no longer whether CFR-TP sleeves work – but whether their own supply chain can reliably manufacture them within the required tolerances and quantities.

If you are developing a rotor or stator sleeve application and want to understand what laser-AFP manufactured CFR-TP sleeves can do for your specific motor architecture, please contact the Alformet team or download our complete white paper on thermoplastic composites in electric motors.