Hairpin Motor

A flat-wire motor (commonly referred to as a hairpin motor) is an electric motor that utilises rectangular (flat cross-section) copper wire windings in place of conventional round copper wire windings. In recent years, spurred by the new energy vehicle (NEV) industry’s escalating demand for higher power density, superior efficiency, and compact packaging, flat-wire motors have progressively superseded traditional round-wire motors to become a mainstream technical route for drive motors in battery electric vehicles (BEVs) and hybrid electric vehicles (HEVs).

Structural Characteristics and Manufacturing Processes

The stator windings of conventional round-wire motors are wound from circular enameled copper wire. Due to the inherent geometric voids between round wires, the slot fill factor remains relatively low. In contrast, flat-wire motors feature rectangular winding cross-sections, distinguished by unique manufacturing processes and structural traits:

Hairpin Structure: Flat-wire windings are typically pre-formed into U-shaped or I-shaped copper segments resembling hairpins, inserted into the stator core slots, and subsequently joined at the open ends via a twist-and-weld process to complete the electrical circuit. 

High Slot Fill Factor: Because rectangular conductors can be arranged tightly, the space utilisation of copper wire within the stator slots (slot fill factor) is significantly elevated from around 65% in conventional round-wire setups to 70%–75% or even higher. 

This architecture substantially shortens the winding end-turn length, reducing copper material usage and resistance losses while improving the overall thermal conductivity of the stator.

Technical Advantages and Performance

Compared with traditional round-wire motors, flat-wire motors deliver notable physical and engineering advantages across several key performance metrics:

Enhanced Efficiency and Power Density: With a higher slot fill factor and lower internal resistance, less thermal loss (copper loss) is generated as current flows through the conductors, resulting in higher overall efficiency under high-load operating conditions. Concurrently, the compact packaging allows for greater power output within the same physical footprint.

Superior Thermal Dissipation: The contact area between adjacent flat wires, as well as between the wires and the stator core, is markedly increased. This enables heat to conduct more readily to the external cooling system, effectively curbing motor temperature rise and bolstering reliability under sustained high-load operations.

Optimised NVH (Noise, Vibration, and Harshness): The inherently higher rigidity of flat-wire windings makes them far less susceptible to displacement or resonance under alternating electromagnetic forces, helping to suppress electromagnetic noise during motor operation.

Challenges and Technological Evolution

Despite their compelling merits, flat-wire motors present specific engineering hurdles in manufacturing and deployment. For instance, flat wires are stiffer and possess larger cross-sectional areas, making them more prone to pronounced "skin effect" and "proximity effect" phenomena at high rotational speeds, which drives up high-frequency AC losses. Furthermore, the dense array of weld joints at the stator ends requires high-precision laser welding, imposing rigorous demands on automotive-grade mass production quality control.

To overcome these bottlenecks, the industry is actively driving technological iterations. For instance, manufacturing has evolved from early discrete hairpin methods to continuous wave winding technology, which provides greater production-line flexibility. Concurrently, high-frequency losses are being suppressed through optimised conductor cross-section designs and multi-strand parallel winding schemes. As the prevailing choice for automakers pursuing high-performance, energy-efficient electric drive units (EDUs), flat-wire motors continue to propel the broader electrification industry toward higher operating voltages and higher rotational speeds.