Why Are High-Efficiency Motors Increasingly Using Flat Wire Motor Stator Winding? Compared with traditional round wire windings, flat copper wire windings can significantly increase the slot fill factor of the motor, thereby improving its overall efficiency. In this article, we’ll explore four types of motor flat wire winding in detail: Hair-pin, I-pin, X-pin, and S-winding.
HairPin Winding Technology
Hair-pin winding is a technology that replaces traditional fine round wire with flat copper hair-pin enamel wires in the stator windings of drive motors. Since the shape of the stator coils resembles a hairpin, this type is commonly known as a “hairpin motor” or a “flat wire motor.”
During manufacturing, the conductor is pre-formed into an “H” shape and inserted into prefabricated stator slots. Then the ends of the winding are twisted and welded together to form a complete winding.
Hair-pin windings use hot fusion welding. The welded end forms a solder ball, with a heat-affected zone of about 8–10 mm². The welding area is roughly 110% of the cross-sectional area, and the pull strength generally ranges between 800–1000 N.
To ensure insulation and structural strength, hair-pin windings are treated with impregnated varnish combined with coating processes.
Advantages
High slot fill rate
Hair-pin windings significantly reduce the space needed for assembly and inter-conductor gaps, achieving around 70% slot fill factor.
Better thermal performance
Flat wires have more contact area with the core and adjacent conductors, improving heat dissipation. Since wire resistance is inversely proportional to cross-sectional area, flat wires reduce DC copper loss, lower temperature rise, and enhance motor efficiency.
Lower operating noise
Flat wires have higher stiffness than round wires, which helps reduce both electromagnetic and mechanical noise during operation.
Disadvantages
Difficult H-shape forming process
Insulation treatment at the winding ends is complex, and gaps between adjacent windings must be minimized. Otherwise, small air gaps may lead to turn-to-turn short circuits or internal hidden defects.
Turn insulation damage risks
To avoid this, insulating paper is inserted between turns, which increases processing complexity and cost.
Despite these challenges, Hair-pin winding is widely adopted both in China and abroad. For example, the rear-wheel drive Tesla Model 3/Y (2022 version) uses 10 layer Hair-pin flat wire motors.
I-Pin Winding
I-pin winding, shaped like the letter “I,” is inserted directly and welded on both sides. It is also referred to as an I-type or dual-chamber winding.
The principle involves alternately inserting two sets of windings in opposite directions into the stator slots, filling the copper wires tightly. The upper and lower windings are arranged in opposite directions and interlace within the stator slots.
Advantages
I-pin winding doesn’t require preforming, and the copper wires are shorter, resulting in lower losses.
It can achieve a slot fill factor of around 74%.
Because of the alternating arrangement, it reduces hot spots and improves heat dissipation.
It enhances the uniformity of the winding and improves current balance across the three motor phases.
Disadvantages
The welding process is more complex.
The end height is larger, space utilization is lower.
X-Pin Winding
X-pin winding is similar to Hair-pin but forms an X-shaped interlaced texture. There is no flat-cutting process. Consistency in pin angles is critical, and the paint removal length is only about 5mm. Mechanical stripping may not ensure tight bonding, leading to welding issues, so laser stripping is the only viable method.
Afterward, the wires are welded to complete the winding. X-pin uses low-temperature welding without forming solder balls. The welding must penetrate downward, with a smaller heat-affected area. The welded area is approximately 80% of the cross-section, and pull strength ranges from 600–800 N.
While impregnated varnish and coating are used for insulation, due to lower welding strength compared to Hair-pin, it’s recommended to use higher-strength end encapsulation for insulation.
Advantages
Compared to Hair-pin, X-pin reduces copper loss by more than 20 mm.
Laser stripping from both sides is sufficient.
Low-temperature welding avoids insulation damage while maintaining high weld strength.
Disadvantages
Higher precision is required.
Wire feeding accuracy must be very high, which makes implementation difficult.
More slots mean narrower pitch and higher creepage risk.
For 800V motors, the creepage distance must be at least 10mm according to cleanliness standards, which is hard to meet with paint film alone.
It is prone to issues like wire breakage and distortion during production.
If X-pin winding can be successfully developed, could it challenge the dominance of Hair-pin, I-pin, and S-winding types? It has the potential to become the leader in flat wire motor windings. United Electronics introduced X-pin in 2022.
S-Winding
S-winding(W-pin winding) arranges adjacent coils in sequence, with each coil bent in an “S” shape. The coils are inserted into the stator using a winding machine. No welding is needed at the ends after forming.
Advantages
No welding at the coil ends results in a smaller end-space dimension.
Excellent NVH (Noise, Vibration, Harshness) performance, minimizing torque ripple and enabling smoother operation.
Superior cooling capability.
The end axial length is reduced by 15%, making the structure more compact and achieving a power density of up to 10 kW/kg.
Disadvantages
It is the most difficult to manufacture.
It requires highly precise winding equipment and advanced techniques.
More copper wire is needed, increasing cost.
Low electromagnetic flexibility limits its application.
BorgWarner developed the S-winding coil forming technology in 2018. This method suits P2 hybrid vehicles with limited installation space. Compared to integral windings, S-winding stators are about 30% shorter, while torque density improves by over 50%.
In June 2021, Changan Auto launched its Blue Whale iDD hybrid system, which uses S-winding motor windings.
Compared to other common processes, this method does not require welding, simplifies assembly, and ensures robust manufacturing.
Would this make it the best choice when performance requirements are high, but space is limited?
Compared Type of Winding For Flat Wire Motor
Each winding method impacts equipment and processing differently. The general process steps include inserting, forming, twisting, flattening, and welding. Below is a comparison based on end size, copper loss, and manufacturing difficulty:
| I-pin | Hair-pin | X-pin | S-winding | |
| End Winding Size | Long | Medium | Short | Short |
| Copper Loss | High | Medium | Low | Low |
| Manufacturing Process Difficulty | Low | Medium | High | High |
With the advancement of China’s “dual-carbon” strategy, green consumption is becoming a major trend, and the automotive industry is shifting toward green, low-carbon development. As motor technologies and production lines continue to evolve, flat wire motors are poised to lead the way in the future of new energy vehicle drive systems.
Flat Wire Motor Stator Winding-Choose Lamnow
Lamnow is a pioneer in electric motor iron core manufacturing, committed to delivering high-performance flat wire stator winding solutions to global customers. Our cutting-edge process technology enhances power density, optimizes heat dissipation, and reduces eddy current losses, empowering electric vehicles and premium industrial equipment with greater efficiency.
We also provide stator winding services for BLDC motors, permanent magnet synchronous motors, and servo motors. With automated production lines and a rigorous quality control system, Lamnow offers fast response, custom design, and industry-leading delivery capabilities to give your motor core a competitive edge.
Contact us now for professional winding support and efficient manufacturing services!
