Hairpin Winding — What Is Hairpin Winding on a Stator? | Reenives Hairpin Winding Supplier

What Is Hairpin Winding on a Stator?

Hairpin winding — also called bar winding, rectangular wire winding, or form-wound winding — is a stator winding technique in which pre-formed rectangular copper conductors, bent into a U or "hairpin" shape, are inserted into the stator slots from one end. The straight legs of each hairpin fill the slot, and the open ends at the opposite side are twisted and welded together to form the complete winding circuit.

The key difference from conventional round-wire winding:

• Round wire — many thin strands wound turn by turn through the slots, leaving irregular air gaps
• Hairpin winding — large rectangular bars, each occupying a defined slot area with minimal air gap

Because rectangular conductors pack more efficiently into a rectangular slot, hairpin winding achieves a much higher slot fill factor — typically 70–80% compared with roughly 40–50% for round wire. This directly translates into:

• Lower DC resistance — more copper cross-section for the same slot area
• Higher power density — more torque or output from the same stator envelope
• Better thermal performance — copper losses are spread over more conductor area and heat path to the lamination is improved
• Improved automation — hairpins can be inserted and welded by machine, reducing labor variance
• Better repeatability — consistent conductor placement improves batch-to-batch motor performance

The trade-offs are equally real:

• AC losses at high frequency — large conductors suffer from skin and proximity effects, so hairpin winding is less attractive at very high electrical frequencies unless strand subdivision or careful bar design is used
• Strict slot geometry requirements — the stator slot must be rectangular with sharp, clean corners and precise width
• Welding complexity — end-to-end welding of hairpins requires controlled processes and inspection
• Insulation handling — the hairpin insulation must survive bending and insertion without damage

Put simply: hairpin winding is a way of replacing many round wires with a few large rectangular bars to get more copper into the same stator slot — and it only works well when the stator core is designed for it.

Hairpin winding stator core — rectangular slot geometry with copper hairpin conductors, manufactured by Reenives with imported high-speed stamping lines.

Types of Hairpin Winding

Hairpin winding configurations vary by hairpin shape, layer count, and connection method. The main types are:

• Single-Layer Hairpin Winding — One hairpin per slot, simpler insertion and welding, lower slot fill than multi-layer designs. Used in smaller motors and cost-sensitive applications.
• Double-Layer Hairpin Winding — Two hairpins per slot (four conductor legs), the most common configuration in automotive traction motors. Balances slot fill, AC loss, and manufacturability.
• Four-Layer Hairpin Winding — Four hairpins per slot, higher slot fill and lower resistance, at the cost of more complex welding and higher AC losses.
• Continuous Hairpin (Wave) Winding — Hairpins are formed as continuous wave shapes rather than separate U-bars, reducing the number of weld joints.
• I-Pin / Bar Winding — Individual straight bars inserted and welded at both ends, a variant used where hairpin bending is impractical.
• Segmented Hairpin Winding — Hairpins inserted into segmented stator teeth, then assembled into a full ring. Improves winding access and slot fill.
• Round-Wire Winding (for comparison) — Conventional wound copper wire, lower slot fill but simpler tooling and lower AC loss at high frequency.

For most drone, eVTOL, and high-performance BLDC motor designs, double-layer hairpin winding is the starting point — it offers a strong balance of slot fill, thermal performance, and production feasibility.

Key Specifications & Data

Below are typical specifications for stator cores intended for hairpin winding, alongside conventional round-wire stators for comparison. Custom values are available upon request.

Data source: Reenives internal testing, industry hairpin winding design references, and supplier material datasheets, 2024–2025. Actual values may vary by design.

Applications & Why Hairpin Winding Is Used

Hairpin winding is chosen where slot fill, power density, and thermal performance matter more than tooling simplicity. Here is where it delivers real value:

• High-Power UAV & eVTOL Motors — Larger UAV and eVTOL propulsion motors benefit from higher slot fill, which means more torque from the same motor envelope and better heat rejection during climb and hover.
• Heavy-Lift Drone Motors — Agricultural and industrial drones carrying heavy payloads need continuous torque without thermal throttling. Hairpin winding improves copper utilization and reduces I²R heating.
• Electric Vehicle Traction Motors — The largest current application for hairpin winding, driven by power density and automated manufacturing.
• High-Efficiency Generators — UAV onboard generators and portable power units benefit from lower resistance and improved thermal margin.
• Industrial Servo & Robotics Motors — Where torque density and repeatability are critical.
• Gimbal & Stabilization Motors — Where smooth torque and precise control matter, though AC loss considerations apply.

Reenives stator cores are used in hairpin winding projects across drone, eVTOL, industrial, and automotive applications — supplying the lamination stack and slot geometry that hairpin insertion depends on.

Hairpin winding stator cores from Reenives are used in high-power UAV, eVTOL, and BLDC motor projects where slot fill and thermal performance are critical.

How to Select the Right Stator Core for Hairpin Winding

Hairpin winding places specific demands on the stator core. If you are evaluating a stator core for hairpin insertion, work through these steps:

1. Confirm the Winding Architecture — Single-layer, double-layer, or four-layer? This determines the number of conductor legs per slot and the required slot depth and width.
2. Define Slot Geometry for the Hairpin — The slot must be rectangular with controlled corner radius, sized to the hairpin cross-section plus insulation and insertion clearance. This is the most critical stator core specification.
3. Tighten Slot Width Tolerance — Hairpin insertion demands tighter slot width tolerance than round wire, typically ±0.03 mm to ±0.05 mm. Loose slots cause vibration and poor thermal contact; tight slots damage insulation during insertion.
4. Control Burr Height in the Slot Area — Burrs on slot edges will cut hairpin insulation during insertion. Specify ≤ 0.02–0.03 mm burr height and verify with incoming inspection.
5. Select Lamination Thickness — For high-frequency operation, 0.20 mm laminations reduce eddy current loss. For lower-frequency, higher-torque designs, 0.35 mm may be acceptable.
6. Specify Epoxy Coating for Insertion Abrasion — The lamination coating must withstand mechanical abrasion from hairpin insertion. H-Class 180°C electrostatic epoxy coating is recommended.
7. Plan for End-Winding and Welding Access — Stator core stack length and end-plate design must leave room for hairpin bending and welding tooling.
8. Request Samples & Test — Reenives provides free stator core samples for qualified hairpin projects. Validate slot fit, insertion force, and coating integrity before committing to production.

FAQ — Hairpin Winding & Stator Core Parameters

Summary — Hairpin Winding Starts with the Right Stator Core