Hairpin Winding

Hairpin winding is a stator winding made of solid, rectangular copper conductors that are bent into U-shaped plug-in coils, inserted axially into the slots of the lamination stack and then twisted and welded at their free ends. The rectangular conductors fill around 70 to 75 percent of the slot with copper, while a round-wire winding reaches 40 to 50 percent. This cuts copper losses and has made hairpin the standard in electric vehicle traction motors; the Baden-Württemberg technology calendar has listed the process as ready for mass production since 2022. This article explains the three designs, the process chain and the limit set by the skin effect. It also shows which insulating parts every hairpin winding needs and which of them GOBA makes as insulation and molded parts.

U-pin, I-pin and continuous hairpin follow the same principle

The name comes from the hairpin that the bent conductor resembles. All designs use enamelled flat copper wire with edge lengths of roughly 1 to 10 mm, as described in a Volkswagen patent on joining hairpin ends. They differ in how the conductor enters the slot and where it is connected.

The fill factors for U-pin and I-pin come from the trade journal E-Mobility Engineering, the values for the wave winding from the guide by the PEM chair at RWTH Aachen University. Depending on the winding scheme, a U-pin stator needs 3 to 16 different hairpin geometries, and a stator with 60 slots and four conductor layers has more than 120 welds. In practice, welded windings rarely go beyond ten layers and 96 slots. The wave winding reaches up to 16 layers and more than 120 slots, because the wire runs through and needs almost no joints.

The principle is old. The first production vehicle with a hairpin stator was the 2008 Chevrolet Tahoe Hybrid, and a research group around Stefan Goetz points to a knowledge base on bar windings that is more than a hundred years old. The breakthrough came with the automation of bending, inserting and laser welding.

Fill factor puts copper into the slot, the skin effect sets the limit

More copper in the same slot cross section means lower ohmic resistance and therefore less heat at the same current. The flat conductors also rest against the slot wall with their full face and transfer heat to the lamination stack better than a loose bundle of round wires. A design example by Ansys compares two motors in the US06 drive cycle: the hairpin machine reaches 94.23 percent average efficiency, the round-wire machine 93.77 percent.

The solid conductor has a downside. At high frequency, alternating current flows only in a thin outer layer, known as the skin effect. The penetration depth in copper is roughly 66 divided by the square root of the frequency in millimetres, so 3.3 mm at 400 hertz. That is the frequency of a four-pole motor at 12,000 revolutions per minute. Traction motors typically run at 200 to 600 hertz electrical frequency, and the conductors close to the air gap are hit hardest. Designers therefore spread the cross section over more, flatter layers and connect branches in parallel; the Ansys example uses eight layers with eight parallel paths.

Is hairpin worth it in small series?

Usually not. Every hairpin geometry needs bending and twisting tools, and the Baden-Württemberg technology profile lists higher costs at small volumes and lower variant flexibility as barriers. KIT is researching kinematic twisting for batch sizes down to one, which has not yet reached series production. Our assessment: hairpin pays off for traction motors with high power density in large volumes. For industrial motors with many variants, for prototypes and for very high-speed machines, the classic round-wire electric motor winding is often the more economical choice.

Making a hairpin winding takes nine steps

The order varies with the line and the design. For a U-pin stator, the typical process chain looks like this:

1. Straighten the flat wire from the reel so that no residual curvature remains.
2. Strip the wire ends by laser, milling cutter or brush, over a length of about 5 to 10 mm.
3. Cut the wire to length and bend it into a hairpin.
4. Insulate the slots: cut and fold the slot liner paper and push it into each slot.
5. Assemble the hairpins into a basket and insert them axially into the lamination stack.
6. Twist the free ends so that the correct welding partners sit side by side.
7. Weld the ends, usually by laser, and interconnect the phases.
8. Coat the weld zone and impregnate the stator with epoxy resin.
9. Test electrically: resistance, high voltage to ground and between phases, surge voltage.

For the insulation, steps four and five matter most. During insertion, the stiff copper conductor slides along the slot liner, and a crease or a torn edge later becomes a ground fault. The PEM guide therefore lists freedom from cracks and creases and a defined overhang of the slot paper as quality criteria. In the wave winding, the slot opening is only about 0.4 mm wider than the wire. There the slot liner goes in as a U or O shape, the O shape is closed after insertion, and a slot liner cover holds the winding in place. How the lamination stack, winding and connections become the finished component is covered in the article on the hairpin stator.

Every hairpin winding has four insulation levels

The insulation of a hairpin winding consists of four layers, each with its own job. If one of them fails, the motor stops.

• The enamel on the flat wire separates neighbouring conductors in the slot, usually polyamide-imide or polyimide, in the premium segment extruded PEEK. It is the turn insulation of the hairpin winding.
• The slot liner separates the winding from the grounded lamination stack and protects the enamel during insertion.
• The phase insulation separates conductors of different phases in the end winding, where the voltage difference is greatest. It usually sits between the strands as a stamped and folded phase paper.
• The coating of the weld zone and the impregnation cover the stripped ends and fill the voids.

The high fill factor leaves little room for insulation. A calculation example: a slot is 5 mm wide and the slot insulation is 0.25 mm thick. Both sides together take 0.5 mm, a tenth of the slot width that is lost to the copper. That is why thin laminates and tight tolerances are in demand in the hairpin stator. Welding heat adds to this and reaches into the upper part of the slot. There, aramid paper tolerates short temperature peaks better than a pure polyester laminate. How the four levels work together with the slot liner cover and impregnating resin is shown in the article on stator insulation.

800 volts shifts the requirements on the slot liner

The inverter feeds the motor with steep voltage edges. Travelling waves form on the supply line, the terminal voltage rises above the rated value, and according to work at TU Darmstadt the first turn of the coil carries the highest stress. In hairpin windings, conductors of different phases lie side by side in defined positions, and at 800 volts DC link voltage the risk of partial discharge rises there. The enamel alone is then often not enough: the PEM guide notes that PAI or PI enamels only partly meet the mechanical requirements, and extruded PEEK reaches a multiple of the breakdown voltage of an enamel insulation.

Four material families are available for the slot liner:

For 800-volt motors, one maker of slot liner tubes names the special Nomex grades 818 and 864 and Kapton MT+, as spiral-wound NKN tubes only 0.13 and 0.19 mm thick without overlap. Our recommendation: plan a laminate with aramid or polyimide content for 800 volts and rate the thermal class with a margin. DMD saves a few cents per part and suits 400-volt machines with moderate temperature; in a high-voltage drive, a breakdown wipes out that saving at great cost. More on the design in the vehicle is in the article on high-voltage insulation.

What GOBA makes for hairpin windings

GOBA does not wind stators. We make the flat insulating parts that sit in every hairpin winding: slot liners as cut and folded strips, slot liner covers, phase papers as stamped and folded parts, and interlayers in the end winding. We process Nomex, NMN, DMD, Kapton and Mylar for this. In stamping and forming, material thickness ranges from 0.023 to 3.0 mm, and we slit rolls in widths from 5 to 1,600 mm with a width tolerance of plus or minus 0.1 mm. A stamped Nomex part with a 50 mm edge length is held to about plus or minus 0.3 mm under DIN ISO 2768-m, and tighter with a suitable tool.

For prototypes we cut without tooling, and samples are often ready within a few days. In series production we stamp with a tool. We prefer to make slot liners and phase papers from the same material batch so that thickness and impregnation behaviour stay identical across the stator. For a reliable quote we need:

• the slot geometry with slot width, stack length and required overhang,
• the voltage level, 400 or 800 volts, and the thermal class,
• the design, U-pin axial or wave winding radial, and the shape of the slot liner,
• the material or the customer's approval status,
• the batch size and the expected annual volume.

Send us your drawing via our industry page Electromobility and Automotive, and you will receive a technical quote for the insulating parts of your hairpin winding.

FAQ on hairpin winding

How high is the fill factor of a hairpin winding?

A hairpin winding fills around 70 to 75 percent of the slot with copper, a round-wire winding 40 to 50 percent. In the wave winding, also called continuous hairpin, the mechanical slot fill factor reaches up to 90 percent and the electrically effective one up to 70 percent.

What is the difference between hairpin winding and hairpin stator?

The winding refers to the process and the copper in the slot. The hairpin stator is the finished component made of lamination stack, hairpin winding, insulating parts and connections. The structure of the component is described in the article on the hairpin stator.

What is the difference between U-pin, I-pin and continuous hairpin?

The U-pin is bent into a hairpin and welded on one side only. The I-pin is a straight bar connected on both end faces. The continuous hairpin is a continuous wire bent in a meander, which goes radially into open slots as a winding mat and needs almost no welds.

What are the disadvantages of hairpin winding?

The solid conductor suffers from the skin effect at high frequency; at 400 hertz the penetration depth in copper is only 3.3 mm. Add high tooling costs, low variant flexibility and more than a hundred welds per stator, all of which must be process-reliable. In small series, round wire is therefore usually cheaper.

Which slot liner does a hairpin winding need at 800 volts?

We recommend a laminate with aramid or polyimide content, such as NMN, Nomex 410 or NKN, and a thermal class with a margin. DMD suits 400-volt machines with moderate temperature. At 800 volts, the risk of partial discharge between phases rises, and special Nomex and Kapton grades exist for this.

Does GOBA supply insulating parts for hairpin stators?

Yes. GOBA makes slot liners, slot liner covers, phase papers and interlayers from Nomex, NMN, DMD, Kapton and Mylar, as tool-free cuts for prototypes and as stamped parts in series production. GOBA does not make the winding itself.