Header image courtesy of Zeiss
Hairpin motors are increasingly used in electric vehicles. They are more efficient, have a higher power density and thermal performance, and are simpler to manufacture than previous types of electric motors.
The advantages brought by hairpin motors are making electric vehicles more competitive not only with previous generations of EVs, but also with combustion engines.
In this article, we will go over the following topics:
- How Do Electric Motors Work?
- What is a Hairpin Motor?
- Hairpin Motor Manufacturing
Before we explore how hairpin motors work, it’s important to understand how EV motors work. Here are EV motor concepts that are crucial to understand why hairpin motors are making a difference.
In hybrid and electric vehicles, electric motors convert the electricity supplied by batteries into motion to move the wheels. Motion is achieved by creating opposing magnetic fields: one in the stator, and one in the rotor. The push and pull force between the stator and the rotor is what makes the rotor rotates.
In the stator, the magnetic field is generated by the electricity running through the copper windings.
But in the rotor, it’s a different story. The two most popular types of electric motors generate the magnetic force differently in the rotor:
- In permanent magnet motors, magnets located in the rotor generate a permanent magnetic field.
- In induction motors, the rotor’s magnetic field is induced. This is done using aluminum or copper bars in the rotor. Since these bars are in the stator’s magnetic field, an electrical current is induced in them, which then generates the rotor’s magnetic field.
Two stators with different types of copper windings. On the left, round wires represent the traditional approach to winding. On the right, the new hairpin technology. Courtesy of Lucid Motors
A hairpin motor is an electric motor whose stator winding is done with hairpins instead of round wires. Hairpins are rectangular and large compared to wires. Their rectangular shape allows for a better fill factor, meaning that the copper winding makes better use of the space (≈20% more efficient).
A rendering of a hairpin (source). Hundreds of hairpins are used in the stators of electric drive systems.
A comparison of hairpin winding and round-wire winding, showing how hairpins achieve a better fill factor (source).
Hairpin winding technology provides a range of benefits:
- Due to the better fill factor, the additional copper generates a stronger magnetic field. This allows a stronger rotation of the rotor and more torque at the wheels.
- A better fill factor means that the stator can be smaller.
- Energy losses when the electric current goes through the copper (called copper losses) are minimal. This is due to the optimized winding path (identical and symmetrical shape of all winding turns).
- The electric motor can use less energy to produce the same amount of power, which helps extend the vehicle's range.
- The symmetrical and solid shape of hairpins makes them easier to wind than round wires, which simplifies assembly a great deal.
- Round wires need stich cords to hold the winding together (a process known as stator lacing). This is not required for hairpins.
- Hairpin motors have a lower risk of failure in the long term. This is because their larger, more solid conductors are less affected by vibrations.
- Hairpins can carry more current than round wires. This better efficiency generates less heat in the winding and in the motor. This simplifies temperature management and improves the reliability and lifespan of the motor.
- Unlike round wires, hairpins can be precisely positioned, leading to a much cleaner design.
Like copper wires, copper hairpins are coated with a protective varnish (called insulating layer) that prevents the electric current from going in all directions when hairpins touch. Hairpin stators typically use one of the following varnishes:
- Polyamide-imides (PAI)
- Polyether ether ketone (PEEK)
- Polyamide-imides with polyimide foil (PAI+FEP)
The following video shows the manufacturing process of a hairpin stator—the defining part of a hairpin motor. Some of the steps you can see include:
- The insulation paper placed in the stator slots to prevent abrasion between hairpins and the steel laminations
- The hairpin production process
- Quality control to ensure tolerances are met
- The laser stripping of the insulating layer
- The hairpin laser welding process
- The impregnation process where part of the stator is covered in a powder coating to protect exposed areas that have been welded
While the traditional round wires used in stators already form a continuous path, hairpins need to be connected to one another to form complete electrical circuit. This is where laser paint stripping and laser welding come in.
The varnish at the end of hairpins needs to be stripped before welding to ensure that it does not penetrate and contaminate the welds.
Mechanical abrasion can be used to remove the insulating layer, but laser stripping is more consistent and precise. Rotating brushes wear away due to friction and require regular maintenance to prevent inconsistent results. They also abrade part of the base metal under the insulating layer, removing copper—an undesirable side effect.
Three connector tabs (left) and a bridge (right) have been laser stripped to remove the epoxy before making the final electrical connections.
After hairpin welding, the side of the stator where the welds are exposed needs protection from oxidation, moisture, and other conductive materials. For this reason, an epoxy powder coating is applied to insulate and protect the areas that have been welded. This process is called impregnation.
To make the final connections, this epoxy needs to be stripped from connector tabs that will be connected to other parts of the electric motor at the assembly stage.
Depending on the stator’s design, different parts may need to be laser stripped and welded. Examples include hairpins, connector tabs, and stator bridges.
You can see here an example of the laser stripping process for thick epoxy coatings.
Hairpin Motors’ Role in the Future of E-Mobility
Hairpin winding is a growing technology that is quickly making its place in the automotive industry to design motors that are more efficient.
Hairpin motors can keep up with larger batteries, use less space, weigh less (and hence reduce the weight of the vehicle). They’re a step in the right direction to make electric vehicles more competitive and affordable to everyone.