Establishing PDIV-free motor designs with PEEK-coated magnet wires

To enhance the efficiency of electric vehicles (EV), manufacturers face the conflicting goals of minimizing battery cost while maximizing the energy potential to extend the driving range. At the moment of writing, three key innovations could help OEMs succeed in this mission.

Firstly, they could increase the operating voltage from 400V to 800V and beyond. This will lead to smaller semiconductor areas, lower the cooling effort, and reduce copper losses. Next, they could boost the efficiency of the powertrain by transitioning to Silicon Carbide (SiC) and Gallium Nitride (GaN) inverters that provide better energy transfer for high-voltage applications. Finally, they could opt for an optimized stator winding design with tighter hairpin bending radii and thinner coatings. This has many advantages, including less energy losses, more effective cooling, and an improved performance-to-weight ratio.

However, all of these solutions are destined to increase the electric stresses on the insulation. This is why choosing the right insulation material is key to avoid partial discharge, and the inevitable premature failure of the electrical application.

To highlight this importance, researchers from Bekaert and Ghent University (EELAB, Belgium) joined forces in an experimental analysis of polyetheretherketone (PEEK) and polyamideimide (PAI) coatings based on industry standard IEC 60034-18-41.

In the realm of electric vehicle (EV) manufacturing, the onset of partial discharge inception voltage (PDIV) is commonly perceived as a precursor to imminent failure. This underscores the importance of the international IEC 60034-18-41 standard, which stipulates the conditions that insulation designs should meet in order to remain PDIV-free throughout the entire lifespan. In other words, the IEC standard prescribes the critical voltage a (part of a) system has to be able to withstand based on their positioning and connection—whether it’s phase-to-phase (P/P), phase-to-ground (P/G), or turn-to-turn (T/T). Additionally, the standard introduced a formula to help determine the critical test voltage, including the overshoot and enhancement factors related to, for example, aging and temperature.

Even though the IEC 60034-18-41 guidelines are crucial to designing and enhancing battery electric vehicles (BEVs), one should keep in mind that these standards apply to all types of electrical motors. As a long-standing supplier to the automotive industry, Bekaert calls for incorporating PEEK as a high-quality insulation material, and taking other specific stresses (such as ambient stresses) into consideration.

According to the IEC60034-18-41 standard, the voltage overshoot factor (OF) depends on the cable length and rise time: for a single pulse with low rise time in combination with long cables, an overshoot factor of up to 2 is possible. Should double pulsing occur, this number can even double.

However, in automotive applications, the integration of converters into the e-machine allows for exceptionally short cable lengths: often as minimal as 5mm, as highlighted by OEM Lucid. This compact configuration significantly mitigates voltage overshoot, making double pulsing a concern only at frequencies that are far beyond typical automotive contexts. Consequently, a conservative overshoot factor of 1.2 is deemed sufficient for electric vehicles. The choice of insulation material choice does not play a determining role in the selection of this parameter.

In assessing electrical systems for automotive applications, the IEC 60034-18-41 standard provides guidelines on calculating the critical test voltage using enamel-coated wires, yet this might not fully apply to alternative materials like PEEK insulation. To bridge this gap, targeted experiments were conducted by subjecting Bekaert’s PEEK-coated magnet wire (Ampact™) to conditions that mimic automotive environments. We were particularly interested in learning more about the temperature and frequency effects. 

As EVs shift towards 800V beyond, OEMs are faced with increasing electrical stresses on the insulation. This means that the choice of insulation material will have a direct impact on the system-wide efficiency. While the existing EC 60034-18-41 standard prescribes the overshoots and enhancement factors that must be considered to be able to run PDIV-free, our experimental analysis found an update is required to unlock the full potential of PEEK as a high-quality coating material for magnet wires.

As demonstrated in the table below, the most important findings include:

• The calculated electrical stress of a PEEK-insulated system that must avoid partial discharge, is lower compared to a PAI or enamel coating.
• Contrary to popular opinion, the increased relative permittivity of PEEK at higher temperatures does not have a negative effect on PDIV.
• The improved aging behavior of PEEK coating insulation requires less over-dimensioning of the insulation.

Finally, it is worth noting that PEEK's extrusion coating process supports thicker insulation layers of up to 300 µm. This allows for higher voltage levels of, for example, up to 1200 V, presenting an innovative alternative for traditional enamel coatings in high-voltage applications.