Stranded vs Bar Windings: The Physics Behind EV Motor Design

In the world of electric vehicle (EV) engineering, choosing between stranded and bar windings isn’t just about cost — it’s about physics. At Munro & Associates, we specialize in the detailed analysis and teardown of EV components to uncover the trade-offs that drive design decisions. This deep dive explores two prominent electric motor winding technologies: stranded (or wire wound) and bar (or hairpin) windings. Each has distinct manufacturing, cost, and performance implications, but the ultimate driver of choice lies in how these motors behave under alternating current (AC) loads.

Why Not One Motor Design?

After a century of motor evolution, one might assume the industry would have landed on a “best” design. But no single motor type reigns supreme. Instead, different applications demand different trade-offs — from cost and manufacturability to resistance behavior at various operating speeds.

Tesla, Nissan, and several other OEMs historically favored stranded winding designs. Meanwhile, bar or hairpin winding — once a niche solution — has become increasingly common in modern EVs, including Tesla’s latest motors.

Understanding Stranded Winding

Stranded winding, often referred to as wire-wound design, relies on bundles of insulated round wires placed into the stator slots of an electric motor. This design has been around for decades and benefits from established, cost-effective manufacturing equipment and a robust supply chain.

Key Advantages:

• Mature Manufacturing: Tried-and-true production methods with widespread tooling availability.

• Consistent Resistance: The AC resistance is approximately equal to the DC resistance, improving predictability and reducing losses at high speeds.

Disadvantages:

• Lower Slot Fill: Due to the round wires and their insulation, less copper can fit into each slot, raising resistance and material costs.

• Long End Turns: The wire loops formed outside the stator core increase resistance and consume additional copper.

• Acoustic and Torque Ripple Issues: Larger gaps between stator teeth, necessary to insert wires, can increase motor noise and reduce torque smoothness.

In essence, stranded winding motors tend to be more efficient at highway speeds but suffer from lower slot fill efficiency, which can raise energy losses during city driving.

Enter Bar (Hairpin) Winding

Bar winding, or hairpin winding, is a more recent innovation in the context of complex multi-turn motor designs. This method uses square or rectangular conductors bent into a hairpin shape and inserted into the stator. Once in place, the ends are twisted and welded to form continuous loops.

Key Advantages:

• High Copper Density: The square conductors maximize slot fill, significantly reducing resistance.

• Shorter End Turns: With less wasted copper, bar-wound motors deliver better efficiency during low-speed operation.

• Lower Material Cost: Less copper per unit of torque output can be used due to high efficiency.

Disadvantages:

• High Equipment Cost: Bar winding requires specialized, proprietary machinery — a barrier for new entrants or smaller manufacturers.

• Complex Connections: Additional components, such as connection rings, are needed to interface the windings with external systems.

• Increased AC Resistance: A fundamental limitation driven by the physics of AC current known as the “skin effect.”

AC vs DC Resistance: The Skin Effect

To understand why AC resistance becomes a pivotal factor, we must examine the physics.

Electric current always travels with a surrounding magnetic field. In AC systems, the constantly changing current creates changing magnetic fields. These, in turn, induce voltages that oppose the flow of current — a phenomenon known as “back electromotive force” or back EMF.

The result is the skin effect: AC current tends to flow near the surface of a conductor, rather than evenly throughout. At higher frequencies — such as those found in high-speed motor operation — the center of the conductor is underutilized. Resistance rises accordingly.

This effect is minimal in stranded windings, where many small wires offer more surface area and better current distribution. But in bar-wound motors, particularly those with large conductors, the increased AC resistance can negate low-speed efficiency gains at higher speeds.

Matching Motor Design to Drive Cycle

Here’s where the trade-off becomes clear:

• Bar-Wound Motors excel at low speeds — city driving and stop-and-go traffic — where low DC resistance yields high efficiency.

• Stranded-Wound Motors dominate at highway speeds, where the skin effect increases AC resistance in bar windings.

This creates a natural engineering dilemma: Should you optimize for urban driving or long-range cruising?

The Modern Compromise: Smaller Bar Conductors

Recent innovations reveal how OEMs are solving this conundrum. By using smaller bar conductors, engineers can strike a balance. A prime example comes from Tesla, which transitioned to bar-wound designs featuring eight 2mm square bars per slot.

The Benefits:

• Reduced Skin Effect: Smaller bars lower the skin depth impact, delaying the AC resistance rise until well above highway speeds.

• Balanced Slot Fill: While some copper density is sacrificed (compared to larger bars), performance remains superior across a wider range of speeds.

• Scalable Efficiency: The result is a motor that retains the low-speed benefits of bar winding while maintaining efficiency during highway operation.

This hybrid approach has emerged as a sweet spot, allowing manufacturers to cater to both urban and long-distance EV use cases.

What’s Next for EV Motors?

Understanding the physics behind motor resistance isn’t just an academic exercise — it allows engineers to anticipate the industry’s direction. With growing pressure on OEMs to improve efficiency, reduce costs, and optimize vehicle range, expect to see wider adoption of fine-tuned bar-wound designs with smaller conductors.

At Munro & Associates, we continue to analyze these shifts firsthand. Whether it’s benchmarking Tesla’s motor evolution or guiding startups through lean design choices, our expertise bridges manufacturing reality with high-performance innovation.

Key Takeaways

• Stranded windings are best for high-speed efficiency, but suffer from lower slot fill and higher material cost.

• Bar windings offer high low-speed efficiency due to better slot fill, but can suffer AC losses at higher speeds due to the skin effect.

• Smaller bar designs strike a new balance — now favored by OEMs like Tesla — offering efficiency across urban and highway cycles.

• Ultimately, physics drives design: understanding AC resistance and skin depth is essential for modern EV motor development.

Ready to Go Deeper?

Explore more teardown insights, cost breakdowns, and design reviews by visiting Munro & Associates or by subscribing to Munro Live. For advanced consulting or benchmarking services, contact us today — where physics, performance, and precision meet.

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