Winding wire for electric mobility: what a traction motor asks of its copper

Electric two wheelers, three wheelers, buses and cars all come down to a motor, and the motor comes down to its winding. Here is what changes when the wire has to work inside an EV, and how we specify for it.

12 August 2026 6 min read Rama Winding Wires

Why EV motors are harder on wire than industrial motors

An industrial induction motor runs at a fixed frequency for years on end. A traction motor is driven by an inverter switching at ten to twenty kilohertz, is asked for peak torque from standstill dozens of times an hour, and is packed into a housing that leaves no room to spare. Every one of those facts lands on the enamel.

Fast switching puts a steep voltage front across the first few turns of each coil, which is where partial discharge starts. Repeated peak load cycles the winding from ambient to 180 °C and back, so the enamel has to flex without cracking. And the push for power density means slot fill of 70% or more, which is only possible with a wire whose diameter is held to a few microns and whose surface slides through the insertion tooling without scuffing.

Class 200 and 220: choosing the covering

For most traction motors we recommend a dual coat: a polyesterimide base for adhesion and flexibility, and a polyamide-imide overcoat for thermal endurance and chemical resistance. That combination is rated Class 200 under IEC 60317-13 and gives a comfortable margin over the 150 to 180 °C hot spot most designs run at.

Where the motor is oil-cooled, or where the inverter uses silicon carbide devices with very fast edges, we move to a Class 220 build with a thicker overcoat, and can supply corona-resistant grades with a filled polyamide-imide layer for the harshest duty. Grade 2 covering thickness is the usual starting point; Grade 3 is available for hairpin designs that need extra dielectric margin at the bend.

Hairpin versus round-wire windings

Hairpin stators use rectangular conductor formed into U-shapes and welded at the ends. They wind faster on automated lines and pack the slot more tightly, but the enamel has to survive forming around a tight radius and then the heat of welding a few millimetres away. Round-wire distributed windings are more forgiving, easier to prototype and still the norm for two- and three-wheelers and for auxiliaries.

We supply enamelled round copper from SWG 8 to 43 for distributed windings today, and work with customers on rectangular sections against a drawing. Whichever route you take, the specification we need is the same: conductor size, covering grade, thermal class, and the elongation and springback limits your winding machine wants.

Beyond the traction motor

An EV carries far more copper than its main motor. On-board chargers and DC-DC converters wind high-frequency transformers and chokes in fine gauges where a smooth, pinhole-free enamel keeps losses down. Power steering, brake boosters, cooling pumps and HVAC compressors each have their own small motor. Coloured enamel helps the assembly line keep multi-winding transformers straight.

Sourcing all of it from one plant means one set of test certificates, one quality system and one call when something needs to change. That is the way we prefer to work with vehicle programmes: a specification agreed early, samples for winding trials, and a supply agreement that tracks your ramp.

Key takeaways

Inverter drive means steep voltage fronts: specify Class 200 or 220 dual coat enamel, corona-resistant where SiC inverters are used.
Slot fill and automated insertion need tight diameter control and a smooth surface; ask for Grade 2 as a baseline.
Chargers, converters and auxiliaries use fine-gauge and coloured wire; one supplier for the whole vehicle keeps documentation simple.

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