Technical Review

Flat Copper Wire (Hairpin) Motors: Why Overload Capacity Jumps ~20%

Wuxi DODO EV · October 17, 2026

If you compare motors side by side, the ones wound with flat copper wire (often called hairpin winding) consistently deliver more punch under load than identical-looking motors wound with round wire. The headline number — roughly 20% more overload capacity — is not marketing talk. It comes from three measurable physical changes in how the copper sits in the motor.

1. Higher slot fill = more copper in the same space

Round wires leave wedge-shaped air gaps where three or four circles meet inside a stator slot. A flat rectangular conductor stacks edge-to-edge and fills those corners. The result is a higher copper fill factor — you simply fit more conducting cross-section into the same stator, which lowers winding resistance.

Motor stator being wound with copper magnet wire on the production bench — Fig. 1 — Stator winding on the DODO production bench

Hand holding flat enamelled copper coil turns — Fig. 2 — Flat enamelled copper (hairpin) wire coil — the rectangular cross-section packs the slot more fully

2. Lower resistance = less heat for the same current

Because more copper fits in the slot, winding DC resistance drops for the same magnetic design. Lower resistance means less copper loss (I²R heating) at any given current. That is exactly what buys you extra overload headroom: the motor heats up more slowly, so it can sustain a higher current for longer before hitting the same temperature limit.

Why "overload capacity" matters: overload = how far above rated power the motor can go for hills, headwinds, heavy loads or quick acceleration without burning. A ~20% higher overload threshold is the difference between a bike that surges uphill and one that throttles back.

Close-up of a wound stator with flat copper windings — Fig. 3 — Stator close-up: dense flat-wire windings fill the slots

3. Better heat path out of the winding

Heat has to escape the copper to reach the stator core and then the housing. A rectangular conductor has more flat surface contact with its neighbours and with the slot walls than a round wire touching at thin lines. That better thermal path pulls heat out faster, keeping the copper cooler and further raising the current the winding can safely carry.

Multiple wound stators lined up on a green workbench — Fig. 4 — Wound stator cores in line for assembly

Servo-style motors with wired harnesses on the bench — Fig. 5 — Finished motors with harnesses, ready for test

Motor stator inside a finned housing — Fig. 6 — Stator in its finned housing

Flat wire vs round wire — at a glance

Factor	Round copper wire	Flat copper wire (hairpin)
Slot fill	Lower (round gaps)	Higher (packed edges)
Winding resistance	Higher	Lower
Copper loss (heat) at same current	Higher	Lower
Heat transfer out of winding	Line contact	Surface contact
Overload capacity	Baseline	≈ +20%
Power density	Baseline	Higher

A concrete example: the YM-120 mid-drive hairpin motor

DODO now supplies a permanent-magnet synchronous mid-drive motor wound with flat copper wire, built with three lifting lugs for easy frame mounting. Its datasheet confirms the point: rated efficiency ≥ 93%, peak torque 43 N·m at 6,000 rpm, in a 48–72 V system with class-H insulation and IP67 protection.

YM-120 mid-drive hairpin motor exploded view and specification poster — Fig. 7 — YM-120 permanent-magnet synchronous mid-drive hairpin motor: exploded view and key specifications

Spec	Value
Voltage	48–72 V
Sensor type	Encoder / Hall sensor
Rated power	1,000–3,000 W
Peak speed	6,000 rpm
Peak torque	43 N·m
Rated efficiency	≥ 93%
Transmission	Chain drive
Cooling	Natural air cooling
IP rating	IP67
Insulation class	Class H

Need a motor that pulls harder uphill and overhauls longer?

Tell us your bike weight, wheel size and target top speed — we will match a flat-copper-wire motor, controller and gear ratio for your platform.

Ask DODO Engineering

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