Windings and copper losses
Where I²R losses come from, and the winding tricks used to minimise them.
- Explain windings and copper losses at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive unit
Overview
Every ampere flowing through a real conductor generates heat, and in a high-power traction motor that heat is one of the central engineering problems.
In depth
Copper loss rises with the square of current, which is why doubling torque (roughly doubling current) quadruples resistive heating in the windings.
Power Loss Calculator
Copper, iron and switching losses in a drive unit.
P_loss = I²R + k_iron × f + switching lossesCopper loss scales with the square of current and dominates at high torque. Iron loss scales roughly with frequency and dominates at high speed. Inverter switching loss scales with switching frequency and device energy.
Example: 300 A through 12 mΩ gives 1,080 W of copper loss alone.
- Halving current quarters copper loss — one reason 800 V systems are efficient.
- Silicon carbide devices cut switching loss by roughly half versus IGBTs.
- Every watt of loss is a watt the cooling system must remove.
Hairpin windings
Modern high-power-density traction motors increasingly use hairpin windings: rectangular copper bars bent into a U-shape, inserted into the slots and welded at the ends. Compared to round wire, hairpins achieve a much higher slot fill factor, meaning more copper cross-section for the same slot size — directly cutting resistance and therefore copper loss.
Key takeaways
- Copper loss scales with the square of current, dominating at high torque.
- Hairpin windings raise slot fill and cut resistance versus round wire.
- Skin and proximity effects raise effective AC resistance above the simple DC value.
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