Electric Motor

Traction motors turn electrical power into wheel torque. Learn PMSM, induction and wound-rotor synchronous motors, torque–speed curves (constant torque and constant power regions), base speed, efficiency maps, gear ratio selection, cooling and rare-earth-free designs.

EV motors deliver maximum torque from standstill, run to 15 000–20 000 rpm and exceed 95 % efficiency at their best points. A single-speed reducer is enough because the motor covers a wide speed range.

Choosing motor type and gear ratio sets acceleration, top speed and efficiency.

An analogy

An EV motor is like a sprinter who is strongest off the blocks (constant torque) and then holds a steady power output as speed rises (constant power).

Motor types

Permanent-magnet synchronous (PMSM/IPM): highest efficiency and power density — most EVs. Induction: no magnets, robust, cheap, lower efficiency at light load — early Tesla, secondary axles. Wound-rotor synchronous (EESM): rare-earth-free, adjustable field. Switched reluctance and axial-flux motors are emerging.

Torque–speed curve

Below base speed the inverter's current limit sets constant maximum torque. Above base speed, voltage is limited, so field weakening keeps roughly constant power while torque falls ∝ 1/ω — like an ideal transmission.

Efficiency maps and gearing

Efficiency varies with torque and speed (peak ≈ 95–97 %). The single reduction ratio (≈ 8–12:1) is chosen so max motor speed gives top speed and the motor spends most time in efficient regions. Some performance EVs use 2-speed gearboxes.

Thermal limits and design trends

Peak ratings last seconds to tens of seconds; continuous power is limited by winding and magnet temperature. Oil spray/jacket cooling, hairpin windings, SiC inverters, 800 V systems and integrated e-axles (motor + inverter + gearbox) increase power density. Magnet-free designs reduce reliance on rare earths.

Formulas

• Power: P = T · ω
• Base speed: ω_b = P_max / T_max
• Wheel torque & force: T_w = T · i · η, F = T_w / r
• Gear ratio for top speed: i = n_max / n_wheel(v_max)
• Copper loss: P_cu = 3 I² R_ph

Worked example: Base speed

Given: P_max = 150 kW, T_max = 310 N·m

Find: Base speed.

• ω_b = 150 000/310 = 484 rad/s = 4620 rpm

Answer: ≈ 4620 rpm

Worked example: Reduction ratio and launch force

Given: Max motor speed 16 000 rpm, top speed 180 km/h; Tyre radius 0.34 m, η = 0.97

Find: Ratio, wheel torque and tractive force.

• Wheel speed at 180 km/h = 50/0.34 = 147 rad/s = 1404 rpm
• i = 16 000/1404 = 11.4
• T_w = 310 × 11.4 × 0.97 = 3430 N·m → F = 10.1 kN

Answer: i ≈ 11.4, F ≈ 10 kN

Common mistakes

• Wrong: Sizing for peak power only. — Right: Check continuous power (thermal) for towing, hills and repeated launches.
• Wrong: Ignoring traction limits. — Right: Launch force above μ × axle load just spins the tyres.

Applications

• Passenger EVs: IPM motors in e-axles, 100–300 kW.
• Performance & AWD: Dual/tri-motor layouts with torque vectoring.
• Commercial & two-wheelers: Hub and central motors across power classes.

Key takeaways

• PMSM dominate; induction and EESM avoid magnets.
• Constant torque to base speed, then constant power.
• i = n_max/n_wheel(v_max); F = T i η/r.
• Continuous rating is thermally limited.

Going further

• Field-oriented control: Controls d/q-axis currents for torque and field weakening (see Inverter).
• Hairpin windings: Rectangular conductors raise slot fill and continuous power.
• Axial-flux motors: Pancake-shaped, very high torque density for performance and in-wheel use.