Torque, speed and power curves
Reading the single chart that defines everything about how a motor drives a car.
- Explain torque, speed and power curves at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive unit
Overview
Every EV motor's capability is summarised in one chart: torque against speed, with power following as their product.
In depth
Torque-speed performance simulator
Drag the operating point across the map and watch torque, power and efficiency respond.
Constant torque up to base speed, then constant power as field weakening takes over.
Two regions
Below base speed, the motor is in the constant-torque region: current, and therefore torque, can be held at its maximum limit. Above base speed, the motor enters the constant-power region: torque must fall roughly in proportion to 1/speed to keep power, and therefore voltage demand, within limits.
Power is torque times angular velocity — in the constant-power region, as speed rises torque falls proportionally so their product stays roughly flat.
Motor Power Calculator
Mechanical shaft power from torque and rotational speed.
P (kW) = T (Nm) × n (rpm) ÷ 9550Power is torque times angular velocity. Angular velocity in rad/s is 2πn/60, and dividing by 1000 for kilowatts collapses the constants into 9550.
Example: 310 Nm at 6,000 rpm → 310 × 6000 ÷ 9550 ≈ 195 kW.
- Below base speed the motor is torque-limited; above it, power-limited.
- Electrical input power is shaft power divided by efficiency.
- Peak power is thermally limited and can rarely be held for more than 10–30 seconds.
Key takeaways
- The torque-speed curve has two regimes: constant torque, then constant power.
- Base speed is the boundary between the two regions.
- Power equals torque times angular velocity at every operating point.
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