Electric Motors - Torque vs. Power and Speed

Work is the result of a force acting over some distance. Work is quantified in joules (Nm) or foot-pounds.

Torque is a rotating force produced by a motor's crankshaft. The more torque the motor produces, the greater is its ability to perform work. Since torque is a vector acting in a direction it is commonly quantified by the units Nm or pound-feet.

Power is how rapidly work is accomplished - work in a given amount of time. Power is quantified in watts (J/s) or horse power.

Note that the driving force of an electric motor is torque - not power. The torque is the twisting force that makes the motor run and the torque is active from 0% to 100% operating speed. The power produced by the motor is zero at zero speed.

Note! that the full torque from zero speed is a major advantage for electric vehicles.

Equations

Torque in Imperial units:

• T(lbf·in) = P(hp) × 63025 / n(rpm)
• T(lbf·ft) = P(hp) × 5252 / n(rpm)
• 1 ft·lbf = 1.356 Nm

Torque in SI units:

• T(Nm) = P(W) × 9.549 / n(rpm)

Conversions:

• 1 hp = 0.746 kW
• 1 lb·ft = 1.355 Nm