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 horsepower. The torque is the twisting force that makes the motor running and the torque is active from 0% to 100% operating speed.

The power produced by the motor depends on the speed of the motor and is

• zero at 0% speed,  and
• normally at it's top at operating speed

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

For full table - rotate the screen!

Electrical Motor Power, Velocity and Torque Equations

Torque in Imperial units can be calculated as

T _inlb = P _hp 63025 / n                          (1)

where

T _inlb = torque (in lb_f )

P _hp =  horsepower delivered by the electric motor (hp)

n = revolution per minute (rpm)

Alternatively

T _ftlb = P _hp 5252 / n                         (1b)

where

T _ftlb = torque (lb_f ft)

• 1 ft lb_f = 1.356 Nm

Torque in SI units can be calculated as

T _Nm = P _W 9.549 / n                        (2)

where

T _Nm = torque (Nm)

P _W =  power (watts)

n = revolution per minute (rpm)

• Torque - Work done and Power transmitted
  

Electric Motor - Torque vs. Power and Speed

power (kW)

speed (rpm)

• 1 hp = 0.746 kW - power converter
• 1 lb ft = 1.355 Nm  - torque converter
  

Electric Motor - Power vs. Torque and Speed

torque (Nm)

speed (rpm)

Electric Motor - Speed vs. Power and Torque

power (kW)

torque (Nm)

Example - Torque from an Electrical Motor

The torque delivered from an electrical motor producing 0.75 kW (750 W) at speed 2000 rpm can be calculated as

T = ( 750 W ) 9.549 / (2000 rpm)

= 3.6 (Nm)

Example - Torque from an Electrical Motor

The torque delivered from an electrical motor producing 100 hp at speed 1000 rpm can be calculated as

T = (100 hp) 63025 / (1000 rpm)

= 6303 < (lb_f in)

To convert to pound-force foot - divide the torque by 12 .

• Car engine torque

Temperature ^oC
K
^oF

Length m
km
in
ft
yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
acres

Volume m³
liters
in³
ft³
us gal

Weight kg_f
N
lbf

Velocity m/s
km/h
ft/min
ft/s
mph
knots

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm