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Angular Motion - Power and Torque

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  • 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.

Power and Torque of Body in Angular Motion

The power of a rotating body can be expressed as

P = T ω

= T 2 π n rps

= T π n rpm / 30                 (1)

where

P = power (W)

T = torque or moment (Nm)

ω = angular velocity (rad/s)

π = 3.14...

n rps = rotations per second (rps, 1/s)

n rpm = rotations per minute (rpm, 1/min)

  • 1 rad = 360 o / 2 π =~ 57.29578.. o

Note! - an object - like an electric motor - can have an active moment without rotation, but without rotation ( ω = 0 ) there is no power produced.

In imperial units

P = T n rpm / 5252                   (1b)

where

P = power (hp)

T = torque (ft lb f )

Example - Torque created by Rotating Motor

An electric motor runs with 3600 rpm with an measured power consumption of 2000 W . The torque created by the motor (without losses) can be calculated by rearranging (1) to

T = 30 P / (π n rpm )

= 30 (2000 W) / (π (3600 rpm))

= 5.3 Nm

Torque Calculator


Download and print Motor - Torque vs. Power and rpm chart

Torque of a Body in Angular Motion

T = I α                    (2)

where

I = moment of inertia (kg m2, lb f ft s2)

α = angular acceleration ( rad /s2)

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