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Start and Running Torque Requirements, Calculations for Electric Motors

When determining the torque requirements for a electric motor, consideration should be given to the load and start time demands during the start duration, operating torque, and peak load torque. The starting torque is dependant on the number of times anelectric motor will have to start in a given time, as well as, the duration of the start cycle. The actual start torque applied should be many times greater than the actual start torque required by the application. The greater difference in torque applied by the motor and the start torque required by the application, the faster the applied acceleration of the electric motor. The time duration required to accelerate a application from a dead stop to operating speed is given by the following:

T = [ N x WR2 ] / [ Ta x 308 ]

Open: Electric Motor Time to Start Calculator


T  = Time to Start (seconds)
N = Velocity at load (rpm)
Ta = Average Torque During start (ft-lbs)
W = Weight (lbs)
R = Radius of Gyration (ft2)
[WR2 =  Rotating Inertia (lbs-ft3)]
308 = Constant derived converting minutes to seconds, mass from weight, and radius to circumference

Running or operating torque is determined by the following equation:

To = [ 5250 x HP ] / N

Open: Electric Motor Running Torque Requirements Calculator


To = Operating or running Torque ( ft-lbs )
HP = Horsepower delivered by electric motor
N = Rotational velocity ( rpm, revolutions per minute )
5250 = Constant converting horsepower to ft-lbs/minute and work/revolution to torque

Use the peak horsepower to determine the maximum operating torque.