How to Calculate DC Motor Torque

By Douglas Quaid
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A motor turns electrical energy into mechanical energy. A direct current (DC) motor uses direct current to induce a magnetic field in a series of wire windings surrounding a magnetic rotor. The magnetic field spins the rotor, rotating the output shaft. Torque is the force with which the shaft spins. Torque is commonly expressed in units of foot-pounds. If you have a socket wrench with a handle one foot long and you turn it with a force of five pounds, you are applying five foot-pounds of torque to the bolt.

Find the power output of the motor. In the English system, power is expressed in units of horsepower; otherwise, it is given in watts. Your motor's documentation should list the power and speed in revolutions per minute (RPM) of the motor at a given supply voltage. The documentation may also include a graph that shows the curve of motor speed dependent on supply voltage.

Calculate horsepower yourself if you do not have the motor's documentation but you know the resistance across the motor's terminals. Divide the supply voltage by the motor resistance to find the motor current. Multiply the voltage, current and efficiency of the motor, and then divide the product by 746. Efficiency is a percentage that accounts for energy lost to friction and heat--no motor converts 100 percent of its input to mechanical energy. The efficiency may be listed in the motor's documentation. If you do not know the efficiency you can leave it out. However, the motor will perform below your calculations.

Divide the horsepower by the speed of the motor in RPM, and then multiply the result with a conversion factor of 5,252 to get the motor's torque in foot-pounds at the given speed and power.

About the Author

Based in Los Angeles but born and bred in Brooklyn, N.Y., Douglas Quaid has been writing for various websites since 2010. He holds a Bachelor of Arts in film from Bard College.