How to calculate the rpm of a motor

Updated July 19, 2017

Electric motors perform work by converting electrical energy into that of mechanical. The mechanical energy is usually manifested by a rotatable shaft or armature. In vehicles, this armature operates propellers and wheels.

RPM, or revolutions per minute, indicates how many turns the object can make, or how fast it can rotate. The force the motor applies to make the objects spin is called a torque. Engineers wish to know how quickly or how powerfully the motor can use torque to do work, and they use horsepower to measure this. To calculate the RPM of a motor, use torque and horsepower.

Study the definition of torque. It is the applied force that causes an armature or shaft to turn. For the motor, where the force is perpendicular, the equation is T = F * R. F is the force in pounds and r is the radius or distance in feet. T is sometimes written as the Greek letter Tau. In British units, torque is normally measured in pound-foot, or lb-foot.

Examine the definition of horsepower or hp. It is a measurement of work that is done in a certain amount of time. In British units, it is ft-lb / s, where s is in seconds and is calculated to be 550ft-lb / s, or 33,000ft-lb / minute.

Learn the formula to find the RPM or speed of a motor. It is RPM = hp * 5252 / T.

Practice calculating the RPM. For example, find the RPM if a car engine of 10hp can apply a torque of 9.53kg-ft to a driveshaft that turns the wheels in an automotive vehicle. The speed is 10hp x 5252 / 9.53kg-ft = 2500 RPM. Automobile engines normally operate from 600 to 6000 RPM.

Things You'll Need

  • RPM calculation problem
  • Calculator
  • Introductory physics textbook
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About the Author

Kim Lewis is a professional programmer and web developer. She has been a technical writer for more than 10 years and has written articles for businesses and the federal government. Lewis holds a Bachelor of Science, and occasionally teaches classes on how to program for the Internet.