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AC Motors: Basic Terminology

Sep 25, 2017

We work with OEMs to help them find AC motors that best suit their application. During this process, it helps to understand the terminology of an electric motor so you will be better equipped to see how each motor differs and determine which one meets your needs.

Below is a brief overview of basic terms related to AC motors, such as NEMA, torque, speed, and horsepower. If you have any questions or need further clarification, contact us and one of our sales engineers will be happy to help.

AC Motor

AC Motors

AC motors create mechanical energy from electrical energy. They’re often found on fans or pumps, and can also connect to larger forms of mechanical equipment such as a mixer, conveyor, or winder. AC motors have a wide range of applications and can be used solo and/or in tandem with multiple motors. Many companies around the world use AC motors for a wide range of applications, including:

  • Commercial
  • Industrial
  • Residential
  • Utility



The National Electrical Manufacturers Association (NEMA) dictates the standard for electrical equipment, which you can find in NEMA Standard Publication No. MG 1. Primarily associated with motors used in the US, NEMA develops standards based on original electrical equipment manufactures and industry best practices. Some large AC motors don’t align with NEMA standards because they’re built for a specific application, so they’re referenced as “above NEMA motors.”  

motor torque torque motor


When an object turns or twists, the force that causes the object to rotate is called torque. For example, if the end of a lever receives a downward force, the pivot point experiences torque as the lever turns.

Torque (τ) equals force multiplied by the radius of the object.

τ = Force x Radius

The English system measures torque in pound-feet (lb-ft) or pound-inches (lb-in). So if a 1 ft. long lever receives 10 lbs of force, the torque is 10 lb-ft. As force or radius increases, so does the torque, so if you increase the radius to two feet, the torque is 20 lb-ft.


Speed is represented when an object travels a certain distance in a specific time frame. Speed is calculated by dividing the total distance of displacement by the travel time.

Speed = Distance / Time

motor horsepower


Although power is sometimes expressed in foot-pounds per second, James Watt made the term horsepower (HP) much more common, starting in the 18th century. When Watt tried to sell steam engines, his customers asked him how many horses a steam engine could replace. To find out, Watt set up a wheel; as a horse walked around the wheel, it lifted a weight. Watt determined that a horse could perform about 550 ft-lbs per second of work. Horsepower equals 550 ft-lbs per second (33K ft-lbs per minute).

Horsepower is calculated with the torque (lbs-ft) and speed:

HP = (T x RPM) / 5,250

If you need help understanding these terms before you purchase an AC motor, contact us and we’d be happy to help.