NEMA Motor Designs – Explained!

National Electrical Manufacturers Association (NEMA), defines four different electrical designs for induction motors. Different motors with the same nominal horsepower may have different start current, torque curves, speeds and other variables. While selecting for an application, all engineering parameters must be taken into account.

The four NEMA designs have unique speed-torque-slip relationships – making them suited for different type of applications.

"The four NEMA designs have unique speed-torque-slip relationships – making them suited for different type of applications"

Design A

In the NEMA Frame sizes Design A usually indicates a special electrical design such as high Locked Rotor or Breakdown torque without the limits on current dictated by Design C or D. Design A is most commonly seen on motors which are derated (Nameplated at a lower HP than their design) or motors requiring high Breakdown torque during short periods without the high slip associated with Design D. Examples include Hydraulic Pump Drives on Injection Molding machines or Compactors.

Design B

This is the most common three phase General Purpose electrical design. With good Torque to Current ratios, this motor is suitable for use on a wide variety of applications. Design B is characterized by low starting current, high locked rotor torque, normal breakdown torque and maximum 5% slip. Common applications include HVAC fans, pumps and blowers.

Design C

Provides a higher level of Locked Rotor torque than Design B with the same level of starting current. The trade-off in design is usually a lower Breakdown Torque and in some cases efficiency. This design is common on conveyors where higher starting torque can prevent the need for selecting the next larger Design B rating just to get the load started. Motors with this design have a maximum 5% slip, low starting current, high locked rotor torque and normal breakdown torque. Common applications include positive displacement pumps, conveyors.

Design D

Provides an even higher level of Locked Rotor torque than Design C with the same Locked Rotor current limit. Full Load Efficiency is poor on Design D. However, Design D motors are used on applications where full load is rarely seen and its ability to efficiently accelerate the load is required. Typical applications are Crane & Hoist, Punch Press and Oil Well Beam Pumps.