Many aspects of a manufacturing system have various NEMA ratings to tell the manufacturers what they are best suited for. Things like size, performance ratings, tolerances, and safety measures come into play for different ratings. Motors in particular can be A, B, C, D, or E motors based on the torque that they offer.
A and B Motors
A and B motors are very similar except that A motors have higher Starting Currents. Both have high efficiency. They also have a large range of Locked Rotor/Starting Torque based on Percent of Rated Load Torque (70-275%). Their Pull-Up and Break Down Torque are also the same (65-190 and 175-300 respectfully). Their Break Downs can reach considerably higher than the other ratings. A and B motors are commonly used for fans, pumps, and blowers where large Starting Torques aren’t necessary and the motor doesn’t need to support a large load.
C motors consistently have high Locked Rotor Torque (200-285%) along with high Pull-Up (140-195%) and Break Down (190-225%). The Locked Rotor Current is average at around 600-700% and the C motors are mid-range when it comes to efficiency. They are best used in machines that require the motor to start under a load such as conveyors, compressors, and reciprocating pumps.
D motors have similar torques to C motors, except that the Break Down Torque can reach up to 275%. They also have a much higher Slip, ranging from 5-8% instead of 1-5%. This creates a stronger torque, but makes the motors very inefficient. They are used for machinery with high peak loads such as elevators, hoists, and punch presses.
E motors are fairly mid-range in Locked Rotor Torque (75-190%), Pull-Up Torque (60-140%), and Break Down Torque (160-200%). They have the highest Locked Rotor current though (800-1000%). Their Slip is also somewhat smaller than the other designs (0.5-3%). These aspects give the E motors the highest efficiency out of the NEMA ratings. They can be used in similar applications to A and B motors like fans, pumps, and blowers with low starting torque.