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.