NEMA Induction Motor Design Classes

Various standard classes (or designs) for motors, corresponding to the torque curves, shown on Picture 1 below, have been developed to better drive various type loads. The National Electrical Manufacturers Association (NEMA) has specified motor classes A, B, C, and D to meet these drive requirements. Similar International Electrotechnical Commission (IEC) classes N and H correspond to NEMA B and C designs respectively.

Picture 1: Characteristics for NEMA designs - Torque curves


All motors except class D operate at %5 slip or less at full load. The standard classes are:

● Class A starting torque is the same as Class B. Drop out torque and starting current (LRT) are higher. This motor handles transient overloads as encountered in injection molding machines.
● Class B (IEC Class N) motors are the default motor to use in most applications. With a starting torque of LRT = 150% to 170% of FLT, it can start most loads without excessive starting current (LRT). Efficiency and power factor are high. It typically drives pumps, fans, and machine tools.
● Class C (IEC Class H) has higher starting torque than class A and B at LRT = 200% of FLT. This motor is applied to hard-starting loads which need to be driven at constant speed like conveyors, crushers, and reciprocating pumps and compressors.
● Class D motors have the highest starting torque (LRT) coupled with low starting current due to high slip (5% to 13% at FLT). The high slip results in lower speed. Speed regulation is poor. However, the motor excels at driving highly variable speed loads like those requiring an energy storage flywheel. Applications include punch presses, shears, and elevators.
● Class E motors are a higher efficiency version of Class B.
● Class F motors have much lower LRC, LRT, and break down torque than Class B. They drive constant easily-started loads.