NEMA Design A B C D for Electric Motor

Different motors of the same rated horsepower may have different starting currents, torque curves, speeds and other variables. NEMA (National Electrical Manufacturers Association) has A, B, C, D four different designs for electric motors.

NEMA Design A: Conventional starting torque (usually 150-170% of the rated torque), high corresponding starting current. Extreme torque is the highest in all NEMA designs. It can treat heavy load in the short period. The rotating-speed difference is lower than or equivalent to 5%. one typical application is the electric motor of plastic injection machine.

NEMA Design B: Is the most common design of electrical motors and ATO single phase AC induction motors are of NEMA design B. Its starting torque is similar to that of design A, but sometimes still lower, providing a relatively low starting current. However, in industrial application, it locks locked rotor and still allows starting load. The rotating-speed difference is lower than or equivalent to 5%. The electric efficiency and full-load power factor are relatively high. The typical applications include pump, fan and machine tool.

NEMA Design C: Provide high starting torque (higher than A and B designs, usually exceed 200% of rated torque). It is usually used to drive the heavy starting load. Almost all these electric motors can operate in full speed without overload. The starting current is low. The rotating-speed difference is lower than or equivalent to 5%.

NEMA Design D: In all NEMA motor designs, it is the electric motor which can provide the highest starting torque. The starting current and full-load speed are low. The high slip value is (5-13%). The motor is suitable for machines that operate at speeds of the motor, without load changes or without drastic changes, such as flywheel energy storage. The subclassification of not a few designs still contains wider rotating-speed difference scope. Such motor category is generally specially customized. The speed-torque characteristics of these motors are diversified. Fixed rotor torque(starting torque) refers to the minimum torque, which is generated when rotor stops. At this moment, it is nominal voltage and frequency. Extreme torque is the maximum torque, which is generated before the motor speed reduces all of a sudden. The nominal speed (nominal voltage and frequency) occurs. The pull-up torque is the minimum torque, which is generated when the motor speed transforms from zero to the speed point. At this moment, extreme torque occurs.