Design of Nema A, B, C and D for Electric Motor Control

Different motors of the same nominal horsepower can have varying starting current, torque curves, speeds, and other variables. Selection of a particular motor for an intended task must take all engineering parameters into account.
The four NEMA (National Electrical Manufacturers Association) designs have unique speed-torque-slip relationships making them suitable to different type of applications:

NEMA design A

• maximum 5% slip
• high to medium starting current
• normal lock rotor torque
• normal breakdown torque
• suited for a broad variety of applications - as fans and pumps

NEMA design B

• maximum 5% slip
• low starting current
• high lock rotor torque
• normal breakdown torque
• suited for a broad variety of applications, normal starting torque - common in HVAC application with fans, blowers and pumps

NEMA design C

• maximum 5% slip
• low starting current
• high lock rotor torque
• normal breakdown torque
• suited for equipment with high inertia starts - as positive displacement pumps

NEMA design D

• maximum 5-13% slip
• low starting current
• very high lock rotor torque
• suited for equipment with very high inertia starts - as cranes, hoists etc.