NEMA A, B, C and D Electrical Motor Design

Different motors with the same nominal horsepower may have different start current, torque curves, speeds and other variables. When selection a particular motor for an intended task all engineering parameters must be taken into account.

The four NEMA (National Electrical Manufacturers Association) designs have unique speed-torque-slip relationships - making them suited for different type of applications.

NEMA design A

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

NEMA design B

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

NEMA design C

• maximum 5% slip
• low starting current
• high locked rotor torque
• normal breakdown torque
• suited for equipment with high inertia and high starting torques at start  - like positive displacement pumps, conveyors
  

NEMA design D

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