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NEMA A, B, C and D Electrical Motor Design

NEMA has established the four different designs A, B, C and D for electrical induction motors.

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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.
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