AC Induction Motor Design (4)

6. NEMA Motor Design Classifications

NEMA defines, in NEMA MG 1-1993, four motor designs dependant upon motor torque during various operating stages:

• Design A: Has a high starting current (not restricted), variable locked-rotor torque, high break down torque, and less than 5% slip.
• Design B: Known as "general purpose" motors, have medium starting currents (500 - 800% of full load nameplate), a medium locked rotor torque, a medium breakdown torque, and less than 5% slip.
• Design C: Has a medium starting current, high locked rotor torque (200 - 250% of full load), low breakdown torque (190 - 200% of full load), and less than 5% slip.
• Design D: Has a medium starting current, the highest locked rotor torque (275% of full load), no defined breakdown torque, and greater than 5% slip.

Design A and B motors are characterized by relatively low rotor winding resistance. They are typically used in compressors, pumps, fans, grinders, machine tools, etc.
Design C motors are characterized with dual sets of rotor windings. A high resistive rotor winding, on the outer, to introduce a high starting torque, and a low resistive winding, on the inner to allow for a medium breakdown torque. They are typically used on loaded conveyers, pulverizers, piston pumps, etc.
Design D motors are characterized by high resistance rotor windings. They are typically used on cranes, punch presses, etc.

to be continued....