NEMA Three Phase Electric Motor Designs

Custom Search
Three phase induction motors are specified by their electrical design type. NEMA  (National Electrical Manufacturers Association) has classified three phase AC motors into design A, B, C, D. A more recent design called E has been added. These designs are suited to particular classes of applications based upon the load requirements typical of each class.

As a motor operates from no load to full load, its torque varies with speed. The relationship between speed and torque is often shown in a graph, called a speed-torque curve. This curve shows the motor’s torque, as a percentage of full-load torque, over the motor’s full speed range, shown as a percentage of its synchronous speed. NEMA classification is based on the speed-torque curves. Typical speed-torque curve for designs A, B, C, and D is shown below:



Characteristics of NEMA Three phase Motor Designs

NEMA Design A Motors
The table below shows the typical characteristics of all NEMA A Motors:

Basic Characteristics
  • High locked rotor torque
  • High locked rotor current

Locked Rotor Torque (% of Full-load Torque)

70 - 275%

Pull - Up Torque (% of Full-load Torque)
65 - 190%

Breakdown Torque (% of Full-load Torque)

175 - 300%
Locked Rotor Current (% of Full-Load Current)
 -

Slip

0.5 - 5%

Areas of Application
Fans, blowers, centrifugal pumps
and compressors, motor-generator sets, etc., where starting torque requirements are relatively low
Efficiency High or Medium


NEMA Design B Motors
The table below shows the typical characteristics of all NEMA B Motors. NEMA design B motor is the most common three-phase AC induction motor design.

Basic Characteristics
  • Normal locked rotor torque
  • Normal locked rotor current

Locked Rotor Torque (% of Full-load Torque)

70 - 275%

Pull - Up Torque (% of Full-load Torque)
65 - 190%

Breakdown Torque (% of Full-load Torque)

175 - 300%
Locked Rotor Current (% of Full-Load Current)
 600 - 700%

Slip

0.5 - 5%

Areas of Application
Fans, blowers, centrifugal pumps
and compressors, motor-generator sets, etc., where starting torque requirements are relatively low
Efficiency High or Medium


NEMA Design C Motors
The table below shows the typical characteristics of all NEMA C Motors:

Basic Characteristics
  • High locked rotor torque
  • Normal locked rotor current

Locked Rotor Torque (% of Full-load Torque)

200 - 285%

Pull - Up Torque (% of Full-load Torque)
140 - 195%

Breakdown Torque (% of Full-load Torque)

190 - 225%
Locked Rotor Current (% of Full-Load Current)
 600 - 700%

Slip

1 - 5%

Areas of Application
Conveyors, crushers, stirring
motors, agitators, reciprocating
pump and compressors, etc.,
where starting under load is
required
Efficiency Medium


NEMA Design D Motors.
The table below shows the typical characteristics of all NEMA D Motors:

Basic Characteristics
  • Normal locked rotor torque
  • High slip

Locked Rotor Torque (% of Full-load Torque)

275%

Pull - Up Torque (% of Full-load Torque)

-

Breakdown Torque (% of Full-load Torque)

275%
Locked Rotor Current (% of Full-Load Current)
 600 - 700%

Slip

5 - 8%

Areas of Application
High peak loads with or without
flywheels such as punch presses,
shears, elevators, extractors,
winches, hoists, oil-well pumping
and wire-drawing motors
Efficiency Low


NEMA Design E Motors
This is the newest NEMA design category. Below are the typical characteristics of  NEMA E motors:

Basic Characteristics
  • Normal locked rotor torque
  • Normal locked rotor current
  • Low slip

Locked Rotor Torque (% of Full-load Torque)

75 - 190%

Pull - Up Torque (% of Full-load Torque)

60 - 140%

Breakdown Torque (% of Full-load Torque)

160 - 200%
Locked Rotor Current (% of Full-Load Current)
 800 - 1000%

Slip

0.5 - 3%

Areas of Application
Fans, blowers, centrifugal pumps
and compressors, motor-generator
sets, etc., where starting torque
requirements are relatively low
Efficiency High