NEMA Three Phase Electric Motor Designs

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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

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