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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:
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 |
|
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 |
|
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 |
|
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 |
|
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 |
This is the newest NEMA design category. Below are the typical characteristics of NEMA E motors:
Basic Characteristics |
|
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 |
