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The National Electric Manufacturers Association (NEMA) provides motor design ratings. The motor design is listed on the data plate by a letter A, B, C, or D. This designation is determined by the type of wire, insulation, and rotor that are used in the motor and are not affected by the way the motor might be connected in the field.
Type A motors have low rotor circuit resistance and have approximate slip of 5 - 10% at full load. These motors have low starting torque with a high locked rotor amperage (LRA). This type of motor tends to reach full speed rather rapidly.
Type B motors have low to medium starting torque and usually have slip of less than 5% at full load. These motors are generally used in fans, blowers, and centrifugal pump applications.
Type C motors have a very high starting torque per ampere rating. That is, they are capable of starting when the full load is applied for applications such as conveyors, crushers, and reciprocating compressors such as air-conditioning and refrigeration compressors. These motors are rated to have slip of less than 5%.
Type D motors have a high starting torque with a low LRA rating. This type of motor has a rotor made of brass rather than copper segments. It's rated for slip of 10% at full load. Normally this type of motor will require a larger frame to produce the same amount of horsepower as a type A, B, or C motor. These motors are generally used for applications with a rapid decrease of shaft acceleration, such as a punch press that has a large flywheel.
These standards are set by NEMA, and a motor must meet all the requirements of the standard to be marked as a type A, B, C, or D. This allows motors made by several manufacturers to be compared on an equal basis according to application.
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