Tom Bishop, P.E. 
EASA Technical Support Specialist 

Motors built to National Electrical Manufacturers Association (NEMA) standards use alphabetical letter codes on the nameplate to designate a number of alternating current (AC) motor characteristics. These characteristics are the code, design, and insulation class.

Read the nameplate carefully as these designations are easily mis­interpreted. Similarly, re-confirm these data items when your customer provides them. For example, the letter “B” could designate a design code, insulation class or kVA code (though highly improbable.) What do these different designations mean? 

Code 
Designated on the nameplate as “Code,” a letter code identifies the locked rotor kVA (kilovolt amperes) on a per-horsepower basis. Codes are defined in NEMA MG 1-10.37.2 by a series of letters from A to V. Gener­ally, the farther the code letter from A, the higher the inrush current per hp. A replacement motor with a “higher” code letter may require different up­stream electrical equipment, such as a larger motor starter. 

Note: Similar letter designations are used for different motor charac­teristics listed on the nameplate. Read the nameplate carefully to avoid misinterpretations between code, design, 
and insulation class. 

Design 
NEMA MG 1 Section MG 1-1.18, defines “design,” which applies to the torque and current characteristics of the motor. Letters are assigned to the defined categories. Most motors are Design B, although the standard also defines Designs A, C, and D. Common headings on nameplates include “Des,” “NEMA Design,” and “Design.” 

Design B has comparatively high efficiency and torque characteris­tics. It should be noted that from the standpoint of efficiency, Design A is the best. However, it is used infre­quently because it has a relatively high starting current, which can cause nuisance tripping of motor protection circuitry and may also require a larger than standard-size starter. Figure 1 compares the four NEMA designs. 

Some motors may not conform to any torque-current characteristics defined in MG 1. The motor manufac­turer may assign them a letter that is not a defined industry standard; or the motor manufacturer may simply not list a design letter on the nameplate. It is important to check for the design letter when replacing a motor in an existing application. 

One of the more common misapplications is the attempt to replace a Design C or D motor with a Design B. The unfortunate outcome is usually that the Design B motor, with its lower starting torque (see Figure 2), cannot accelerate the load up to operating speed.

Insulation Class
Often abbreviated “Ins. Cl.” on nameplates, it is an industry standard classification of the thermal endurance of the motor winding. Insulation class is indicated by a letter designation such as “A,” “B,” “F,” or “H” (see Figure 3), depending on the winding’s ability to withstand a given operating temperature for a given life. Insulation classes of a letter deeper into the alphabet perform better. For example Class F insulation has a longer nominal life at a given operating temperature than Class A, or for a given life it can survive higher temperatures.

It is worth noting that manufacturers produce some motors using a higher insulation class than indicated on the nameplate. For example, a motor wound using Class F insulation may be listed for a Class B rise. The reason for doing so is to provide a more thermally robust winding capable of better handling real-world operating conditions. (For similar reasons, many EASA service centers upgrade winding insulation to Class H.)

Operating temperature is a result of ambient conditions plus the energy lost in the form of heat (causing the temperature rise) as the motor converts electrical to mechanical energy. The ultimate temperature in the winding is the sum of the ambient plus the winding temperature rise. For example, if a motor is rated with a 1.15 service factor and has a class B (130° C) insulation system, the temperature rise according to NEMA MG1 is 90° C, and the maximum ambient limit is 40° C.

The winding total temperature would be 90° + 40°, or 130° C. Operating at temperatures above rated will reduce winding life, generally reducing life by half for every 10° C increase.

For an expanded explanation of motor temperature rise, see the article titled “Understanding Motor Temperature Rise Limits” in the November 2003 issue of CURRENTS.

Categories: Design/theory/application, Nameplates

Tags: Nameplate Design letter(s) Motor design

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