Electrical Apparatus: Should you be afraid of Design E motors?

Never fear-what is all the fuss about?

" A MATURE INDUSTRY"-- that's how the electric motor industry is often described. Yet both technological and political changes occur in the motor business that are sometimes widely misunderstood. That's certainly true for the NEMA "Design E" polyphase induction motor (advance word of which appeared in "Energy efficient motors: We haven't seen anything yet!", EA July 1993).

During the spring of 1999, some writers warned at length of the pitfalls or misapplications possible in using these motors. One wrote that a NEMA E user faces various motor starting problems. "Always use caution when directly replacing existing motors" with the new design, he adds, because they "may have starting torques too low . . . and starting currents excessively high."

According to another, "increased locked-rotor current, reduced . . . torque of Design E motors can result in costly errors in retrofit/replacement situations . . ."

NEMA standards for Design E, approved in 1993, do indeed include higher locked-rotor current for many ratings. That's evident from Figure 1. Users should keep in mind, however, that the NEMA figures are maximum limits, rather than actual values. In any event, the National Electrical Code has taken account of these higher current limits in its provisions for motor circuit protection.

Similarly, NEMA standard torque limits for Design E motors are often lower than those for the older Design B. Table I is illustrative. Remember, though, that these torques are minimum values. Just as with Design B, actual motors would be built to develop higher torques than these.

How much higher? That's going to vary with the designer and the technology available. Safe, timely acceleration of any load depends upon properly matching motor capability and load requirement. Simply relying on a published standard minimum value, while making no attempt to verify what the load needs, is playing Russian roulette.

So where are they?

Whatever the validity of the alarms that are being sounded, the publications referred to are clearly implying that the NEMA E design is a readily available, commercially established product that is being foisted on the unwary motor purchaser, who is then likely to find it unsuitable for the application.

Such an implication has no basis in fact. No motor manufacturer in the U.S., or overseas supplier catering to the U.S. market, has yet offered a line of NEMA E motors as either a standard product or an optional specialty. Such motors are simply not on the market. They have never been on the market and they are not likely to be on the market.

At a recent trade show, 16 different motor manufacturers were asked if they were (or planned to be) offering NEMA Design E motors. The answers, all negative, ranged from "What's NEMA Design ET' to "We're not aware of anyone doing it," "We haven't talked about it," or "We could do it if requested, but there's been no demand." Said one: "There's a lot of resistance to it." Claimed another, "Nobody's doing this unless they're keeping it a secret." Among persons familiar with the development, the consensus seemed to be that complete lack of user interest in Design E products means that manufacturers see no reason to offer them.

To understand why that's true, we begin by recalling how Design E found its way into NEMA standards.

When the 1992 Energy Policy Act (EPACT) was created, it included this provision in Section 2105:

- "The Secretary [of Energy] shall conduct a five-year program, in accordance with Sections 3001 and 3002 of this Act, to increase the efficiency of electric drive technologies, including adjustable speed drives, high speed motors, and high efficiency motors."

- "Within one year after the date of enactment of this Act, the Secretary shall solicit proposals for projects under this section."

Sections 3001 and 3002 cite the 1980 Stevenson-Wydler Technology Innovation Act, allowing for various joint ventures involving industry and the Department of Energy. "An objective of any commercial application program ... shall be to accelerate the transition of technologies from the research and development stage.

At that time, the motor manufacturing members of NEMA had one readily available technological means of achieving significantly higher efficiency than EPACT required. That was to design motors with higher lockedrotor current. Such "magnetically stronger" designs inherently reduced the stator and rotor 12R losses-normally the highest in any standard motor.

Precedent existed for those higher currents. The International Electrotechnical Commission had already Standardized on values well above NEMA Design B limits. Figure 1 shows what was adopted for Design E-much the same as the IEC standards.

The presumption was that the existence of the Design E standard would satisfy the Department of Energy that the motor industry was working toward efficiencies above the letter of the law.

The DOE was-and is-not concemed with locked-rotor current (nor with accelerating torque, which tended to be lower for Design E because of lower rotor resistance and other design modifications).

Since the early 1990s, most manufacturers have developed a second line of standard motors, with efficiencies beyond the EPACT requirements, that are variously called "premium efficiency" or some other designation undefined by standards or legislation. These tend to offer the higher torques and lower locked-rotor currents of NEMA Designs A or B. These products are not being identified or marketed as "NEMA Design E"-they may bear no design letter.

Just because a motor has higherthan-EPACT efficiency doesn't make it a Design E product. Compliance with NEMA standards requires that a product sold under any "Design Letter" have that letter stamped on its nameplate (MGI-10.40.1). Therefore, if a motor is to be marketed as "Design El" that design letter must appear on the nameplate. Otherwise, it can't be called a "Design F' machine.

Thus, in the absence of either a need or a demand for Design E motors, users should have no fear of being somehow waylaid by the motor industry, to be unwittingly sold this peculiar machine that threatens so much grief. If ever a tempest existed in a teapot, this is it. M

By Richard L. Nailen, P.E., EA Engineering Editor