Analysis of Design E motor characteristics reveals surprising properties that may have a significant impact on your choice of "energy-efficient" motors.

In response to demands for increased energy efficiency, NEMA has issued specifications for a Design E motor. To evaluate this motor for possible applications (as with any motor), you should understand its performance characteristics.

Design B or Design E?

Properly evaluating a motor for a particular job requires that you have an understanding of the relationship between its speed and torque capabilities and current requirements. These characteristics can be controlled by manufacturing techniques to configure motors for specific applications. NEMA has established speed-torque parameters, as shown in Fig. 1, for squirrel-cage induction motors. The most commonly used motor is the Design B general-purpose motor, with its torque peaking at approximately 80% of synchronous speed, and having a normal starting current of approximately six times the full-load current.

Many manufacturers' ("Brand X") existing premium efficient Design B motors already have exceeded efficiency values in NEMA Table 12-10 for some time. (See sidebar below.) In fact, some approximately match the new Design E figures. ([ILLUSTRATION FOR FIGURE 2 OMITTED], on page 32.) Also, it's very important to realize that the Design E motor torque curve is very similar to the Design B curve in sizes above 3 hp. (Actual Design B torque values, in many instances, are even higher than the minimums).

What then, will be the difference between a new Design "E" motor and a Design "B" motor that already exceeds the efficiency values in NEMA Table 12-10?

Two critical differences

In addition to horsepower, voltage, synchronous speed, and frame size, there are two important motor characteristics that affect the suitability and installation of a motor:

* Locked-rotor torque, which defines the magnitude of the load that the motor can start; and

* Locked-rotor current, which is the peak current that flows through the windings during start-up.

Locked-rotor torque. According to the new NEMA standard specifically in Sec. 12.38.4, Design E minimum allowable locked-rotor torques for specific hp ratings are lower than Design B values (as shown in NEMA Table 12-2), particularly in the smaller sizes. This is shown in Fig. 3, on page 34. Therefore, it's conceivable that replacing a general-purpose Design B motor with a Design E motor, whether because of failure or a desire to increase efficiency, could result in the Design E motor being unable to start the load previously being successfully started by the Design B motor.

This possibility is increased by the fact that motor manufacturers often build their Design B motors with locked-rotor torques significantly above the NEMA allowable torque minimums. In fact, some Design B motors actually deliver Design C locked-rotor torque values.

A Design E motor built at or near minimum locked-rotor torque values could develop as much as 35% less locked-rotor torque than a conservatively built Design B motor with the same horsepower. This could force the selection of a higher-horsepower, higher-priced Design E motor to start the load, absorbing some of the savings anticipated from operating a more energy-efficient motor in the same application.

Also, NEMA Design E locked-rotor torque requirements are identical to international IEC minimums for "N" or Normal Duty motors. This could allow the Design E motor to be manufactured and competitively marketed to compete with offshore products.

The revised NEMA standard also reveals that the minimum allowable breakdown and pull-up torques of the Design E motor are somewhat lower than the Design B motor, especially in the smaller sizes up to 10 hp. This might also dictate the selection of a larger Design E motor to replace a Design B motor for some applications.

Locked-rotor current. Design E motor allowable locked-rotor currents are basically higher, and sometimes significantly higher, than Design B locked-rotor currents. ([ILLUSTRATION FOR FIGURE 4 OMITTED], on page 34.) Relaxing the restraints on current limits is a another way to increase motor efficiency. (See sidebar, "What's involved in manufacturing a premium-efficient motor?" on page 31.) However, a Design E motor could require a larger motor starter or special overload protection to accommodate the greater inrush current experienced during start-up.

Design B motors are built with maximum locked-rotor currents that are below NEMA maximum values (normally within six times full-load current). A Design E motor, however, could draw a locked rotor current as much as 55% higher than that of a conservatively built Design B motor with the same horsepower. (Refer again to [ILLUSTRATION FOR FIGURE 4 OMITTED].)

The National Electrical Code calls for a starter capable of handling a current equal to six times the rated-load current. Under the Design E standard, maximum locked-rotor current can reach eight to nine times the rated-load current.

Many control manufacturers have pointed out that existing Design C starters and motor controls can handle these higher currents. However, if replacing a Design B motor with a Design E motor requires replacing the starter or controller as well, some of the projected energy savings from improved motor efficiency would be lost.

Proceed with caution

The advent of a new standard, the concern with offshore motors designed to different standards, and the continuing pressures from the government and utilities for greater motor efficiency suggest that the motor business will experience continuing change.

Each motor manufacturer seeks to produce more efficient, less costly products to satisfy a rapidly changing market. The Design E motor may be just such a desired product.

However, as Design E motors are introduced, you should exercise care in selecting and applying these motors to avoid costly errors, especially in retrofit/replacement. For example, if a Design E motor is being considered to replace an existing, general-purpose Design B motor, you should compare torque characteristics and current values to ensure that the Design E can do the job without requiring a higher-horsepower rating or a costly motor control upgrade.

RELATED ARTICLE: WHAT'S INVOLVED IN MANUFACTURING A PREMIUM-EFFICIENT MOTOR?

As manufacturers have developed more efficient motor designs within existing NEMA standards, practically all of the most effective techniques to increase motor efficiency have been applied and reapplied. These include variations in lamination steel, slot shape, air gap, ventilation system, etc.

One of the simplest ways to reduce motor [I.sup.2]R losses is to reduce the effective turns in the winding. (This is also a way to squeeze more horsepower out of a particular frame size.) However, the price to be paid is higher locked-rotor current (which may exceed existing inrush standards), diminished service factors, and more severe power system voltage disturbances whet the motor is started.

While NEMA requires open-type Designs A, B, and C motors to have service factors of 1.15 (NEMA Table 12-4), the Design E motor may have a service factor of 1.0 (Sec. 12.51.2). Similarly, Designs A, B, and C are required to meet the no-load sound power levels specified in Sec 12.53.3. Design E is not.

In late 1992, the Energy Policy Act of 1992 (EPACT) was passed; it stated that commonly used Design A and Design B general-purpose motors must be manufactured to meet certain "energy efficient" design requirements by October 24, 1997. This date is fast approaching. To meet this mandate, a great deal has happened within the motor industry.

In the early editions of MG-1, NEMA Standard for Motors and Generators, a Design B motor had to meet or exceed efficiencies listed in Table 12-6B (subsequently renamed Table 129) to display the words "Energy Efficient" on its nameplate. In 1993, a Table 12-6C (subsequently renamed Table 12-10) was added to MG-1 to meet EPACT requirements. The values of Tables 12-9 and 12-10 are represented here in the graph at right.

Note that the efficiencies of Table 12-10 are approximately 0.5% to 4.3% higher than those from 12-9.

In 1994, NEMA issued a revision to MG-1, 1993 that included specifications for a new NEMA "Design E" motor. The revision includes a new Table 12-11. The efficiency values in this table are the full-load efficiencies prescribed for Design E Motors, which exceed Table 12-10 efficiencies. Thus, Design E motors certainly qualify as energy-efficient motors.

Jean J. Revelt is Director of Marketing, Motor Div., Lincoln Electric Co., Cleveland, Ohio.