Design E Motor: You May Have Problems
Sep 1, 1999 12:00 PM, By John DeDad, Editor-in-Chief
You must compare torque characteristics and current values to ensure the Design E can do the job without requiring a higher-horsepower rating or a costly motor control upgrade.
Caution is the word. Yes, motor manufacturers seek to produce more efficient, less costly products to satisfy a rapidly changing market. The Design E motor may be just such a desired product. As these manufacturers introduce Design E motors, you must be careful in selecting and applying them to avoid costly errors, especially in retrofit or replacement. For example, if you're considering replacing a general-purpose Design B motor with a new Design E motor, you must compare torque characteristics and current values to ensure the Design E can do the job without requiring a higher- horsepower rating or a costly motor control upgrade.
Design B or Design E? NEMA established speed-torque parameters 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 62 the full-load current.
Many manufacturers' existing premium efficient Design B motors already exceed efficiency values in NEMA Table 12-10. (See sidebar, "History Leading up to the Design E motor.") In fact, some approximately match the new Design E figures. Also, you must realize the Design E motor torque curve is 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 will be the difference between the new Design E motor and a Design B motor that already exceeds the efficiency values in NEMA Table 12-10?
Here's what to watch out for. With 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 the motor can start; and
• Locked-rotor current, which is the peak current flowing 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 torque for specific hp ratings are lower than Design B values (as shown in NEMA Table 12-2), particularly in the smaller sizes. It's possible 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 that same load.
This possibility increases because motor manufacturers often build their Design B motors with locked-rotor torque values above the NEMA allowable torque minimums. In fact, some Design B motors actually deliver Design C locked-rotor torque values.
So, 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 you to select a higher-horsepower, higher-priced Design E motor to start the existing load. The result: The loss of some of the savings anticipated from operating a more energy-efficient motor in the same application.
The revised NEMA standard is noteworthy in another aspect. It reveals that the minimum allowable breakdown and pull-up torque values of the Design E motor are somewhat lower than those of the Design B motor. This is true in smaller sizes up to 10 hp. Again, this may force you to select a larger Design E motor to replace a Design B motor for some application.
Some Design E characteristics are beneficial. For example, NEMA Design E locked-rotor torque requirements are identical to international IEC minimums for "N" or Normal Duty motors. This allows manufacturers to make and market Design E motors to compete with offshore products.
Locked-rotor current. Relaxing the restraints on current limits is another way to increase motor efficiency. (See sidebar, "What's involved in manufacturing a premium-efficient motor?") This is why Design E motor allowable locked-rotor currents are higher, and sometimes significantly higher, than those of Design B motors. So, you may need a larger motor starter or special overload protection to accommodate the greater inrush current of the Design E motor during start-up.
Manufacturers build Design B motors with maximum locked-rotor currents below NEMA maximum values (normally within 62 full-load current). A Design E motor, however, can draw a locked rotor current as much as 55% higher than that of a conservatively built Design B motor with the same horsepower.
The NEC calls for a starter capable of handling a current equal to 62 the rated-load current. Under the Design E standard, maximum locked-rotor current can reach 82 to 92 the rated-load current.
Many control manufacturers point out that existing Design C starters and motor controls can handle these higher currents. But, if you're replacing a Design B motor with a Design E motor, you may have to replace the starter or controller as well. And, this will diminish the projected energy savings from improved motor efficiency.
Sidebar: History Leading Up to the Design E Motor
In late 1992, Congress passed the Energy Policy Act of 1992 (EPACT); 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. To meet this mandate, a great deal 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 12-9) to display the words "Energy Efficient" on its nameplate. NEMA added a Table 12-6C (subsequently renamed Table 12-10) to MG-1 to meet EPACT requirements. We're showing the values of Tables 12-9 and 12-10 in the accompanying graph. 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 included 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.
Sidebar: What's Involved in Manufacturing a Premium-Efficient Motor?
As manufacturers develop more efficient motor designs within existing NEMA standards, they apply (and reapply) practically all of the most effective techniques to increase motor efficiency. These include variations in lamination steel, slot shape, air gap, ventilation system, etc.
One of the simplest ways to reduce motor I2R 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 when you start the motor.
While NEMA requires open-type Designs A, B, and C motors to have service factors of 1.15 (NEMA Table 12-4), a Design E motor may have service factor of 1.0 (Sec. 12.51.2). NEMA requires Designs A, B, and C meet no-load sound power levels (Sec. 12.53.3). Design E is not required to do so.