Heading Off a Kilowatt Crunch

Aug. 7, 2008
With the stroke of a pen, legislators have forced electric motors to get more efficient.

There’s more to know …
Authors: Emmanuel Agamloh, Ph.D., Motor Systems Engineer;

Kitt Butler, Director, Motors and Drives

Advanced Energy, Raleigh, N.C.: advancedenergy.org

The DOE framework document that specifically defines small motors was first published in June of 2007 and can be found here: tinyurl.com/43a49u

A summary of motor-evaluation services from Advanced Energy can be found here: tinyurl.com/4dcyyt

Calculating electric-motor efficiency: tinyurl.com/2tgcte

DOE document: Buying an energy-efficient motor: tinyurl.com/4xg44x

Edited by Leland Teschler

By Emmanuel Agamloh, Ph.D., Motor Systems Engineer
By Kitt Butler, Director, Motors and Drives, Advanced Energy

On December 19, 2007, the Energy Independence and Security Act of 2007 (EISA) became law. This new legislation is vast in scope. It’s designed to “move the U.S. toward greater energy independence and security, increase the production of clean renewable fuels, increase the efficiency of products, buildings, and vehicles, promote research on and deploy greenhouse gas capture and storage options, ….” Included in the 310 pages of legislation is one important page on Electric Motor Efficiency Standards that, for the first time, will impact how machine designers select motors.

The old minimum energy-efficiency standards mainly applied to three-phase general-purpose induction motors. Under the old act, the energy-efficiency levels for induction motors were known as Epact levels and can be found in NEMA MG-1 Table 12-11. The new regulation restates the definition of General Purpose Electric Motors as defined in 10 CFR 431 from the Energy Policy Act of 1992 (Epact) and classifies them as Subtype I. These motors “manufactured (alone or as a component of another piece of equipment)” will be required to have nominal full-load efficiencies that meet the levels defined in NEMA MG-1 (2006) Table 12-12.

This represents a significant boost in motor efficiency and, for the first time, requires OEMs to comply. For instance, a 1.5-hp four-pole TEFC induction motor would be expected to have a minimum efficiency of 84% under the old legislation and 86.5% under the new one.

In addition, the law goes on to define a new category of General Purpose Motors, Subtype II, as motors incorporating design elements of a general-purpose motors (subtype I) that are configured as:

  • U-frame motors
  • Design C motors
  • Close-coupled pump motors
  • Footless motors
  • Vertical solid shaft normal thrust motor

(tested in a horizontal configuration) • Eight-pole motors (900 rpm) • Polyphase motors less than 600 V (e.g., 575 V) Subtype II motors between 1 and 200 hp manufactured alone or as part of another piece of equipment shall have a nominal full-load efficiency not less than as defined in NEMA MG-1 (2006) Table 12-11. Each Fire Pump motor manufactured alone, or as a piece of equipment, must also comply with table 12-11. NEMA Design B motors with horsepower ratings above 200 hp and not greater than 500 hp must comply with NEMA MG-1 table 12-11. The effective date for compliance for all these new regulations is approximately January 2011. Motors manufactured after Dec. 19, 2010 must comply with the new rules defined in EISA.

In addition to these new laws for polyphase motors, small motors (including single phase) are going through a public rulemaking process at the Dept. of Energy that is expected to establish minimum efficiency standards for the first time. It is unclear if this will impact OEMs, as the legislation above clearly does, but it’s possible. A framework document specifically defines small motors. The public can comment and help direct DOE in this rule making process. DOE’s final ruling on small motors is expected to happen close to when the polyphase regulations go into effect (approximately January 2011).

What does this all mean for OEMs, and what should machine designers do today to prepare?

First, OEMs must build-in energy-efficient motors to meet the new regulations when they take effect. This may not be easy, particularly for applications that have matured through hands-on tweaking. On the other hand, the need for a change to a more-efficient motor can be a blessing for OEMs “hooked” to a motor vendor for various reasons. The prospect of getting your motor business may bring out the competitive juices of existing and prospective vendors.

Second, energy-efficient motors could have design specifications differing from those of standard motors. Yet, energy-efficient versions should be able to handle applications without sacrificing performance, reliability or safety. This means conducting a thorough review of equipment design while considering the new motor specifications.

For example, highly efficient three-phase motors such as the NEMA Premium models are designed to have higher speeds than motors with Epact (the 1992 Energy Act) level efficiency. This speed difference may be a consideration for certain applications such as centrifugal loads. Also, in a bid to boost efficiency, it is more likely some highly efficient motor designs may sacrifice locked rotor torque. This compromise is less likely in standard designs. The difference in torque performance may pose a problem for some applications. So an evaluation of torque qualities must be factored into the specification review.

Motor types
Most analyses of motor types begin with a classification into the two broad categories of ac and dc. Most of these are electronically controlled but, for the purposes of clarity, electronically controlled motors would belong to a different and special third category. The accompanying figure shows the different types of motors identified by the Small Motor and Motion Association (SMMA).

Induction motors include single and three-phase versions. Their stators hold copper wires distributed around their circumference in lap or concentric windings. The three-phase induction-motor stator is typical for most three-phase ac motors. The rotors are cast aluminum made in a squirrel-cage configuration. Single-phase induction motors are less efficient than threephase models and typically go into equipment that does not use high power or torque.

Direct-current machines have been widely used because they are relatively simple to control. (High starting torque is another of their strong points.) Today, improvements in power electronics have simplified the control of ac motors to a point where ac is taking over many of the industrial applications that dc formerly dominated. However, there are still niche applications for dc motors, especially in transportation. It is not uncommon to find dc motors operating golf carts, forklifts, and other material-handling equipment. Hand tools almost exclusively carry universal motors, a type of series dc motor that runs from both ac and dc.

Permanent-magnet motors come in both brushed dc and ac versions and serve in various kinds of equipment. The high cost of magnet material has hampered the wide application of these motors. These days, magnet costs are dropping and these motors are becoming more attractive in certain applications.

Most current research in motors focuses on permanent- magnet types and their control. Currently, the permanent- magnet synchronous motor is a strong contender for plug-in hybrid electric vehicles. Brushless-dc motors increasingly find application not only in servosystems but also high-performance equipment such as greens mowers.

New motor technologies such as the switched reluctance motor (SRM) are yet to find widespread use. The SRM is a robust electronically controlled motor that rivals the induction motor in ruggedness: The rotor has no windings and consists of a stacked-lamination core with salient poles. The problem with SRM has been noisy operation and a control scheme that is relatively complicated. But these motors are increasingly applied in heavy off-road equipment and have been proposed in smaller equipment such as washers or automotive systems that can tolerate noise.

Smarter selection
OEM designers always look for motors that are low priced, compact, run quietly, are reliable and weigh little. Efficiency, though important, often takes a back seat in these considerations. In fact, motor manufacturers said in a recent survey that their customers ranked availability, reliability, and price as the top three concerns. But this is about to change as the cost of energy rises and the new efficiency legislation takes hold.

Recently, some motor manufacturers have been promoting induction motors with cast-copper rotors. These motors have been touted as having efficiencies well above the NEMA premium efficiency levels prompting experts to speculate about the possibility of a super premium efficiency category. Advanced Energy recently tested several of these motors with promising results. Although tests substantiated some of these claims, the motors with cast-copper rotors are currently only available in ratings of 20 hp or less. Thus the proposed new level of legislation may remain at levels in Table 12-12 until efficiency improvements above the NEMA Premium level can cover at least the range of 1 to 200 hp.

So far, efficiency regulations only apply to induction motors. Some other motor types actually have higher efficiencies. Generally speaking, PM (permanent-magnet) motors are an example. Because the excitation comes from permanent magnets, one set of windings can be omitted so the losses can be lower than in induction motors. A recent IEEE paper entitled “Permanent Magnet Motors for Industrial Energy Savings” backed up the theory, demonstrating the superior efficiency of PM motors. Author M. Melfi ran tests in which he replaced the squirrel-caged rotor of an induction motor with rotors having two permanent magnets. Melfi saw a significant efficiency improvement in the PM design. He also figures there is room for improvement by perfecting the design going to higher horsepower ratings as well.

Currently, single-phase motors have no minimum energy- efficiency standards, but that’s changing. The DOE is now looking at the area. Some of the work to be done includes zeroing in on standards for the testing. Today’s standard for testing single-phase motors, IEEE Std. 114, is undergoing review. The Canadian standard on single-phase motors, the CSA C747, is also going through a review process. The IEEE standard review process is expected to be complete before the end of the year, in time for voting by the IEEE Electrical Machines Committee. This activity is independent of the DOE process but is important. The general expectation is that this would be the ultimate standard for evaluating efficiency of single-phase motors when a rule is established.

Motor-drive techniques
There’s more than one way to field energy-efficient equipment that meets regulatory requirements. Sometimes it is advantageous to switch to a different category of motor — say, from a shaded-pole motor to a permanent split-capacitor motor. These changes may also involve a change in control scheme.

For instance, single-phase induction machines (specifically, permanent split-capacitor motors) and universal motors are widely used in clothes washers. They are equipped with simple voltage-control techniques. Contrast this with high-end, high-performance machines where three-phase motors are more common. Here variable- frequency control (VFD) schemes can be found.

Several OEMs have explored using SRMs as a possible alternative. However, SRM-control schemes still need work before this kind of swap will become practical. On the other hand, three-phase induction motors are readily available. VFD-control techniques for these motors have been improving significantly and now work well. More important, VFD electronics costs have been dropping. Their result: Three-phase motors are increasingly attractive for low cost models.

An OEM accustomed to a universal motor with simple triac control may now have to learn the ropes with three-phase induction motors and VFD control to get better energy efficiency. OEMs familiar with threephase/ VFD configurations may have to consider competing technologies like brushless dc. In short, there will be a learning curve in motor and controller technologies.

The learning curve is even more daunting if motors in existing equipment have been reliable in the past. Here the OEM faces the tough situation of moving successfully to a new, relatively unfamiliar motor and making it work without encountering warranty issues. The tough calls that arise in the process can be difficult to deal with.

In our experience, OEMs know their equipment extremely well but are not necessarily well versed in the motors. One OEM summed this up recently; “We make compressors, we are not motor experts.” For these companies, the motor-evaluation process can be broadly divided into three steps. First and foremost, a motor-build and inspection analysis (MBIA) should be a precursor to actual testing. This is basically a tear-down analysis of a sample motor to ensure its quality-of-build meets minimum standards for that type of motor and the application in question.

If the quality is below minimum standards, there usually is no benefit in proceeding further with that particular motor. But in a few cases, there is an overriding reason the OEM still wants to work with the motor vendor. Here, Advanced Energy can give a number of recommendations for design/manufacturing modifications to bring the motor up to par.

Second, the sample motor undergoes a performance characterization to determine whether it meets its nameplate specifications and torque requirements of the intended application. If this step is successful, the third step consists of endurance tests to gauge motor behavior under a rigorous application duty cycle. Here internal motor defects can become evident. Some of these defects may not be readily visible during the tear down analysis or the short performance tests already conducted. The endurance tests are designed with the application and the warranty period in mind.

A motor gets aligned on one of the dynos in the Advanced Energy motor test center.

Motor Build Inspection Analysis (MBIA) underway at Advanced Energy labs. Here engineers disassemble motors to better assess the quality of internal features.

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