Energy-Efficient Motors Deliver Savings
In simplest terms, energy-efficient electric motors are high-quality versions of standard motor products. They pack more of "active" electric materials (steel laminations and copper) into essentially the same physical package—hence carry a price tag 15-30% higher than their less-efficient cousins.
In simplest terms, energy-efficient electric motors are high-quality versions of standard motor products. They pack more of 'active' electric materials (steel laminations and copper) into essentially the same physical package—hence carry a price tag 15-30% higher than their less-efficient cousins. That's a premium often worth paying, with electricity charges taking nearly all of many motors' lifetime operating cost.
The purchasing process for electric motors tends to mask true lifecycle cost. Since lower procurement price is the goal of most purchasing departments, buying less costly and lower efficiency motors seems attractive at first, without attention to huge operating expenses that accrue over a long product life. Such costs are well understood by plant engineers and maintenance personnel, yet they're not typically involved in what's purchased. Also, company business climate can dictate minimum spending, just to keep the plant running. Top management and purchasing people need to become more aware of the impact of equipment efficiency on business.
Why target electric motors for energy efficiency? They account for a dominant portion of electric energy consumed by industry—as well as a substantial share of total electricity used in developed nations. For the U.S., estimates by the Department of Energy (DOE) are as much as 63% industrial usage and 25% total usage (see more figures in Online Extra article). While focus is on motors here, concern for energy efficiency also applies to other elements of the total power conversion system, as discussed later.
Induction motor focus
Due to magnitude of electricity usage and a 'workhorse' role in industry, ac induction (asynchronous) motors have so far received virtually all efficiency efforts, including development of products, standards, and legislation. While energy-efficient (EE) motors have been available for at least 20 years, for petrochemical and other severe-duty industrial applications (per IEEE Std. 841), it took more recent legislation and other initiatives to bring energy issues into focus.
The U.S. has led the way with the only known law for EE motors, with the Energy Policy and Conservation Act (EPAct) of 1994—effective Oct. 1997—that sets nominal and minimum efficiency levels for general-purpose 3-phase motors of 1-200 hp size sold or imported into the U.S. This remains a surprising step, given energy-usage concerns in Europe and numerous technology directives issued by the European Union (EU). EPAct applies to open (ODP) and enclosed (TEFC) foot-mounted 2, 4, and 6-pole motors running on 230/460 V supply. EPAct efficiency values reach 95.0% for a 200-hp, 4-pole unit.
The EU and European Committee of Manufacturers of Electrical Machines and Power Electronics (CEMEP) followed shortly after EPAct with a classification and labeling agreement (not law) for nominal efficiency of 2 and 4-pole motors in the 1.1-90 kW (1.5-120 hp) range operating at 400 V and 50 Hz. This standard defines three efficiency classes: Eff1 (high efficiency), Eff2 (improved efficiency), and Eff3 (standard)—a lower efficiency class that the EU is pushing manufacturers to discontinue.
A voluntary agreement among some 36 European manufacturers promised to cut the joint share of Eff3 motors to 50% by the end of 2003, which has been accomplished, explains Sven Sjöberg, member of CEMEP and VP of motor marketing at ABB in Sweden. 'Reduction of Eff3 motors has been much more successful than the 50% demanded by the EU Commission,' he says. 'However, due to the cost of moving from Eff2 to Eff1 motors, increase of Eff1 motors' share has not been that dramatic (7.8% for 2-pole motors; 5.2% for 4-pole motors in 2003).' Sjöberg puts ABB's share of Eff1 motors produced at 17.7% and 16.6%, respectively, with no Eff3 units made in the last three years.
The motor efficiency bar was raised in mid-2001 by the harmonization of standards from the National Electrical Manufacturing Association (NEMA) and the Consortium for Energy Efficiency (CEE) into a trademarked specification known as 'NEMA Premium.' The specification expands coverage to ODP and TEFC motors in the 1-500 hp range (2, 4, and 6-pole) at inputs up to 600 V. NEMA Premium (NP) also expands its scope to medium-voltage 250-500 hp, special-purpose motors with inputs to 5 kV and mountings not covered in EPAct. NP nominal efficiencies are higher than EPAct, topping out at 96.2% for the largest low-voltage motor—a 500-hp, 4-pole unit.
As more standards and efficiency initiatives evolve, other countries adopt or generate their own EE motor standards (see more online).
U.S. EPAct law was a major step forward, but it had limited scope. It could be expanded, but that would require another 'act of Congress'—unlikely in the current agenda. One result is that NEMA Premium has become the de facto efficiency standard. 'No changes are expected to EPAct regulations in the foreseeable future,' says Robert Boteler, director of marketing for Emerson Motor Technologies. 'NEMA Premium has gained acceptance with a wide range of stakeholders, which continues to increase.' He doesn't believe that federal standards will be expanded to cover additional motor products.
Enforcement of EPAct has gone reasonably well. The U.S. DOE, under its authority for EPAct, works with motor manufacturers to ensure that regulations are followed. 'To date, 63 compliance numbers have been issued, over half of them for imported motors,' adds Boteler. It indicates products meet compliant efficiency.
John Malinowski, Baldor Electric Co. product manager for ac and dc motors, concurs about rising sales of NEMA Premium motors, helped by regional incentives. 'Rebate programs are alive on the West Coast, Northeast, and parts of the Midwest,' he says. As for European developments, Malinowski states that CEMEP recently indicated raising motor efficiency levels, 'but this is not yet defined.' CEMEP Eff1 band begins at a point equivalent to our EPAct, but doesn't define any higher efficiency level, such as NEMA Premium.
Rockwell Automation likewise lauds the NP program. 'NEMA Premium continues to gain recognition as the highest standard for induction motor efficiency,' explains Dale Basso, product manager for Reliance ac motors. 'Most utility incentive programs and customer specifications calling for premium efficiency, reference NEMA Premium as the standard to meet.'
Basso also mentions incentive rebates offered through electric utilities in some states, encouraging users to apply premium-efficient motors and variable-speed drives. 'These utilities consider incented savings as 'purchasing' future capacity growth as opposed to having to 'build' it,' he adds.
So far, the EU Commission favors voluntary agreements and has not 'pushed' for any motor efficiency legislation, explains ABB's Sjöberg. The Commission wants to increase Eff1 motor share up to 30%, to which CEMEP replied that some subsidy to end-users is needed from the EU or local governments.
Testing: not so simple
Complying with efficiency standards requires verification by test. However, no universal test method exists because of regional practices or origin of the standard. Moreover, the seemingly simple concept of determining efficiency (ratio of power out to power in) breaks down in practice due to measurement accuracy. Rigorous motor testing also is physically complex and costly—especially for larger units.
Two main test methods in use are: IEC Standard 60034-2 and IEEE Standard 112, Method B. Mainly used in North America, IEEE 112 B is being referenced and adopted elsewhere. IEC 60034-2 is simpler to use because it merely assumes a fixed value for stray load losses (0.5% of motor input). Reportedly, the IEC method overestimates efficiencies by up to 2% for motor sizes up to 10 kW and underestimates them for sizes above 700 kW. 'IEEE 112 B and equivalent Canadian Standards Association C390 test methods continue to be the only standards that actually measure all losses in the motor and accurately reflect true efficiency,' states Malinowski. Thus, results from different tests cannot be directly compared.
He says, CEMEP (using the IEC approach) doesn't embrace higher efficiencies provided by NEMA Premium motors, which CEMEP suggests are impossible on 50 Hz machines. 'In fact they are possible and actually might be higher,' says Malinowski. 'Less friction, windage, and lower stray load losses because of the lower speed are inherent advantages of 50 Hz.'
System view, lifecycle cost
Electric motors comprise a prime energy-efficiency target—particularly for myriad applications requiring constant speed and long running times. If the application calls for speed and load changes, adding an adjustable-speed drive (ASD) nets great energy savings. Motor efficiency remains vital, but may not mean choosing the highest efficiency model available.
'Highest efficiency gains will not come from motors alone, but a redesign of the system,' says Malinowski. He cites possible 20% efficiency improvement by changing gear reducer design compared to raising motor efficiency by 1-2%. Such redesign may further allow use of a substantially smaller motor with more savings.
Emerson's Boteler, says 'The greatest potential saving opportunities are in the system, which is driven by the motor. System gains are not as easily understood as motors, requiring input of people who understand operations and processes to allow them to select and apply products to optimize efficiency.' He also mentions large efficiency gains (30-40%) possible from improved gearing design and use of an ASD—if it fits the application.
Rockwell's Basso relates cost-effective energy savings to proper application and management of motors. He mentions an industry association campaign, Motor Decisions Matter (MDM), that offers customers insights to many underlying factors of energy efficiency, along with motor management programs of manufacturers and service providers. 'Tools now exist that allow users to measure load and efficiency of their motors without removing them from the application,' he states. 'These tools serve the motor repair/replace planning process.'
Capital cost of equipment weighs heavily on investment for efficiency. 'Many companies still do not understand the concept of lifecycle cost. Often there is no central group responsible for energy use and efficiency standards,' comments Malinowski.
ABB's Sjöberg notes knowledge isn't enough to overcome initial cost concerns. He believes total lifecycle cost is understood by some end-users—in particular those specifying Eff1 motors in the process industries that run more than 6,000 hours per year. 'But most end-users are not willing to pay about 10-25% more for Eff1 motors compared to Eff2 motors,' he adds.
A major development for induction motor efficiency is replacement of traditional aluminum rotor bars with copper bars. Nearly 60% higher conductivity for copper is the primary draw. However, tradeoffs like a more complex higher temperature die-casting process, some axial dimension growth, and higher-cost motors must be justified. Still, the need to meet emerging efficiency standards worldwide makes copper-rotor induction motors a reality.
Some large motors—above-NEMA units (250 hp and larger)—already use copper-bar rotors, according to Malinowski. For smaller general-purpose motors, the technology is under prototype development in the U.S. 'The long anticipated cast-copper rotor is getting closer. It is estimated that a copper (instead of aluminum) rotor can reduce losses of a motor by 10-15%, says Basso of Rockwell.
The copper industry has made significant strides to develop methods for die-casting rotor bars in copper instead of aluminum, explains Emerson's Boteler. Information, marketing, and technical development activities of the Copper Development Association (CDA) are aiding the process. 'Motor manufacturers will be monitoring the copper bar process closely to determine how it could affect future designs, looking at total motor performance that includes efficiency as well as other parameters necessary to meet motor users' requirements,' he says.
Actually, some non-U.S. companies are already there, producing commercial products. SEW-Eurodrive offers a line of Eff1 (and global equivalent) copper-rotor motors up to 37 kW made in Germany. Tim Schumann, corporate engineer at the company's U.S. facility, verifies availability of DTE/DVE Series copper-rotor motors, and notes the tough die-casting process that had to be mastered to produce the copper bars. 'These motors are priced 15-20% higher than an EPAct-quality motor of the same size,' he adds.
Other motor types, sizes?
Economic reasons have focused efficiency regulations and standards on 3-phase induction motors. Why not expand certification to other motors like single-phase designs or other types of smaller polyphase motors? Besides the huge task, several factors work against the proposition. Efficiency issues have not been sufficiently validated for these motors in terms of how they're used (load-speed profile, operating hours, energy savings), acceptable test methods are not available and, again, up-front costs remain a drag.
Rockwell's Basso mentions the case of permanent magnet (PM) synchronous (servo) motors, which are increasingly associated with adjustable-speed drives. He says, '15-25% of motor losses can be eliminated with permanent magnet rotor technology. The PM magnet rotor goes beyond copper, by eliminating all the rotor losses, and can lead to smaller motors for a given power output.'
There is indication that well-made PM synchronous motors have efficiencies comparable to EE induction motors. However, dynamic performance rather than efficiency is the main consideration for these motors. 'At this point, users simply are not concerned about PM magnet motor efficiency,' explains Basso.
Perhaps with added awareness for the cost of inefficiency and development of acceptable new test methods, smaller motors and types other than 3-phase induction may come under the efficiency microscope in the future.
Motors are heavy‘consumers’
Electric motors are justifiably targeted for efficiency improvement because of their heavy consumption of electric power nationwide—see main article. Motors employed in industry, represented by a huge installed base of 3-phase induction motors, take even larger share of total industrial power usage. Experts estimate that in some U.S. process industries, such as mining and cement production, motors use over 70% of total electric power.
Moreover, this is a worldwide concern. In Australia, 30% of electric demand reportedly goes to 3-phase induction motors. This motor type in the 1-200 hp range typically accounts for 10-20% of Canada’s total annual electricity consumption. Finland is said to consume 80% of its electric power generation to run induction motors.
More efficiency-related standards evolving IEEE Std. 841-2001, “IEEE Standard for Petroleum and Chemical Industry-Severe Duty Totally Enclosed Fan-Cooled (TEFC) Squirrel Cage Induction Motors-Up to and Including 370 kW (500 hp),” is currently being revised and will adopt NEMA Premium efficiency levels, according to John Malinowski, product manager for ac and dc motors at Baldor Electric Co . “These are rugged motors find wide application outside of petrochemical plants, including pulp and paper, mining, cement processing, and other process industries,” he says.
Malinowski notes that Australia and New Zealand have a very progressive ongoing energy-efficiency program. These countries have announced new, higher efficiency standards for motors to their Minimum Energy Performance (MEPS) requirements. From April 1, 2006, MEPS levels for 3-phase motors will be revised to more stringent levels. The “High Efficiency” level from 2001 will become the new MEPS (minimum) level in April `06 and—as of April 1, 2005—a revised “High Efficiency” level also has come into force. New MEPS and high efficiency requirements, along with transition arrangements, are detailed in AS/NZS 1359.5-2004 (published in September 2004). The standard, based on both IEC and IEEE 112 testing methods, supersedes AS/NZS 1359.5-2000.
See more at www.energyrating.gov.au/motor2.html
New copper-rotor motors offered by SEW-Eurodrive , described in the main article, are intended to meet standards such as Australia’s MEPS requirements, among other evolving efficiency-related initiatives, explains Tim Schumann, corporate engineer at the company’s U.S. facility.
Another standard, IEC 61972, attempts to resolve differences between European and North American efficiency levels, which derive from different testing methods. Measurement method of IEC 61972 is based on IEE 112 B. Although IEC 61972 is in force, it’s unpopular in Europe because of extra procedural steps and its calculation results that yield lower motor efficiency values, according to SEW-Eurodrive. Australia and New Zealand currently apply IEC 61972.
CEMEP’s (European Committee of Manufacturers of Electrical Machines and Power Electronics) voluntary agreement with 36 European motor manufacturers to promote Eff1 and Eff2 motors—as well as cut the joint share of Eff3 motors being produced (see main article)—has been renewed and extended to 2008.
Next phase of energy conservation, with a system view in mind, is being addressed by U.S. industry and association groups, such as Electrical Apparatus Service Association Inc. (EASA) , Motor Decisions Matter (MDM) , and large system integrators, for example, Emerson Process Management, explains Robert Boteler, director of marketing for Emerson Motor Technologies . “At Emerson, multiple divisions work to optimize and support customer systems needs whether it be motors, drives, controls, measurement, flow, or valves,” he says.
The Consortium for Energy Efficiency (CEE) originally defined efficiency levels for motors in EPAct’s 1-200 hp range, but at somewhat higher values. Later CEE’s spec was combined into the National Electrical Manufacturing Association (NEMA) initiative to form the current NEMA Premium program for motors in the 1-500 hp range (see main text). Presently, NEMA Premium represents the highest efficiency standard for induction motors. Utility rebate programs often reference CEE’s guidelines. A summary of CEE member utility programs is listed on the organization’s Web site, and also linked from MDM ’s site.
An ongoing U.K. energy saving project—called the ECA (Enhanced Capital Allowance) program—allows end-users to depreciate the total cost for Eff1 motors during the first year of operation. Even this incentive has not given substantial lift to wider use of Eff1 motors, according to Sven Sjöberg, member of CEMEP and VP of motor marketing at ABB in Sweden.
The EU Commission also has been running the Motor Challenge Programme (MCP) since 2003. MCP is a voluntary program through which industrial companies commit to improve energy efficiency measures of motor-driven systems. “Under this program, CEMEP continues to supply statistics showing the development of motors in different efficiency classes and also where most of the motor suppliers are endorsers, which means that we promote the MCP,” adds Sjöberg.
Those interested in energy-efficient motors and related topics will find numerous available sources. Here are a few suggestions.
IEEE Paper PCIC-2003-22 , “Advances in Construction Techniques of AC Induction Motors Preparation for Super-Premium Efficiency Levels. (Authored by Baldor Electric Co. engineers.)
Cut losses, see more efficiency
It starts with overall product quality: more precise tolerances/components, higher quality materials, and good manufacturing. However, quantity as well as quality characterizes energy-efficient (EE) motors. Compared to standard products, EE motors contain a lot more of high-grade steel laminations and copper—so-called 'active' electric materials (see diagram). EE motors raise efficiency by cutting power losses inherent in all motors. Power losses typically fall into five categories:
Stator resistance (I 2 R )—current losses in stator windings;
Rotor resistance (I 2 R )—current losses in rotor bars and end rings;
Iron core —magnetic losses in laminations, inductance, and eddy-current losses;
Friction and windage —mechanical drag in bearings and cooling fans; and
Stray load —magnetic losses in air gap between stator and rotor; also miscellaneous losses due to motor manufacturing quality.
The first three loss categories are most fertile for reduction through newer designs and application of active electric materials. More, and thinner gauge, special steel laminations help cut core losses, while copper rotor bars (see main text) reduce rotor resistance. All efficiency measures add to motor cost.
EE motor characteristics include a flatter efficiency curve at reduced motor loads and somewhat higher base speed due to lower slip than a standard motor. Less slip may or may not benefit some applications. Larger motors achieve higher efficiency values than smaller size motors.