Motor-Driven Systems Efficiency Update
Electric motors have received predominant attention from energy-efficiency standards and regulations due their vast industrial usage and well-documented design, performance, and test specifications. Major efficiency improvements have been obtained at the motor level but are reaching diminishing returns and economic limits. Recent developments focus on the larger “driven system” connected to the motor for continued efficiency gains.
Minimum efficiency performance standards (MEPS) have been enacted for ac induction motors in various developed countries. That motor type has drawn the main attention because of its huge installed base worldwide. Motor MEPS represent one area where the U.S. has taken global leadership—with Australia, Brazil, and Canada, among others, also taking strong positions. More recently, the European Union has followed suit with mandatory MEPS. In addition, numerous countries have voluntary motor efficiency standards, some of which are making their way toward law.
In the U.S., efficiency standards for induction motors have been widened in stages. Implemented in 1997, EPAct 1992 (Energy Policy Act) applied to general-purpose motors in the 1-200 hp (0.75-150 kW) range. EPAct 2005 mandated that government motor purchases in the 1-200 hp range be at higher NEMA Premium motor efficiency levels. Energy Independence & Security Act (EISA 2007) went into effect in Dec. 2010, extending coverage to motors up to 500 hp (375 kW) rating and to motor designs/varieties previously exempted from MEPS. Motor varieties added in EISA include U-frame, NEMA Design C, close-coupled pump motors, footless motors, vertical solid shaft normal thrust motors, eight-pole (900 rpm), and polyphase motors with voltage of not more than 600 V (other than 230 or 460 V).
Full-load efficiency values for energy efficiency motors are listed in National Electrical Manufacturers Association (NEMA) Standard Publication Motors and Generators (MG 1-2011), table 12-11, and for Premium efficiency motors (random wound and form wound) in tables 12-12, 20-B (motors through 500 hp), and 20-C (medium-voltage motors), respectively.
A further initiative of the U.S. Dept. of Energy (DOE) seeks to draw smaller electric motors into the energy-efficiency picture. The so-called DOE Final Rule, “Energy Conservation Program: Energy Conservation Standards for Small Electric Motors,” published in March 2010 in the Federal Register (10 CFR Part 431), covers general-purpose, open drip-proof, three-phase electric motors—typically below 1 hp rating, but extending up to 3 hp for some types.
Specifically, this DOE ruling applies energy conservation standards to motors ¼ through 3 hp with 2-, 4-, and 6-pole designs and frame sizes 42 through 56. Single-phase, capacitor-start motors of the same power range and pole count, as well as applicable IEC motors and corresponding frame sizes, are also included. Effective date of the ruling is March 2015.
DOE’s small motors efficiency ruling has not met with enthusiasm from motor manufacturers. Objections come from various reasons, such as more diverse applications, different motor types/designs, less recognized testing methods, etc.—relative to the industry’s wide experience with larger induction motors (see Ref. 1, online).
Europe on board
Meanwhile, a series of International Electrotechnical Commission (IEC) standards has been developed in the European Union (EU), covering various areas of ac induction and other motor types. Table 1 summarizes regulations that make up multi-part standard IEC 60034.
Table 1: IEC Global Motor Energy Efficiency Standards
Rating and performance
Standard methods for determining losses and efficiency from tests (excluding machines for traction vehicles)
Specific methods for determining separate losses of large machines from tests—Supplement to IEC 60034-2-1
Efficiency classes of single-speed, three-phase, cage-induction motors (IE-code)
Selection of energy-efficient motors including variable speed applications—Application guide
Specific test methods for determining losses and efficiency of converter-fed AC motors, controlled by a variable frequency drive (VFD)
Source: IEA, 4E Electric Motor Systems and Control Engineering
Motor size range coverage in IEC 60034-30 has been widened over the range of 0.12-500 kW, for 50 Hz units. Higher power ratings up to 800 kW will have flat efficiency limits. More motor types are now included: all fixed-speed, wound-rotor synchronous, single-phase designs, and virtually all brake motor types; as well as variable-speed permanent magnet (PM) synchronous motors, within the speed range of 1,000-5,000 rpm.
IEC 60034-30 defines four international efficiency (IE) classes for ac induction motors: standard efficiency (IE1), high efficiency (IE2), premium efficiency (IE3), and super-premium efficiency (IE4). IE classes correspond roughly to certain MEPS specified in the U.S., namely IE2 to EPAct and IE3 to NEMA Premium values. A still higher IE5 class has been proposed without a qualifying commercial motor being specified or available. However, that motor technology is envisioned as one able to lower losses by 20% compared to IE4 class.
While ac induction motors have been the primary technology of efficiency standards, it’s increasingly harder for that motor type to meet higher IE requirements. This is where other motor types come into play—such as PM synchronous. Upcoming standards will look at line-fed versus converter-fed motors using VSDs (for example, IEC 60034-2-3). Drive harmonic losses and sine filter efficiency will need to be assessed when the VSD is included in the motor system. IEC Technical Committees are also working on other motor topologies like switched reluctance, electronically commutated, and motors specifically built for VSD operation, to be covered in future IEC efficiency standards.
Based on IE class definitions, European Union regulation EC No. 640/2009 has implemented mandatory energy-efficiency requirements in Europe, according to the following timeline:
- IE2 efficiency on June 16, 2011, for motors of 0.75-375 kW power range
- IE3 efficiency on Jan. 1, 2015, for motors of 7.5-375 kW power range
- IE3 efficiency on Jan. 1, 2017, for motors of 0.75-375 kW power range.
The second and third phases of this regulation pose an interesting connection to variable-speed drives. (Also read: “Adding a drive to meet energy regulation.”)
Highest efficiency electric motors provide the largest benefit to applications where motors run at near constant speeds and operate for a large number of hours per year. Among such applications are process gas production and power plant air circulation fans. However, for many other applications, parts of the larger motor-driven system require efficiency evaluation before implementation or assessment for redesign or retrofit.
Going beyond motors
While MEPS developments have highlighted electric motors, the connected equipment has a profound effect on overall system efficiency. As a simple illustration, consider a motor and gearbox combination. Even with quite high-efficiency components—95%-efficient motor and gearbox—the result is an overall “system” efficiency of 90.2%. Obviously, other connected elements would further reduce system efficiency.
It is for this reason that efficiency efforts for motor-driven equipment are gaining traction. However, developing full system efficiency standards becomes progressively harder. Efficiency data are limited or insufficient for the myriad industrial and commercial systems in service. Complexity and the large number of variables present in such systems compound the problem. Another development area is assessment and improvement of partial load performance (for example, at 50% and 25% loads), whether for motors or connected equipment.
International efficiency standards do not exist for full motor systems, but some developments have taken place for motor-pump, motor-fan, and motor-VSD combinations (see sidebar). Reference 2, a comprehensive energy policy publication of the International Energy Agency, discusses regional standards and testing standards for motor-pump/fan systems in Europe.
Several EU efficiency regulations are in draft form for pumps (including building water circulator systems) and fans in combination with a motor and drive. A sampling of upcoming energy-efficiency (EE) regulations includes increasingly stringent energy-efficiency index (EEI) values for circulator pumps with effective dates of January 2013 and August 2015. For fans in the 125 W to 500 kW power range, an Eco-design draft regulation is in place, also with two-tier efficiency requirements to become effective in 2012 and 2015 (Ref. 2).
In the U.S., pumps, fans, and compressors have no regulatory standards at present. However, DOE is working on updating its “best practice” methods for system efficiency, which includes the above motor-driven equipment. NEMA and a consortium of energy advocates are part of that effort (see Table 2), noted John Malinowski, chairman of NEMA’s Motor & Generator Section and senior product manager – AC motors at Baldor Electric Co., a member of the ABB Group. Given the complexity of system-level efficiency, cooperative initiatives make sense to assemble the expertise and synergy of different organizations to develop best practices. In time, such best practices can lead to EE standards.
Table 2: Some Energy Advocates Associated with U.S. DOE and NEMA
ACEEE - American Council for an Energy-Efficient Economy
ASE - Alliance to Save Energy
ASAP - Appliance Standard Awareness Project
NEEA - Northwest Energy Efficiency Alliance
NRDC - Natural Resources Defense Council
Malinowski said the consortium and many other invitees are working to update various DOE publications developed by the agency’s Industrial Technology Program (recently renamed Advanced Manufacturing Office).
Related activities include development of software tools to help implement EE systems, training programs, and technology conferences that educate users about these systems. Notable biennial conferences include Motor Summit, held in Zurich, Switzerland (next edition in Dec. 2012), and Energy Efficiency in Motor Drive Systems (EEMODS), held for the first time in the U.S. in Sept. 2011 and scheduled next for Brazil in 2013. Another conference to promote energy-efficient systems is being organized by NEMA, DOE, and others for the April-May 2013 timeframe, Malinowski added.
Tools, retrofits, education
Various software tools and best practices are already available. The following examples are free online software tools available from DOE’s Advanced Manufacturing Office:
- Fan system assessment tool (FSAT)—helps quantify energy use and savings opportunities in industrial fan systems (Ref. 3).
- Pump system assessment tool (PSAT)—enables calculation of potential energy and cost savings for pumping systems. Pump performance data come from Hydraulic Institute standards and motor performance data from NEMA’s MotorMaster+ database (Ref. 4).
- AirMaster+—helps analyze energy use and potential savings in industrial compressed air systems as a baseline for existing application or to model future ones (Ref. 5).
Other notable collaborative initiatives in the arena of pumps and compressed air systems include Pump Systems Matter (PSM) and Compressed Air Challenge (CAC). PSM and CAC assist users of these respective systems to improve energy efficiency and cost savings through product-neutral information, training, and education (Refs. 6 and 7).
The Air-Conditioning, Heating, and Refrigeration Institute (USA) has recently issued AHRI Standard 1210 (I-P), Performance Rating of Variable Frequency Drives, for the control of ac induction motors. The standard includes speed/torque test point requirements for drive system efficiency and power line harmonics, as applicable to constant and variable-torque applications.
In Europe, future standards will address increasingly wider systems. One example presented at Motor Summit 2010 by Dr.-Ing. Martin Doppelbauer, convenor of IEC TC2 working group 31, is IEC 528xx, which will cover efficiency of variable-speed drives, power drive systems, and “complete drive systems” (Ref. 8).
Power transmission efficiency is an additional area of major energy savings potential. It merits detailed attention when implementing any new system. However, worthwhile retrofit opportunities can also arise. A case in point is upgrading of cooling tower fans used in various industrial and commercial facilities. Baldor Electric’s Malinowski mentions the replacement of an inefficient worm-gear reducer and two-speed induction motor combination with an IE4-class, direct-drive interior permanent magnet synchronous motor (see photo) and VSD to save more than 50% on electric usage and obtain other benefits. A relatively simple retrofit eliminated the need for the gear reducer.
In summarizing the perspective for motor-system efficiency, Malinowski said, “We have used component replacement for efficiency upgrades for many years. Although this is a good practice, it yields only modest savings.” What’s needed is to look at the complete “system” from the power distribution transformer to the smart starter-motor and drive, and continue to the mechanical transmission components and driven load.
“Optimization of all components within the complete system—as well as use of process control—can lead to strong double-digit savings, increased productivity, and reduced downtime,” Malinowski concluded.
- Frank J. Bartos, PE, is a Control Engineering contributing content specialist. Reach him at braunbart(at)sbcglobal.net
See the links below and "Adding a drive to meet energy regulation" at the bottom of this article.
Ref. 2 - "Energy-Efficiency Policy Opportunities for Electric Motor-Driven Systems," International Energy Agency (2011)
Ref. 3 - Fan System Assessment Tool
Ref. 4 - Pump System Assessment Tool
Ref. 5 - Air Master+
Ref. 6 - Pump Systems Matter
Ref. 7- Compressed Air Challenge
Ref. 9 - "Looking at electric drive efficiency"