Efficiency to the Masses —of Electric Motors, that Is

Energy efficiency is hardly a new theme. In a technologically expanding world, it has become crucial to come to grips with energy consumption yet still use energy wisely. A basic question to answer is, "Where can energy be saved efficiently"?Electric motors, particularly in industrial usage, represent a large portion of electric energy consumption.

07/01/1998


KEY WORDS

 

  • Motors and motor control

  • Standards and regulations

  • AC motors

Sidebars:
EPAct-related Motor Terms

Energy efficiency is hardly a new theme. In a technologically expanding world, it has become crucial to come to grips with energy consumption yet still use energy wisely. A basic question to answer is, "Where can energy be saved efficiently "?

Electric motors, particularly in industrial usage, represent a large portion of electric energy consumption. It is said that well over 50% of electricity generated in the U.S. goes to power electric motors of all sizes. And industry consumes a significant part of it. Probably at the high end of the range, Finland uses 80% of its electric energy to power ac induction motors—the workhorse of industry.

Energy efficient ac induction motors are likewise not a novelty. Most motor manufacturers have some time ago introduced such "premium"-efficiency products in the 1-200 hp (0.75-150 kW) range. These met a market need, not a legal regulation. But they also carried a premium price tag versus standard-efficiency motors. Thanks to enlightened users and consideration of life-cycle energy savings rather than initial cost, their numbers have grown—to nearly 20% of units sold in this size range, according to U.S. Electrical Motors, Div. of Emerson Electric (USEM, St. Louis, Mo.).

One large user of premium-efficiency motors is the DuPont Co. Among more than 100,000 motors driving production equipment throughout DuPont facilities, over 2,000 are energy-efficient NEMA- frame motors from Rockwell Automation/Reliance Electric Co. (Cleveland, O.) that saved over $500,000 in one year.

However, something more was needed to bring into line the millions of other ac induction motors designed for "standard" efficiency.

Making it legal

The U.S. and its North American neighbors have taken the lead to put energy efficiency into legislation. This is somewhat of a surprise in view of concerns for energy availability and cost in other parts of the world.

Motor efficiency legislation in the U.S.—part of broader energy conservation issues—stems from the Energy Policy and Conservation Act (EPCA) of 1975, passed in response to the oil crisis of the early 1970s. EPCA led to the more familiar Energy Policy and Conservation Act (EPAct) enacted in 1992 and which became effective in October 1997. (See CE , Sept. '97, p. 147 on EPAct relative to electric motors.)

EPAct's intent is clear. Save electric energy through efficiency regulation of the most numerous motor sizes and types in use—not all motors. Motors covered by EPAct include 1-200 hp general-purpose T-frame, single-speed (6 pole/1,200 rpm, 4 pole/1,800 rpm, 2 pole/3,600 rpm) foot-mounted, polyphase (3-phase) squirrel-cage induction motors of NEMA (National Electrical Manufacturers Association) Design A and B, continuous rated, operating on 230/460 V and constant 60 Hz line power as defined in NEMA Standard MG1-1993.

Nominal efficiency

Efficiency requirements in EPAct come from the so-called "Table 12-10'' of NEMA Std. MG-1. The table (below) sets nominal efficiencies for 113 motor models, which is a matrix of 19 sizes, three speeds (number of poles), and two enclosure types—less one speed variation for the smallest open model. Note that EPAct efficiencies are well below those of premium-efficiency motors.

Nominal efficiency is the average efficiency of a population of motors identical in design and manufacture. Material and production variations can make the efficiency of a given motor different from its nameplate indication.

It's interesting that the NEMA standard also defines a minimum efficiency for these motors to limit deviation from the nominal value. But "neither EPAct nor the DOE [U.S. Department of Energy] proposed rule makes reference to the minimum efficiency for each nominal efficiency," says Roger H. Daugherty, consulting engineer at Rockwell Automation/Reliance Electric.

Minimum efficiency in the NEMA MG-1 specification is based on losses (not efficiency).

EPAct 92 concentrates on general-purpose (GP) motors, while exempting definite-purpose (DP) and special-purpose (SP) motors. However, definitions in technology can vary among users and organizations. DOE uses an expanded interpretation for the GP category to include motors defined as definite purpose by current industry practice but, in fact, usable in most GP applications, explains Dr. Daugherty. "Definition of SP motors is the same common one found in NEMA MG-1," he adds (see motor definitions box).

Examples of the GP motor category's wider definition include motors with thermal protection, roller bearings, and intermediate power ratings. However, an extension to no later than Oct. 25, 1999 has been allowed for these motors to comply.

Thus, coverage is not limited to the standard power ratings tabulated in EPAct. After the extension date, intermediate motor sizes that fall midway (or above) between two standard ratings must meet the efficiency of the next higher rating; if below the midpoint, they must meet the next lower efficiency rating in the table.

IEC 'equivalents,' other legislation

Motors of IEC (International Electrotechnical Commission) design—or IEC equivalents to NEMA frame sizes—are also included. IEC Design N motors have comparable operating characteristics to NEMA Design A and B (diagram). IEC motors are imported for sale in North America, or part of imported equipment, or locally built for special needs. Inclusion of IEC motors in the DOE enforcement rule serves a further purpose, according to Rob Boteler, director of marketing at U.S. Electrical Motors. "Including IEC-equivalent motors provides consistency between EPAct and similar legislation in Canada," he says.

Proposed DOE rules provide standard kilowatt equivalents to motor horsepower for IEC-style motors in the nominal efficiency table. Conversion by the standard formula is also allowed. Intermediate power ratings of IEC-style motors are handled by the same "fit" rules as for NEMA motors.

NRCan (Natural Resources Canada), part of Canada's Energy Efficiency Regulations, appears to be even more stringent than EPAct. From input supplied by USEM, Canada already covers motors with intermediate power ratings, roller bearings, and thermally protected types—as well as motor voltages in the 200-575 range. Integral gear motors (not part of EPAct) will be added to NRCAN in 1999. Explosion-proof motors are to be covered in 1999, same as in EPAct.

Mexico is likewise on the scene. Its legislation, called NOM 74, matches EPAct's efficiency levels, says Mr. Boteler. There's indication that other countries are becoming interested in adopting efficiency ratings along EPAct's lines, as well.

Where's the loss

To make a motor more efficient the ratio of power output to input must be increased. The difference between those two quantities make up motor losses—generally defined as no-load and load-dependent losses (see table). Friction and windage losses (bearing friction, fan and rotor windage, etc.) and core losses comprise no-load losses, so-called because their magnitude is basically independent of motor loading. GE Industrial Control Systems (GEICS, Fort Wayne, Ind.) defines core losses as a combination of hysteresis and eddy current losses in the motor's steel core.

At-load losses include three effects: Stator losses (product of stator input current squared and stator resistance at operating temperature); rotor losses (product of induced rotor current squared and rotor resistance); and stray-load losses . Stray losses come from "additional harmonic and circulating current losses in the magnetic steel and windings," according to GEICS. They're inherent to the motor's design and manufacturing processes.

GE puts no-load losses typically at 30% of total losses, while load losses account for the remaining 70%. Leeson Electric Corp. (Grafton, Wis.) notes similar figures—1/3 or less of total motor losses for friction, windage, and core losses. Stator, rotor, and stray losses increase with load, accounting for 2/3 or more of all losses in a motor operating under typical loading, says Leeson.

Looking at efficiency specifics

What does it take to design and manufacture motors to meet EPAct? The common, short answer from most manufacturers is: add more "active materials"—namely, steel, copper, and aluminum—relative to standard motors.

All motor manufacturers stress the importance of higher quality, low-loss steel laminations for stators and rotors. Steel quality plus using more and thinner laminations help cut hysteresis and eddy currents losses. Michael Offik, Reliance Electric's product manager for modified motors, notes that all such design refinements went into Reliance's EPAct-compliant motors. "The result increases length of the lamination stack, but clever design can keep it in the same motor frame," says Mr. Offik.

Longer stators (and rotors) also result from the need to provide much larger slot areas to house an extra volume of copper windings. The added windings cut stator I2R losses. Similarly, "rotor I2R losses are improved through redesign of the rotor slots to increase conductor cross section," according to GEICS.

GE, Leeson, and others report that stray losses can be reduced by attention to various design details. Examples are tighter, more uniform control of the air gap, and finer finish on the rotor surfaces.

For most motor producers, EPAct meant a critical overview of their existing products. Joe Howard, senior application engineer at GEICS, says, "EPAct translated into performing a benchmark between our standard and high-efficiency motor lines. It was also an opportunity to go back to fundamentals, allowing us to look at processes and variables more closely." Among tasks deemed necessary at GE, Mr. Howard lists precise evaluation and location of motor losses, use of SPC to continuously manage manufacturing processes via statistical methods, and the upgrading of test procedures.

A chance to step back and re-examine the whole production process was also the case at Baldor Electric Co. (Fort Smith, Ark.). Baldor further looked at electrical and mechanical aspects of design—including laminations, windings, and the manufacture/assembly of rotors, according to Randy Waltman, vp of engineering. "An overall better motor is the result. User benefits include a cooler running motor with improved rotor balancing to one-half of NEMA vibration requirements [though not part of EPAct]" he says.

At MagneTek Motors & Generators (St. Louis, Mo.) EPAct's impact meant "redesign of the bulk of the product line. This contrasts with 'first-cost' methods of earlier designs," explains Jerry Calvert, product manager of NEMA-frame motors. MagneTek's basic design concept for EPAct requirements is said to have a "different magnetic circuit." Novelty seems to be in how the materials are used, although Mr. Calvert does not elaborate. New manufacturing processes further provide better control of air gap size and concentricity.

"All motor manufacturers are affected by EPAct," comments Mark Hodowanec, senior product engineer at Siemens Energy & Automation's motor facility in Norwood, Ohio. He notes that better quality steel for laminations and more copper are basic to improve motor efficiency, but sound design principles and good process control are just as important. "The idea is to add the least amount of active materials from the cost side and still meet the necessary efficiency requirements," explains Mr. Hodowanec.

Motors sold as well as made in the U.S. are covered by EPAct. Some foreign manufacturers recognize the implications and are ready with products. WEG Exportadura S.A. (Jaraguá do Sul, SC, Brazil)—the large, rapidly expanding Latin American motor manufacturer—includes NEMA models in its offerings. J.P. Silva, sales manager, special projects at WEG Electric Motors Corp. (Rochester, N.Y.) states, "Meeting EPAct requirements should not be a big problem for a manufacturer that makes premium-efficiency motors. EPAct efficiencies are relatively lower than for 'premium' motors of the same size."

VEM Motors GmbH (Wernigerode, Germany) displayed references to EPCA in its stand at Hannover Fair '98. The company stresses exports, mainly for motors built into OEM equipment. It can meet efficiency requirements for OEM orders going to North America.

At what cost?

There is a premium to be paid to obtain higher motor efficiency. A good deal of this comes from the higher grade laminations and other active materials added.

"Average price increase varies from six to 30% depending on frame size and manufacturer," says Tim Albers, product manager at USEM. "Smaller frames have the largest percentage increases in cost." Other manufacturers project pricing of EPAct-compatible motors to fall between that of previous standard-efficiency and premium-efficiency models, which command premiums of 20-40%.

Among specific product lines that have evolved from EPAct's initiatives are Baldor's Standard-E motors; High Efficient, KE from GEICS; MagneTek's E-Plus line, Reliance's ExMaster, and USEM's World Motor consisting of several motor styles.

Such products add benefits beyond efficiency rating—typically less motor heating, less vibration, and quieter operation. Due to less heat generated, GEICS claims increased motor insulation and bearing lubrication service life.

Verifying efficiency

Testing and certification are crucial to the success of EPAct. EPAct and its supporting DOE rules address these areas, but some issues remain to be finalized. Basic motor designs require testing for efficiency and losses in accordance with IEEE 112 Method B (Canadian Standards Association C390-93C is acceptable). Significantly, a complete dynamometer test is included. Equipment and methods used must meet DOE and National Institute of Standards and Technology (NIST) standards. NIST, which is part of the U.S. Department of Commerce, plays a strong part in developing the test procedures.

Since testing of all basic motor models and types would be time- and cost-prohibitive, statistical sampling of the number of motors to be tested is allowed. "Alternative efficiency determinations" are allowed, based on engineering or statistical analyses, computer simulation, math modeling, or other analytical evaluation of performance data.

While EPACt doesn't specify minimum efficiencies (see earlier text), NEMA minimum efficiencies are used in the DOE rules to "provide a reasonable estimate of measurement uncertainties and product variabilities likely to be encountered during actual testing."

Motor testing must be done in a lab with accreditation from one of three sources: the National Voluntary Laboratory Accreditation Program (NVLAP), a foreign organization recognized by NVLAP, or an organization classified by DOE as an accreditation body. NVLAP works under the auspices of NIST.

If their test facilities become accredited, EPAct lets manufacturers do the required testing in their own laboratories. Alternative correlation methods can be used for some motors. GE and MagneTek are among motor manufacturers that have NIST-approved in-house testing labs.

Certification to EPAct rules will be a concern for motors imported to the U.S. While several efficiency measurement standards exist worldwide, results can be significantly different due to the way stray losses are calculated. For example, IEEE-112 (EPAct's method) determines stray losses indirectly, while IEC 34-2 assumes a value that's 0.5% of input power. Japanese standard JEC-37 does not include stray losses. The foreseeable answer is for non-U.S. testing agencies to be accredited to IEEE-112's methods.

Still some questions remain. For example, "How would imported motors with unacceptable testing be handled? Who pays for the corrections?" asks Siemens E&A's Mr. Hodowanec. Concerns about the ability of customs agents to verify test compliance from nameplate data is also mentioned by USEM's Mr. Boteler and Reliance's Mr. Offik.

Possible changes ahead

Some fallout may result from slight changes in motor characteristics due to higher efficiency. These motors run at somewhat higher speed and power, possibly causing concern for variable-torque pump, fan, and blower applications. Higher starting torque could be a mixed blessing: a concern where softer starting is desired, but a benefit to some constant-torque applications. Starting currents can also be higher. An existing motor starter should continue to work as long as the motor design type remains the same. None of these effects appears to be a major concern, but will require greater attention on the part of the motor supplier and the end-user.

EPAct is here to stay. And it will likely expand its coverage to include more sizes and types of motors. One example is NEMA Design C motors, which until recently have not drawn attention because of their small numbers in use.

Required Nominal Efficiency
(General-Purpose Motors under Full Load)

Source: EPAct-92 and U.S. DOE rules in Title 10 CFR Part 431.

Norminal Full-Load Efficiency (%)

Open Motors

Enclosed Motors

Motor Horsepower/Standard kW equivalent

6 pole

4 pole

2 pole

6 pole

4 pole

2 pole

1/0.75

80.0

82.5

80.0

82.5

75.5

1.5/1.1

84.0

84.0

82.5

85.5

84.0

82.5

2/1.5

85.5

84.0

84.0

86.5

84.0

84.0

3/2.2

86.5

86.5

84.0

87.5

87.5

85.5

5/3/7

87.5

87.5

85.5

87.5

87.5

87.5

7.5/5.5

88.5

88.5

87.5

89.5

89.5

88.5

10/7.5

90.2

89.5

88.5

89.5

89.5

89.5

15/11

90.2

91.0

89.5

90.2

90.0

90.2

20/15

91.0

91.0

90.2

90.2

91.0

90.2

25/18.5

91.7

91.7

91.0

91.7

92.4

91.0

30/22

92.4

92.4

91.0

91.7

92.4

91.0

40/30

93.0

93.0

91.7

93.0

93.0

91.7

50/37

93.0

93.0

92.4

93.0

93.0

92.4

60/45

93.6

93.6

93.0

93.6

93.6

93.0

75/55

93.6

94.1

93.0

93.6

94.1

93.0

100/75

94.1

94.1

93.0

94.1

94.5

93.6

125/90

94.1

94.5

93.6

94.1

94.5

94.5

150/110

94.5

95.0

93.6

95.0

95.0

94.5

200/150

94.5

95.0

94.5

95.0

95.0

95.0


Typical Electric Motor Losses

No-load losses

%Total

Source: GE Industrial Control Systems

Friction, windage

14

Core losses

16

Load-dependent losses

Stator I2R

33

Rotor I2R

15

Stray-load losses

22

Total

100%




EPAct-related Motor Terms

Only the first of three basic motor categories is covered under EPAct.

General-purpose motor —any motor designed in standard ratings with either: (1) standard operating characteristics and mechanical construction for use under usual service conditions, such as those specified in NEMA Std. Publication MG1-1993…, and without restriction to a particular application or type of application; or (2)…for use under unusual service conditions, or for a particular type of application, and which can be used in most general-purpose applications.

Definite-purpose motor —any motor designed in standard ratings with standard operating characteristics for use under service conditions other than usual, or for use on a particular type of application and which cannot be used in most general-purpose applications. Examples are TENV (totally enclosed, nonventilated) and TEAO (totally eclosed, air over) types. [Examples and italics added.]

Special-purpose motor —any motor designed for a particular application, and either: (1) designed in nonstandard ratings with special characteristics or mechanical construction, or (2) one that has special operating and special mechanical construction

Source: Title 10 CFR Part 431—Energy Efficiency Program for Certain Commercial and Industrial Equipment.



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