Energy-efficient motors may now get respect

By Frank J. Bartos, CONTROL ENGINEERING May 1, 2001

M aking electric motors more efficient has never had greater incentive than today. Recent realities of energy costs and supply problems underline the need to act.

Electric motors consume over 60% of all electricity used by U.S. industry. In some specific industries, such as mining, it can be as much as 90%. Elsewhere in the world, for example, in Finland, motors account for 80% of total electric power use. Due to such statistics and reasons explained in the main feature article, a major focus of energy efficiency programs is on general-purpose ac induction motors-the workhorse motors of industry.

Efficiency in electric motors can be viewed in several ways: as a relationship of power output relative to power input or as a measure of losses incurred in the process of converting electrical energy to mechanical energy. From another perspective, energy-efficient motors consume less electrical power than standard motors to produce the same shaft output power.

What makes motors efficient?
Basically, an energy-efficient electric motor is a higher quality product. It embodies more and higher-grade electric materials, and is more precisely manufactured than a standard motor. The main points of efficient design-such as thinner gauge, better quality steel laminations, more and larger cross section copper windings, and more optimal fan design-are covered in the main article. Higher grade insulation for the windings is another quality factor. Non-hygroscopic Class F insulation is typically used with premium-efficiency motors.

How the motor is manufactured contributes to operating efficiency. Variations in the machining and assembly process can affect stray losses. Tighterdimensional tolerances, including smaller air gap between rotor and stator, also help. More accurate rotor balancing contributes to longer bearing life, and hence longer motor life.

How much can efficient motors save?
Potential savings in energy costs from replacing lower efficiency motors with energy-efficient ones-or selecting the higher efficiency alternative in a new installation-depend on many variables. Number and size of motors, operating hours and electricity costs are all involved, plus a gamut of installation, process, and control details. It’s not a simple decision.

However, here is an illustration of typical cost savings possible using single motor examples and a simplified equation for calculations. Differential cost (or savings) per year from operating a more efficient motor is given by the relation:

D$ = 0.746 HP x LF x N x $P [DE/E 1 x E 2 ]
where, HP is motor size, hp
LF is the application’s load factor
N is motor running time, hr/yr
$P is electricity cost, $/kW-hr
E1 is efficiency of the less-efficient motor
E2 is efficiency of the more-efficient motor
DE = E2 – E1, or difference in motor efficiencies
(Efficiencies expressed in decimal units)

Example 1: Two 50-hp motors are available with 0.89 or 0.93 (EPAct) efficiency, to run 4,380 hours per year (50% of the time) at 0.8 load factor in a plant where cost of power is 8 cents per kilowatt-hour. Then,

D$ = 0.746 (50) 0.8 x 4,380 x 0.08 [0.04/(0.89 x 0.93)] = $505 per year.

This is the saving from operating the more efficient motor. Longer running hours and/or higher cost of power would increase the amount saved.

Example 2: Two 100-hp motors are available with 0.91 or 0.954 efficiency, to run 6,570 hours per year (75% of the time) at 0.8 load factor in a plant where cost of power is 10 cents per kilowatt-hour. Then,

D$ = 0.746 (100) 0.8 x 6,570 x 0.10 [0.044/(0.91 x 0.954) ] = $1,987 per year.

As this example shows, larger motor sizes lead to greater operating cost savings.

Use of higher efficiency motors, especially in installations with numerous motors, can result in additional savings from lower electrical power demand-besides the above operating cost savings.

The above equation can be used with kW-rated motors and electricity costs in other currencies as well. Just replace the 0.746 HP term with the kW rating of the motor and use euro/kW-hr or other cost factor instead of $P.

Legislation, variable-speed use
Legislative efforts and initiatives by industry associations are pushing the use of energy-efficient motors. Industry programs promote an educational role for users, as well. The U.S. Energy Policy and Conservation Act (EPAct) of 1992, which finally became effective in October 1997, first put into law nominal efficiency requirements for applicable motors. Canada has similar legislation in place. A voluntary agreement for efficiency labeling of induction motors made and sold in Europe started in 1999 (see more in Products section, below). These and other efficiency initiatives are discussed in the main article. An original diagram compares the major initiatives.

Although EPAct has been on the books for more than three years, it left some issues to be resolved-for example, in the area of compliance testing and verification of covered motors. Here, inconsistencies remain about test methodology, dynamometer calibration, rating calculations, statistical sampling, and other factors. However, work is ongoing to resolve the issues and harmonize some procedural differences across industry segments.

A provision of EPAct allowed the building of standard-efficiency (noncomplying) motors right up until the law’s effective date of Oct. 24, 1997. Creative marketing and production managers at various motor companies took EPAct at its word, and ‘maxed out’ final production of the lower cost motors. This has led to anecdotal stories of ‘production to inventory’ and warehouses stuffed with motors just prior to the start of EPAct. Presumably, this overstock of standard-efficiency motors has been consumed in the interim, and only energy-efficient motors of the applicable size range are now being made, sold, and used in new installations.

Energy-efficient motors target mainly constant-speed applications with long running times. However, they can be called on for variable-speed operation using adjustable-frequency drives (AFDs). These so-called ‘inverter-duty’ motors then need to limit their usable speed range, largely because the shaft-driven fan becomes progressively less effective for motor cooling as running speed is reduced.

For example, Emerson (St. Louis, Mo.) recommends the following limitations for its energy-efficient U.S. Electrical Motors when used with AFDs. For its highest grade (Premium Efficient) motors typical limits include 10:1 speed range with variable-torque loads, 4:1 speed range with constant-torque loads, at 1.0 service factor and m600 V supply. Users are also cautioned to watch motor-to-drive cable lengths. Cable length limits decrease with supply voltage and drive switching frequency (see specific manufacturer values). Similar cautions apply to Emerson’s Energy-Efficient (World Motor) class, except for more restrictions on speed range at constant torque (2:1), supply voltage (m400 V), and even shorter motor/drive cable lengths.

Other incentives
Energy rebates from electric utilities and suppliers provide another incentive to some users of higher efficiency motors. Approximately 30 U.S. electric utilities presently offer incentives to increase the efficiency of power distribution grids, according to Chris Cockrill, project manager with the U.S. Department of Energy, Best Practices, Motor Systems. These incentives are demand-side energy management tools left over from earlier electrical utility programs, rather than recent steps to fight ongoing energy shortfalls. ‘Utilities that still have these programs are reevaluating them and will likely continue them because they’ll be valuable on a near-term basis to reduce power demand on the utility grid,’ adds Mr. Cockrill.

Savings from rebates can be as much as 15% of the motor’s purchase price, according to Baldor Electric Co. (Fort, Smith, Ark.). Utilities also offer incentive programs to decrease energy use at the application level. Adjustable-speed drives and soft motor starters become significant here.

Most motor manufacturers offer more than one class of efficient motors. With the coming of EPAct, North American manufacturers have added a line of motors to meet the law’s requirements. At the same time, they upgraded their existing efficient motor lines under a higher efficiency class. Various manufacturers designate these motor products as ‘extra,’ ‘high,’ ‘plus,’ ‘premium,’ ‘super,’ ‘ultra,’ etc., but they have no uniform efficiency meanings.

In Europe, a recent government-manufacturer agreement-voluntary at present-has introduced three efficiency classes for ac induction motors in the range of 1.1-90 kW (1.5-120 hp) at 400 V/50 Hz. These classes are Eff1 (high-efficiency), Eff2 (improved-efficiency), and Eff3 (standard motors). Production volumes of the three classes are being tracked. The agreement mandates that use of Eff3 motors be reduced by 50% by the end of 2003, and eventually eliminated.

All major European motor suppliers participate in the agreement. For example, ABB (Zürich, Switzerland) now produces motors only in the two highest efficiency classes. Likewise Siemens Automation & Drives Group (A&D, Erlangen, Germany) makes energy-efficient Eff1 and Eff2 motors exclusively, as part of its Totally Integrated Automation program.

Here is a sampling of products in the energy-efficient motor sector.

Emerson (St. Louis, Mo.) World Motor, under the U.S. Electrical Motors brand, comes in ODP (open, drip-proof) and TEFC (totally enclosed, fan-cooled) designs for 230/460- or 460-V operation at 60 Hz (also usable on 190/380 or 380 V at 50 Hz). It meets standard NEMA requirements, including 40 °C ambient, design B, class B temperature rise at full load. World Motor satisfies EPAct 92 (U.S.), CSA C390 (Canada), and NOM 74 (Mexico) efficiency requirements, and carries CE-mark certification for the European Community’s low voltage directive.

TEFC motors have two types of construction: Unimount or Hostile Duty. For 180 frame size and larger, Unimount has an extruded aluminum shell, while Hostile Duty motors feature a cast iron shell and end bells. On 56 and 140 frames, it’s rolled steel shells for both products, with end bells being the variation (aluminum for Unimount; cast iron for Hostile Duty).

Rockwell Automation Power Systems (Greenville, N.C.) offers E-Master Energy Efficient and XE Premium Efficient motors, intended for increasingly demanding applications. E-Master general-purpose motors are available in a 0.75-450 hp size range with cast iron frame; and 0.25-30 hp sizes with rolled-steel construction. XE (extra efficiency) Premium motors range over 0.75-600 hp size (cast iron); and 0.25-2 hp (rolled steel). LXE Series includes extra efficiency motors above the 600-hp rating.

Leeson Electric (Grafton, Wis.), part of Regal-Beloit Corp., has been making its ‘Wattsaver’ line of premium-efficiency motors for general-purpose and inverter-fed (adjustable speed) applications since October 2000. Overall ratings are in the 0.5-350 hp range for three-phase operation. TEFC models and ODP models (to 100 hp) are available. Sizes 1-125 hp meet or exceed efficiency levels of the Consortium for Energy Efficiency (CEE) that average 1-2% higher than EPAct values.

Baldor Electric ‘s (Fort, Smith, Ark.) Premium Efficiency Super-E motors are available in a ‘chemical processing’ version up to 400 hp. This is in addition to the general-purpose version in 1-500 hp sizes. Another version of the line in the 1-250 hp range at 460 V operation has been recently upgraded to exceed IEEE Std. 841 requirements. IEEE Standard 841 (revised in 1994) applies to high-efficiency TEFC, horizontal and vertical, single-speed, squirrel-cage polyphase induction motors, through 500 hp, in NEMA frame sizes 143T and larger. Commonly known as severe duty motors, they are intended for petroleum, chemical, and other harsh applications. Baldor claims it was the first manufacturer to upgrade its entire premium-efficiency line to meet CEE motor efficiency criteria.

WEG Electric Motors Corp. (Suwanee, Ga.; Jaraguá do Sul, Brazil) offers TOP Premium Efficiency TEFC, IPW55-rated motors that are designed to work in moist or dusty environments without shortening their useful life. Power rating of the TOP motor line is 1-500 hp at 208-230/460, 460 or 575 V operation. These NEMA design B motors feature Class F insulation for all frame sizes, Class B (80 °C) temperature rise, and service factor of 1.25 to 100 hp (1.15 for M125 hp). Another WEG motor line-World Energy Guardian TEFC IP55-meets or exceeds EPAct efficiency ratings.

Leroy-Somer (Angoulême, France), a division of U.S.-based Emerson, opted to make Eff2 class energy-efficient motors its standard ac motor offering as of January 2000. In effect, this raises the efficiency of a standard Leroy-Somer 1.1-kW (1.5-hp equivalent) motor to 83%, compared to 76% for the previous generation. For a larger, 11-kW motor, comparable figures would be 91% versus 88%. The company also offers a line of higher efficiency motors that meets Eff1 classification in the new European Motor Efficiency Labeling agreement.

Invensys Brook Crompton (Huddersfield, U.K.) also complies with the European agreement on motor efficiency labeling, and states it was the first to label all of its applicable motors in compliance with the agreement. The company has made Eff2 classification the standard for its T Series motors. Brook Crompton also supplies W+ Series motors in Eff1 class over the full range of 1.1-90 kW covered by the European efficiency agreement.

ABB (Zürich, Switzerland) recently launched new M3000 and M2000 Series ac induction motors that correspond to European efficiency classes Eff1 and Eff2, respectively. M3000 Series comes in a power range of 11-710 kW, with IEC frame sizes 160-400 in cast iron, as well as 0.18-75 kW ratings for IEC sizes 63-250 in aluminum. Intended for rugged continuous processes, M3000’s size range extends beyond the covered efficiency agreement. M2000 Series is available in 0.25-55 kW cast iron models (IEC sizes 71-250) and 0.06-3 kW aluminum models (IEC 56-100). These Eff2 motors are intended for shorter runtimeapplications. ABB no longer makes Eff3 class motors.

Comments? E-mail