Introduction to Premium Efficiency Motors

This article is excerpted from "Premium-Efficiency Motors and Transformers", a CD-ROM available from CDA by going to the Energy Effficiency Publications and follow the instructions for ordering.

There is a capital investment that can repay many times its original value over the next 20 years. At the same time, it can improve equipment reliability, reduce downtime and repair costs, and result in lower releases of carbon dioxide to the atmosphere.

The investment is straightforward: install electric motors having the highest electrical energy efficiency commensurate with your needs. Energy-efficient motors pay for themselves in a few years or sometimes even a few months, after which they will continue to pile up savings worth many times their purchase cost for as long as they remain in service. That's another way of saying that operating costs, not just first cost, are what you should look at when buying a new motor.

The rule applies to all motors, although this article is limited to the widely used motors that fall under the requirements of the Energy Policy Act of 1992 (EPAct) and to those that additionally meet or exceed the National Electrical Manufacturers Association's NEMA Premium™ efficiency ratings. We'll explain what the ratings mean in a moment.

Before we begin, however, there are three factors to keep in mind, whether you're replacing an old worn out motor or specifying a motor for a new piece of equipment. First, energy-efficient motors only provide savings when they're running, and the more the motors run, the more energy and money they save. Second, maximum savings - and the fastest returns on investment - are attained in regions of the country where utility rates are highest. Even so, energy-efficient motors are highly recommended even in low energy-cost areas because they eventually provide savings that more than adequately justify their cost. And third, remember that any motor selection has to be properly engineered for its intended application.

It can even be worthwhile to replace fully serviceable standard efficiency (pre-EPAct) motors, including ones that were recently overhauled. That's certainly not a simple decision, and it should only be made after conducting a thorough analysis of the economic and technical factors involved. On the other hand, large companies like Cummins Engine and Weyerhaeuser did perform such analyses and found that the savings were large enough to make motor replacement programs a part of their corporate energy policies.

Background

Until the energy crises in the 1970s, most general-purpose motors were designed to provide rated output and operating characteristics at reasonable cost, period. Efficient operation was at best a secondary consideration. As energy prices began rising, however, manufacturers began promoting improved motors they called "high-efficiency" and "energy-efficient", although the terms were not specifically defined at the time.

Old-style "standard efficiency" motors remained popular because they generally cost less than the new models. Purchasing agents were seldom inclined to spend a little more money up front in order to save on energy costs later on. Because of the national energy implications of motor efficiency, Congress enacted the Energy Policy Act of 1992, which granted the USA Department of Energy (DOE) the authority to set minimum efficiency standards for certain classes of electric motors. EPAct rules for motors became effective Oct. 24, 1997. All covered motors sold in the USA after that date are required to have efficiency ratings equal to or better than those listed in NEMA MG 1-1993, Table 12.10. EPAct covers general-purpose motors rated from 1 to 200 hp; 2-, 4- and 6-pole (3600, 1800 and 1200 rpm); horizontal; T-frame; single speed; continuous duty, 230V, 460V or 230/460V; NEMA Designs A and B. Efficiencies of these so-called "EPAct motors" are from one to four percentage points higher than the previous "standard-efficiency" motors.

EPAct didn't bring about the manufacture of an entirely new type of motor; it simply set standards for motors that could be sold in the U.S.A. EPAct also provided grandfather protection to existing standard-efficiency motors no matter how often they were rewound or repaired.

NEMA Premium™ Motors, the New Standard

EPAct was a step in the right direction, but its requirements were based on minimum efficiency levels that industry and the DOE agreed were reasonable at the time. In fact, many motors that were available before EPAct became law exceeded the statute's minimum requirements, and as motor manufacturers continue to improve their products, they are now able to offer significantly more efficient motors, sometimes at little if any cost premium, model for model.

In June 2001, NEMA granted such "better-than-EPAct" motors special recognition by creating a designation called NEMA Premium™. Going a step beyond EPAct, NEMA Premium applies to single-speed, polyphase, 1 to 500 hp, 2-, 4-, and 6-pole (3600, 1800 and 1200 rpm) squirrel cage induction motors, NEMA Designs A or B, 600V or less, (5kV or less for medium voltage motors), and continuous rated.

The Consortium for Energy Efficiency (CEE), a non-profit organization that includes many electric utilities among its members, recognizes NEMA Premium motors up to 200 hp as meeting their criteria for possible energy efficiency rebates.

Many motors exceed NEMA Premium efficiency ratings; however, some such motors are manufactured by companies that are not members of NEMA and who may therefore not use the NEMA premium trademark. Other manufacturers, while they may be NEMA members, voluntarily choose not to apply the label to their products. The point is that, while the NEMA Premium label assures the buyer of a certain minimum yet high level of efficiency, lack of the label does not necessarily imply that the motor doesn't meet the high standards. It therefore pays to check nameplate efficiencies and use tools such as MotorMaster+, MotorSlide, or other free publications from Copper Development Association to help identify those motors that do offer high efficiency, possibly even exceeding that of NEMA Premium.

Defusing the Energy Explosion

It goes without saying that more-efficient motors will consume less energy and reduce their owners' electric bills over the long run, but a rapid return on investment is most likely when the motors operate at high duty cycles. Motors that operate intermittently may or may not save enough to justify replacement except in cases where utility rates are especially high. But, in evaluating motors that operate at a high duty cycle, or continuously, replacement with energy-efficient motors can usually result in very rapid payback, and save many times their initial cost.

• On a nation wide basis, high-efficiency motors promise truly enormous energy savings. The DOE estimates that there are about 12.4 million motors of more than 1 hp in service in U.S. manufacturing facilities. ¹ CEE reports that about 2.9 million of these motors fail each year, of which 600,000 are replaced. According to DOE estimates, potential industrial motor system energy savings, using mature, proven, cost-effective technologies range from 11-18 percent of current annual usage or 62 to 104 billion kWh per year in the manufacturing sector alone. This savings is valued up to $5 billion. It would also avoid the release of up to 29.5 million metric tons of carbon equivalent emissions to the atmosphere annually.
• Industrial electric motor driven systems used in production account for about 679 billion kWh, or about 23% of all the electricity sold in the USA. Motors used in industrial space heating, cooling and ventilation systems use an additional 68 billion kWh. Process motor systems account for 63% of all electricity used in industry. ¹
• It is estimated that the NEMA Premium™ motor program could save over 5,800 GWh (5.8 billion kWh) of electricity and prevent the release of nearly 80 million metric tons of carbon into the atmosphere over the next 10 years. That would be the equivalent of keeping 16 million cars off the road. ²

Today, many industrial organizations are seeking ways to display their concern for the environment. Establishing and implementing a policy to use high-efficiency motors is one way to demonstrate environmental concern (and, at the same time, save energy and money). In fact, numerous companies have already won national recognition by including high-efficiency motors in their corporate energy policies.

As to the cost of the motors themselves, a number of utilities and state agencies now offer incentive programs in the form of rebates and cost-sharing programs that encourage their customers to install the efficient devices. Utilities benefit from these demand-side management programs because the improved motors reduce the need to bring new power sources on line.

What does "efficiency" mean?

Electric motors are simply devices that convert electrical energy into mechanical energy. Like all electromechanical equipment, motors consume some "extra" energy in order to make the conversion. Efficiency is a measure of how much total energy a motor uses in relation to the rated power delivered to the shaft.

A motor's nameplate rating is based on output horsepower, which is fixed for continuous operation at full load. The amount of input power needed to produce rated horsepower will vary from motor to motor, with more-efficient motors requiring less input wattage than less-efficient models to produce the same output. Electrical energy input is measured in watts, while output is given in horsepower. (This convention applies in the USA; output power for motors manufactured in other countries may be stated in watts or kilowatts.) One horsepower is equivalent to 746 watts.

There are several ways to express motor efficiency, but the basic concept and the numerical results are the same. For example:

The ratio describes efficiency in terms of what can be observed from outside the motor, but it doesn't say anything about what is going on inside the motor, and it is what's happening inside that makes one motor more or less efficient than another. For example, we can rewrite the equation as:

"Losses" stands for all the energy "fees" the motor charges in order to make its electrical-to-mechanical energy conversion. Their magnitude varies from motor to motor and can even vary among motors of the same make, type and size. In general, however, standard-efficiency motors (pre-EPAct) have higher losses than motors that meet EPAct standards, while NEMA Premium motors, or better, have lower losses still.

Types of Losses

Energy losses in electric motors fall into four categories:

• Power losses
• Magnetic core losses
• Friction and windage losses, and
• Stray load losses.

Power losses and stray load losses appear only when the motor is operating under load. They are therefore more important - in terms of energy efficiency - than magnetic core losses and friction and windage losses, which are present even under no-load conditions (when the motor is running, of course).

Figure1. A typical NEMA Design B motor showing components that can be modified to increase the motor's efficiency: (a)Stator windings; (b) Rotor length; (c) conductor bars and end rings; (d) air gap; (e) laminations; f) bearings; (g) fan.

Power losses, also called I²R losses, are the most important of the four categories and can account for more than one-half of a motor's total losses. Power losses appear as heat generated by resistance to current flowing in the stator windings and rotor conductor bars and end rings.

Figure 2. Conductor bars, end plates and fan in a typical squirrel cage motor. The steel rotor laminations have been removed by etching.

Rotor losses, another form of power losses, are also called slip losses because they are largely - but not entirely - dependent on the degree of slip the motor displays. Slip is the difference in rpm between the rotational speed of the magnetic field and the actual rpm of the rotor and shaft at a given load.

S =	N s - N
	N s

Where:

S = Slip
N = Output speed under load and
N _s = Synchronous (no-load) speed, rpm

Rotor losses are reduced by decreasing the degree of slip. This is accomplished by increasing the mass of the rotor conductors (conductor bars and end-plates) and/or increasing their conductivity (see below), and to a lesser extent by increasing the total flux across the air gap between rotor and stator.

Conductivity is an important characteristic of the rotor. Conductor bars in large motors are normally made from high-conductivity copper. Conductor bars in small-to-intermediate size motors, up to about 200 hp, depending on manufacturer, are in the form of a die-cast aluminum "squirrel cage" that gives these motors their common name. Increasing the mass of the die-cast bars requires changes in the slots in the rotor laminations, through which the bars are cast, and that changes the rotor's magnetic structure. Lowering rotor I²R losses in what are typically aluminum alloy squirrel cage motors is therefore not a simple task.

Figure 3. Cross-section of a die-cast copper motor rotor. The blue area represents the surface of one of the rotor laminations, through which the copper has been cast.

The fact that high-efficiency motors tend to have less slip (run faster) than standard-efficiency motors must be taken into account in certain applications. For example, energy consumption by centrifugal loads such as fans and rotary compressors is proportional to the cube of rotational speed. If such loads are driven at the higher speed of a low-slip, high-efficiency motor directly replacing a standard motor, energy consumption can actually increase. This situation can sometimes be resolved by lowering rotational speed with a variable-speed drive, gears or pulleys. There are other parameters, such as torque or starting current, that can vary among motors of the same nominal horsepower. It is important to properly engineer the application of any motor to the intended task.

Magnetic core losses arise from hysteresis effects, eddy currents and magnetic saturation, all of which take effect in the steel laminations. Magnetic losses can account for up to 20% of total losses. With proper design, use of better materials and stringent quality control, these losses can be reduced considerably.

Figure 4. Three different efficiencies for the same horsepower rating. Top: standard-efficiency pre-EPAct motor; lower left: EPAct-level motor; lower right: NEMA Premium efficiency motor. Notice that the rotor and stator lengthen (and the amount of copper in the motor rises) as efficiency increases. (Courtesy: Toshiba)

The most effective means to reduce hysteresis and saturation losses is to utilize steels containing up to 4% silicon for the laminations in place of lower-cost plain carbon steels. The better magnetic properties offered by silicon steels can reduce core losses by 10 to 25%. Reducing the laminations' thickness also helps: substituting 26-ga or 29-ga steel for the 24-ga steel found in standard-efficiency motors lowers core losses by between 15 and 25%. Lengthening the lamination stack, which reduces the flux density within the stack, also reduces core losses. Eddy current losses can be reduced by ensuring adequate insulation between laminations, thus minimizing the flow of current (and I²R losses) through the stack.

MotorMaster+ Makes it Easy to Decide Which Motor to Choose

Many business owners hesitate to replace old motors because the capital cost of a new motor usually exceeds the cost of repairing the old one. This is a valid concern, but it is important to recognize that motors themselves may be quite inexpensive compared with the cost of power they consume.

Example # 1: Replacing a Serviceable Standard-Efficiency Motor

To illustrate that point and also compare the true cost of owning motors of several efficiencies, we'll use software called MotorMaster+, which was developed for the DOE by engineers at Washington State University.

For this example, assume you have a serviceable standard-efficiency (pre-EPAct), 5-hp, 1800-rpm, 208-230/460-V, general-purpose, T frame, TEFC, NEMA Design B motor, one that might have been produced until not too many years ago. Assume the motor operates 8000 hours (11 months) per year at 75% of full load, and that power costs $0.075/kWh, the national average. Such motors have an average efficiency rating of 84% at full load. (Efficiency ratings for motors of this type at 75% loading range from 81% to 88.8%, averaging 84.06%.) Millions of old motors like this remain in service today, most of them having been rewound several times. Unfortunately, rewinding cannot and does not improve a motor's efficiency beyond the motor's original nameplate rating.

Using the operating parameters and power cost given above, MotorMaster+ calculates the motor will consume 26,644 kWh of energy annually, and the annual cost of operating this motor will be:

[0.746(W/hp) X 5(hp) X 0.75(load factor) X 8000(h/y) X $0.075/kWh)]/0.84(efficiency) = $1998.21,

or about 7 to 9 times the cost of a new replacement motor! The old motor's operating cost will total $39,960 over a 20-year lifespan.

MotorMaster+ can compare that motor with one that just meets EPAct's minimum efficiency requirements (87.5% at full load for this size and type of motor). A motor of this type would cost around $233 after a typical 35% discount from list price. Operating continuously for 8000 h/y at 75% load, and at 88.2% efficiency (efficiency usually peaks near 75% of full load), the annual cost to operate the EPAct motor would be:

(0.746 X 5 X 0.75 X 8000 X 0.075)/0.882 = $1,903.06.

This is $95 less per year ($1,900 less over 20 years) than the standard-efficiency model it replaces. MotorMaster calculates a simple payback period of 2.44 years.

If on the other hand we upgrade to a NEMA Premium motor that has an efficiency of 90.5% at 75% of full load, annual energy and cost savings rise to 1914 kWh and $144, respectively, over the standard model. One such motor would cost approximately $302 after discount, and it would pay back its purchase price in only 2.10 years.

The table below lists comparisons made by MotorMaster+ for the cost of owning and operating motors of various sizes against keeping a standard-efficiency motor in service. For each size, comparisons are made using average efficiency values for standard-efficiency motors, and nameplate efficiency values for commercial motors that meet EPAct efficiency requirements and for motors that qualify for the NEMA Premium designation, respectively. All motors listed are 208-230V/460V, general- purpose, 4-pole, TEFC, T-frame, NEMA Design B types. Cost comparisons are based on 8000 h/y at 75% of rated power, and a $0.075/kWh utility rate.

In most cases, replacing an older, serviceable, standard-efficiency motor with an EPAct-minimum motor results in significant energy and cost savings, as well as payback periods of three years or less.

Replacing that serviceable, standard-efficiency motor with a NEMA Premium motor usually increases the savings and decreases the payback period. Remember: a payback in 2.4 years is equivalent to a 34% return on investment.

Example #2: Replacing a Failed Motor

A different type of comparison to consider involves choosing between two different motors for a new application, or when replacing a failed motor. The savings and payback figures will be significantly better than those listed above, because the initial capital cost must be expended in any event, so only the difference in initial cost between motors that meet EPAct requirements and ones that meet or exceed NEMA Premium standards enters the equation. For the operating parameters used above (8000 h/y, 75% load, $0.075/kWh), paybacks can be achieved in as little as 7 to 12 months for some motors, with 1-2 years being more typical, as in the following table.

Several other examples, worked out using MotorMaster+ software, can be found elsewhere on this Web site.

Example #3: Motors for OEM Equipment

OEM equipment suppliers sometimes - but not always! - offer their products with a choice of motors. If the customer looks at first-cost price alone and selects the cheapest option, it is likely that the equipment will be fitted with an EPAct motor (assuming it qualifies under the EPAct-listed types described above), or one even lower efficiency. However, a smart customer will run the numbers through MotorMaster+ and specify the most energy efficient motor that meets the equipment's requirements at a reasonable first cost. Paybacks will be similar to those listed in Example #2. An example describing one major industrial company's successful experience with this practice can be found in Cummins Case Study.

Additional Savings

Another point to consider: premium efficiency motors are generally made to higher manufacturing standards and tighter quality controls than the old standard-efficiency motors they are meant to replace. The new motors run cooler because they generate less I ²R heat, producing less stress on windings. This is generally taken to be an indication that the motors will last longer, and it can translate in reduced downtime and lower repair costs over the life of the motor.

The MotorMaster+ calculations summarized in the table, above, make no allowance for incentives offered by some utilities, which can be substantial. The calculations also ignore the sharp increase in utility rates recently seen in certain regions. When these factors are taken into account, payback periods may become as short as a few months.

While the savings described above are impressive, they are only an indication of what can be gained in an entire facility, even if it only operates a few motors. Many companies have examined their entire motor inventory, including motors in service as well as those held in reserve, to determine which ones could be replaced profitably. In at least one instance, a company instituted a corporate energy policy to replace all standard-efficiency motors rated at less than 50 hp, regardless of how recently they had been rewound. That action may or may not be suitable for all organizations since it involves many factors, such as utility rate structures, that can vary significantly.

References

1. United States Industrial Motor Systems Marketing Assessment Executive Summary, U.S. Department of Energy, December 1998.