By David T. Brender & John Crowie

Motors do two things: 1) they consume electrical energy, and 2) they perform mechanical work. Maintenance staff often are so focused on the latter that they pay little heed to the former. Since motors typically last several decades, they consume significant electrical energy over their lifetimes; as a result, the operating cost of a motor is typically many times its purchase cost.

Motor manufacturers recognize that energy efficiency is an important selling point for their products. The U.S. Department of Energy works hard to educate consumers about the energy efficiency of air-conditioners, refrigerators, washers, dryers and other home appliances. Furthermore, major campaigns are directed at industrial users of motors, such as motors in climate control systems, materials handling systems, and process equipment.

As a result, plant engineers, facilities managers, energy managers and manufacturing engineers typically are well-versed about motor efficiency, and they understand that the savings accrued can exceed the cost of the motor in a short time, often in a year or less.

For electrical contractors, it is important to understand the importance of energy efficiency in today’s commercial buildings and that one of the simplest ways to realize greater energy efficiency is through the use of high efficiency, especially NEMA Premium TM-efficiency motors.

As with any product selection, there are tradeoffs. Some design decisions have been legislated into law by the Energy Policy Acts (EPAct) of 1992 and 2005. Beyond the minimum standards established by law, significant gains in efficiency are possible.

However, rather than purchasing “the most efficient motor possible,” electrical contractors should seek to understand how motors vary and what is the total cost of additional percentage points of efficiency.

The High Stakes of Motor Selection
Calculations suggest that operating costs can be as much as 98 percent of the total cost of a motor over its life. Stated differently, the purchase price of a motor might account for only one or two percent of the total cost, including energy costs, of running a motor for 10 to 20 years.  

Compared with standard-efficiency motors, a high-efficiency motor (one that exceeds EPAct minimums by a wide margin) can pay back its original cost many times over through energy savings. Replacing even a fully serviceable, standard-efficiency, older (pre-EPAct) motor with a high-efficiency motor can pay big dividends in the long run.

Whether or not to replace any specific motor should be decided after a proper analysis of the economics as well as technical factors.  The return on investment can be quite rapid for a motor that operates at a high duty cycle. Replacing a motor that operates nearly continuously with an energy-efficient motor can result in energy savings many times the initial cost of the motor within a few months.

Even motors that operate intermittently can save enough energy to justify replacement, especially where utility rates are high.

 It is worthwhile to review the main types of energy loss in motors and the features of high-efficiency motors. Additional and more detailed information is available from the DOE, various state agencies, utilities and motor manufacturers as well as the Copper Development Association (www.copper.org/energy).

Efficiency of a Motor
When electric motors convert electrical energy into mechanical energy they consume some amount of energy to make the conversion. Efficiency is a measure of the power delivered to a motor shaft (output) compared with the electrical power a motor actually uses (input).

A motor's nameplate rating provides electrical input ratings, which are measured under specific conditions at the full rated load. Efficient motors produce the same output as inefficient motors of the same horsepower rating but require less input wattage. (The U.S. convention rates an output of one horsepower equal to 746 Watts. Output power for motors manufactured in other countries may be stated in watts or kilowatts. Electrical energy input is measured in watts in the USA and elsewhere.)

Typically, a motor’s efficiency is determined by dividing the power output (in watts) by the power input (also in watts) and multiplying the result by 100 to obtain a percentage. (Where motor output is expressed in horsepower, multiply horsepower by 746 to obtain the output in watts.) Most motors have their nominal efficiencies stated right on the nameplate.

There are important differences in efficiency measurement protocol between the USA and other countries. U.S. efficiency measurement is made under the IEEE 112b methodology, which incorporates actual dynamometer testing. Other countries, including European, use non-equivalent methods of measuring efficiency. Efficiency comparisons between motors are only valid if the same testing protocol is used. Generally speaking, the IEEE 112b protocol is more stringent and, therefore, generates efficiency ratings more representative of a motor's in-service performance.

Types of Losses
Unfortunately, the percentage value of efficiency doesn't explain what is going on inside a motor or why motors are inefficient. Losses represent the energy “fees" that a motor charges to make its electrical-to-mechanical energy conversion.

Types of energy losses in electric motors include power losses (made up of stator and rotor I 2R losses), stray load losses, magnetic core losses, friction and windage losses, and a catchall category called stray losses. In terms of energy efficiency, the most important losses are power losses and stray load losses because they vary with load. The magnetic core losses, and friction and windage losses, which are present even under no-load conditions, are less important for energy efficiency.

Power and magnetic losses together account for about 70 percent of total losses of a motor, and they will be the focus of the present discussion. Stray load losses are miscellaneous, load-related, residual losses, which are a function of many complex factors and account for about 15 percent of the total losses. Improved motor design can reduce these stray load losses, but these losses, as well as friction losses at motor bearings and windage losses from internal fans, are not discussed further here.

Power Losses
Power losses (also called I²R losses) can account for one-half or more of a motor’s total losses. These losses appear as resistance heating resulting from electrical current in the stator windings, rotor conductor bars and rotor end-rings. About two-thirds of the power losses are due to stator losses. Motor manufacturers have made significant gains in efficiency by increasing the mass of stator windings, which lowers their electrical resistance and reduces I²R losses.

Typically, high-efficiency motors contain about 20 percent more copper compared with standard efficiency models of equivalent size and rating.

The other one-third of the power losses are due to rotor losses. Rotor losses are mainly from motor slip, which is the difference between the rotational speed of the magnetic field and the actual rotation of the rotor and shaft at a given load. Rotor losses can be reduced by increasing the mass of the rotor conductors (i.e., the conductor bars and end plates), the conductivity of the rotor conductors, and, to a lesser extent, the total flux across the air gap between rotor and stator.

Conductor bars in large motors are normally made from high-conductivity copper, but in small-to-intermediate size motors (from a few horsepower up to about 200 hp) conductor bars are made from aluminum, usually in the form of die-cast “squirrel cages.” Increasing the mass of the die-cast bars requires changing the shape of the slots in the rotor laminations through which the bars are cast, which changes the rotor’s magnetic structure. Lowering I²R losses in typical aluminum-alloy squirrel cage motors is not a simple task. Copper, which has higher electrical conductivity than aluminum, is a much better conductor material.

Copper has not traditionally been used in smaller motors because, until recently, it has been notoriously difficult to die cast. Fortunately, a process to die cast copper rotors has recently been developed, and motors with even higher efficiencies than the best models currently available are now being manufactured.

High-efficiency motors tend to have less slip (and therefore run faster) than standard-efficiency motors. This feature 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. Other parameters such as torque or starting current can vary among motors of the same nominal horsepower. It is important to engineer properly the application of any motor to the intended task, taking into account all engineering parameters under various operating conditions.

Magnetic Losses
Magnetic core losses arise from hysteresis effects, eddy currents and magnetic saturation. These all take effect in the steel laminations. Magnetic losses can account for up to 20 percent of total losses. With proper design, use of better materials and stringent quality control, these losses can be reduced considerably. The most effective means to reduce hysteresis and saturation losses is to use steels that contain up to 4 percent silicon for the laminations in place of lower-cost plain carbon steels. The better magnetic properties offered by silicon steels could reduce core losses by 10 to 25 percent. Reducing the thickness of the laminations also helps; for example, substituting 26-ga or 29-ga steel for the 24-ga steel in standard efficiency motors would lower core losses by 15 to 25 percent. Lengthening the lamination stack reduces the flux density within the stack and, therefore, reduces core losses as well. Eddy-current losses can be reduced by designing the stack with insulation between laminations to minimize current (and I²R losses).

Choosing High-Efficiency Motors
The way to make the correct economical choice is to evaluate the life-cycle cost, which includes the purchase price plus the value of future operating costs. The longer high-efficiency motors are run, and the higher their duty cycle, the more energy they will save. The greatest savings are attained where utility rates are highest.

Another point to consider is that high-efficiency motors are made to higher manufacturing standards and tighter quality controls than the older standard-efficiency motors they typically replace. The new motors run cooler because they generate less I 2R heat, so there is less thermal stress on the windings. The motors can be expected to last longer with reduced downtime and lower repair costs over the life of the motor. It has been estimated that every 10-degree increase in operating temperature reduces a motor's service life by one-half.

OEM equipment suppliers sometimes offer their products with a choice of motors. If the equipment buyer looks at first-cost price only and selects the cheapest option, then the equipment might be fitted with a lower-efficiency motor. In order for competing vendors to offer bids based on the same high-efficiency components, a smart buyer will run a life-cycle-cost analysis and specify the most energy-efficient motor that meets the equipment's requirements at a reasonable first cost.

Evaluating life cycle cost is relatively easy using various software tools. Examples of such calculations, along with a wealth of additional information about high-efficiency motors, can be found in the energy efficiency area of CDA’s website (www. copper.org/energy). David T. Brender, P.E. is national program manager, Electrical; and John Cowie is project manager, Copper Rotor; Copper Development Association, 260 Madison Avenue, New York, NY 10016. Phone: 212-251-7200. Fax: 212-251-7234.
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Premium-Efficiency Motors & Transformers CD-ROM

This CD-ROM explains some of the mechanical aspects of how more-efficient motors and transformers are made, how they differ from standard-efficiency products, and why they are worth the premium you may pay. It also examines properly selecting the right capacity unit for the job (oversizing a motor or transformer can result in significant energy losses) and going beyond the Energy Policy Act and other practices to help save energy, protect the environment and add to your company's bottom line.

Importantly, the CD-ROM explains how to perform the simple math to analyze the total cost of ownership and some tools available to help you make the right selection. Several examples of motor and transformer life-cycle-cost calculations are presented.

The CD-ROM (A6070) is made for Windows 98, 2000 or XP. It includes about 40 minutes of video as well as printable (PDF) documents. The first copy is free to U.S. addresses. Call toll-free, 888-480-MOTR. For non-U.S. orders, please see the CDA Publications List.
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Noteworthy Facts

Source: Motor Decisions Matter, Energy Efficiency/Usage Fact Sheet, www.motorsmatter.org

 

• Electric motor systems account for 23 percent of all electricity consumed in the United States and almost 70 percent of manufacturing sector electricity consumption.

 

• In 1998 the U.S. Department of Energy reported that only 11 percent of customers have written specifications for motor purchases and only two-thirds of those customers included efficiency in their specifications.

 

• Motor electricity consumption can approach 90 percent of some industries' (e.g. pulp and paper, textiles) total electric bill.

 

• Premium efficiency motors are 1-4 percent more efficient than motors meeting federal minimum efficiency standards. Because many motors operate 40-80 hours per week (or more), even small increases in efficiency can yield huge energy savings. Many motor manufacturers, electric utilities and state and regional programs now recognize NEMA Premium™ as a common definition for premium efficiency motors.

 

• The average motor easily consumes 50-60 times its initial purchase price in electricity during its 10-year life.

 

• Motor energy costs can exceed $1 million annually in large industrial plants. In steel plants, energy costs can exceed $6 million.

 

• According to the U.S. Department of Energy, greater attention to motor system management can reduce motor energy costs by 18 percent while also boosting productivity, reliability and profitability.

 

• In 1992, the Energy Policy (EPAct) established minimum efficiency standards for industrial electric motors. Because of EPAct, standard efficiency motors bought today are likely to be more efficient than older motors; premium-efficiency motors offer additional savings.

 

• For most motors, the purchase price represents just 2 percent of its lifetime cost. Electricity accounts for nearly 98 percent.

 

• When all appropriate applications for premium-efficiency motors are realized, they will save approximately 4 billion kWh/yr. and $200 million in annual energy expenditures.

 

• The energy saved by using premium efficiency motors is expected to decrease harmful emissions by the following amounts each year:

• Carbon dioxide: 6 billion pounds
• Sulfur dioxide: 77 million pounds
• Nitrogen dioxide: 22 million pounds

 

• According to the Department of Energy, it is estimated that the NEMA Premium™ motor program could save over 5,800 gigawatts/hours 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.

 

• Over 1.2 million integral electric motors are sold each year.

 

• Each year, more motors are repaired than are sold new. For every new motor sold, approximately 2.5 motors are repaired. It is estimated that motors are repaired on average every 5 to 7 years. Since motors are frequently operated for 20 to 30 years, a motor may be repaired 3 to 5 times in its service life.

 

• Recent research has linked quality repair practices to greater retained motor efficiency and reliability. If properly repaired, most motors can be restored to their original efficiency. However, improper repair of motors can decrease efficiency by up to 5 percent. The Electrical Service Apparatus Association and the Department of Energy offer guidance on how to maintain motor efficiency during repair.