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| Motors: Making Motor Repair/Replacement Decisions |  to download printable version (91k) |
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The prevailing view among facility managers is that it is cheaper to repair failed motors above 15 horsepower (hp) than to replace them. While this is usually true in terms of first cost, when all the relevant factors are considered, replacement with an energy-efficient motor makes economic sense in many situations. One such factor is the efficiency degradation that is typical when motors are repaired. Field surveys confirm that typical motor repair practice reduces motor efficiency by up to 5 percent. Rewinding a motor can preserve and, in rare cases, slightly improve its efficiency, if skillfully done. However, in the rewinding process the motor efficiency can be degraded, greatly increasing operating cost and energy consumption. The magnitude of this problem can vary widely from one rewind shop to another, and can only be properly identified when efficiency measurements are taken before and after rewinding. Quality assurance programs developed by the motor repair industry aim to improve field practice so that a motor will emerge from a repair shop with as small an impact on efficiency as possible. Given federal regulations that went into effect in 1997, mandating minimum efficiency levels for the most common motors, it's likely that even the least-efficient replacement motor will be more efficient than the original motor when it was brand new. Moreover, motors are often oversized for the function they perform, meaning that they operate below the full-load efficiency stated on their nameplate, and thus can be replaced by smaller and less costly motors. These factors often combine to create opportunities for significant efficiency improvements and resulting energy cost savings by replacing rather than repairing motors when they fail. Thus the best economic decision for a given motor is not always as straightforward as it might seem. Economic Comparison of Rewinding Versus Replacement Most analyses of the comparative economics of rewinds versus replacement consider only a few parameters, including first cost, the difference in nameplate efficiency between the failed motor and a potential replacement, duty factor, electricity price, and demand charges. A more comprehensive analysis should consider the following: New energy-efficient motors typically cost about two to three times as much as a repair job in motors up to 200 hp. Figure 1 expresses this relationship in terms of marginal cost per horsepower. The cost-effectiveness of rewinds tends to improve at larger motor sizes because labor requirements for rewinding increase more slowly with motor size than do materials requirements for new motors. Figure 1: Marginal cost for motor replacement versus motor repair
Although a new energy-efficient motor costs more than a rewind, it typically pays back quickly in reduced energy costs. Figure 2 shows a conservative assessment of the cost of saved energy (CSE is defined as the cost of obtaining energy savings divided by the amount of energy saved) from high-efficiency replacement versus rewinding, for totally enclosed, fan cooled (TEFC) motors. It shows a CSE of less than 2.5¢ per kilowatt-hour for motors up to 200 hp at duty factors of 4,000 hours per year or more. (The average duty factor of commercial and industrial motors is over 4,000 hours per year.) While the numbers shown in Figure 2 reflect the assumption that the rewound motor is 2 percentage points less efficient than its nameplate (due to damage from past or proposed rewinding), the potential that frequently exists for motor downsizing is ignored, and the figure is based on the "average" energy-efficient motor, not the most efficient one available. When these other factors are considered, the cost of saved energy will be less than shown in Figure 2. Open drip-proof motors will have lower CSE than TEFC units, relative to rewinds, because they cost less than TEFC motors. Figure 2: Cost of saved energy of energy-efficient motors versus rewinds of standard motors
Figure 3 shows how the payback for replacement gets more attractive if one assumes that the old motor was 75 percent loaded (a common occurrence in the field) and that the new motor can consequently be one standard size-increment smaller. This downsizing reduces the capital cost premium of the high-efficiency unit and allows it to operate on a more efficient part of its load curve than the original motor.
When Is Rewinding More Economical than Replacement? Conditions that tend to favor rewinding over replacement include the following:
Resources to Help the Repair/Replace Decision Horsepower breakpoint charts. Some motor consultants and service professionals advocate the use of horsepower breakpoint charts as a tool for determining a threshold below which all motors would be replaced and above which all motors would be repaired. An example horsepower breakpoint chart is available at: www.advancedenergy.org/root/industrial/publications/horsepower.pdf. Although breakpoint charts can be useful because they give simple rules for deciding whether to repair or replace, they account only for motor duty cycle and the price of electricity in determining the repair/replace threshold. The charts make assumptions about such critical parameters as:
As a result, horsepower breakpoint charts can provide only a rough guide and will result in poor decisions in some cases. Software. Fortunately, the U.S. Department of Energy (DOE) Office of Industrial Technologies has developed a much more precise tool for making repair/replace decisions. MotorMaster+ 3.0 is a computer program that greatly simplifies the process of making smart decisions when specifying motors for new applications or comparing the costs of motor repair with replacement. The software includes a database containing information on over 20,000 motors. It is available free of charge through the DOE Best Practices Clearinghouse, 1-800-862-2086, or http://mm3.energy.wsu.edu/mmplus/default.stm. The Motor Decisions Matter (MDM) campaign. A national campaign to encourage the use of sound motor management and planning as a tool to cut motor energy costs and increase productivity, MDM is sponsored by a consortium of motor industry manufacturers and service centers, trade associations, electric utilities and government agencies. The MDM Web site at www.motorsmatter.org provides information and links to a variety of resources that are useful in making informed repair vs. replace decisions. Information on average rewind costs. The cost to rewind a given motor can vary considerably from shop to shop. In Canadian field studies, rewind prices varied by as much as a factor of 2. Vaughen's Price Publishing Co. provides average costs for motor rewinds by motor size, speed, and frame type at www.vaughens.com.
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