Making Motor Repair or Replacement Decisions

The prevailing view among many facility managers is that it is cheaper to repair failed motors larger than 15 horsepower (hp) than to replace them. Although this is usually true in terms of first cost, the best economic decision for a given motor is not always as straightforward as it might seem. 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 common when motors are repaired. Motor repair can preserve and, in rare cases, even improve efficiency slightly, if skillfully done. But field surveys have shown that it's more common for motor repair practices to reduce motor efficiency by up to 2 percent. Reduced efficiency, of course, translates into greater energy consumption and increased operating cost.

The effects of motor repairing on efficiency can vary widely from one repair shop to another and can only be properly identified when efficiency measurements are taken before and after the repair. 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.

Another factor that frequently tips the balance in favor of replacement rather than repair is the fact that in many commercial and industrial applications, a motor's purchase price is a small fraction of the total cost of owning and operating the motor over its useful life (its “life-cycle” cost). The purchase price of a motor that runs at least a few thousand hours a year will usually amount to no more than 3 to 5 percent of its life-cycle cost, with electricity purchases accounting for over 90 percent of that cost. That being the case, it will often make economic sense to replace a failed motor with a premium-efficiency unit, which will reduce life-cycle costs by cutting energy costs.

A third factor is that 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, less costly motors. These factors often combine to create opportunities for significant efficiency improvements and the resulting energy cost savings by replacing rather than repairing motors when they fail.

Economic comparison of repair versus replacement
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Most analyses of the comparative economics of repairing 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 also consider:

  • Life-cycle cost, cost of saved energy, or at least a simple payback analysis
  • Actual versus nameplate efficiency of the existing motor (actual efficiency may have been degraded by prior repairs)
  • Motor capacity versus peak loading (oversized motors operate at low efficiency)
  • Expected lifetime of the repaired motor versus that of a replacement

New premium-efficiency motors typically cost about two to three times as much as a repair job for motors up to 200 hp. The cost-effectiveness of repair tends to improve at larger motor sizes because labor requirements increase more slowly with motor size than materials requirements for new motors do.

Although a new premium-efficiency motor costs more than a repair, it typically pays back quickly in reduced energy costs. Figure 1 shows a conservative assessment of the cost of saved energy (CSE, the cost of obtaining energy savings divided by the amount of energy saved) from high-efficiency replacement versus repairing for totally enclosed, fan-cooled (TEFC) motors. It shows a CSE of less than 2.5 cents per kilowatt-hour for motors up to 200 hp that operate 4,000 hours per year or more. (The average run time of commercial and industrial motors is over 4,000 hours per year.) Though the numbers shown in Figure 1 reflect the assumption that the rewound motor is 2 percentage points less efficient than its nameplate (due to damage from past or proposed repair), 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 1. In addition, open drip-proof motors will have a lower CSE than TEFC units, relative to repairs, because they cost less than TEFC motors.

Figure 1: Cost of saved energy of energy-efficient motors versus repairs of standard motors
This comparison assumes 1,800 rpm totally enclosed, fan-cooled (TEFC) motors at 75 percent load, a discount rate of 5 percent, and an investment life of 10 years.

Payback for replacement gets more attractive if the old motor is less than 75 percent loaded (a common occurrence in the field) and the new motor can consequently be one standard size 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 repair more economical than replacement?
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Motor repair tends to be preferable to replacement when:

  • The original motor is a premium-efficiency unit, and little gain in efficiency will be achieved by replacing it
  • The application has a low duty factor (it only runs a few hours per year)
  • The original motor is a specialty design for which a high-efficiency replacement is hard to get
  • The repair shop can guarantee and verify that its repair will not degrade efficiency

If you do decide to repair a motor, look for a repair shop that has an industry-recognized quality assurance program. Two such programs are the EASA-Q Quality Management System for Motor Repair developed by the Electrical Apparatus Service Association (the national association of motor repair shops) and the Proven Efficiency Verification for Motor Repair (PEV) program developed by Advanced Energy, a Raleigh, North Carolina–based organization that conducts research on industrial process efficiency. EASA-Q outlines equipment, calibration, and repair practices necessary for maintaining motor efficiency. The PEV program adds on-site verification and annual before-and-after repair testing in a nationally accredited motor laboratory.

Resources to help the repair-or-replace decision
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Horsepower breakpoint charts. Some motor consultants and service professionals advocate the use of horsepower breakpoint charts as a tool for determining a company-wide horsepower threshold, below which all motors would be replaced and above which all motors would be repaired. Given a maximum acceptable payback period and information about a motor’s average load, it’s possible to plot motor horsepower versus running hours in a way that illustrates when it’s advantageous to repair a motor and when it makes more sense to replace it (Figure 2). Advanced Energy offers guidance in its Horsepower Bulletin (PDF) on creating a horsepower breakpoint chart using information specific to your company.

Figure 2: Horsepower breakpoint chart for 3,600 rpm TEFC motors
Based on motor price data from 2004 and motor repair data from 2005, Advanced Energy’s Horsepower Breakpoint Tool illustrates when to repair and when to replace a failed motor. In this example, the curve indicates that 30 horsepower and smaller totally enclosed, fan-cooled (TEFC) motors that operate at least 4,000 hours per year should be replaced with NEMA Premium motors, whereas larger motors should be rewound.

It’s important to note that these breakpoint charts should be used only as guidelines. Recent increases in the price of copper, steel, and aluminum have driven prices up for both repair and replacement and may have shifted the breakpoint for any particular motor application.

The Motor Decisions Matter campaign. Motor Decisions Matter (MDM) is a national campaign that encourages the use of sound planning and motor management to cut motor energy costs and increase productivity. The campaign is sponsored by a consortium of motor-industry manufacturers and service centers, trade associations, electric utilities, and government agencies.

Information on average repair costs. The cost to repair a given motor can vary considerably from shop to shop. In Canadian field studies, repair prices varied by as much as a factor of two. Vaughen’s Price Publishing Inc. provides average costs for motor repairs by motor size, speed, and frame type.

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