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.