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
When Is Repair More Economical than Replacement?
Resources to Help the Repair-or-Replace Decision