By James Piper, P.E.
November 2014
Motors and motor drives are among the single largest users of electricity in most commercial and institutional facilities. And while in most cases, motors are among the most reliable pieces of equipment found in facilities, failures do occur. When they do, engineering and maintenance managers are faced with an important decision — repair the failed motor replace it.
On the surface, the repair-or-replace decision might seem simple, but it can be challenging. Motors use energy. New standards have improved the efficiency of new motor designs to as high as 95 percent for general-purpose motors up to 200 horsepower (hp). Replacing a failed motor with a new, higher-efficiency motor would save energy, and replacing a large, older motor with a higher-efficiency model would save even more energy. So replacement would seem to be the answer.
But replacement is not the only option, and in some cases, it might not be the best option. Motors can be rebuilt. Depending on the level of damage to the motor, rebuilding can be a much more economical option than replacement.
Managers need to take other factors into consideration, such as the cause of the motor failure, the motor's condition, the loading of the motor, the economics of the application, and the time it will take to obtain a replacement motor.
The standard practice for most managers when a motor fails is to have a technician remove the motor and have it rebuilt or replaced without investigating the cause of the failure. In many applications, managers can get away with this approach, and the new or rebuilt motor functions properly for many years. But this does not always happen, nor is it always the best course of action. Too often, the new or repaired motor once again fails, usually for the same reason the original motor failed.
For example, the motor might be installed in a location with high levels of dust and dirt. If the motor had an open enclosure, it could be the exposure to dust and dirt contributed to its failure. Replacing it with another motor that has open enclosure would subject the replacement motor to the same environmental conditions that caused the original motor to fail.
Managers need to look at the history of the motor. Have there been previous failures in this application? A history of failures indicates that the wrong type of motor might have been installed for the application or that there might be significant power issues with the location that are contributing to the motor failure. Unless managers take steps to identify and correct these issues, motor failures are likely to continue even after replacement or rebuilding.