When a motor fails, plant maintenance and engineering teams must decide between rewinding, repairing, or replacing the motor, and the best economic decision for a given asset is not always clear. What factors do industry practitioners evaluate in order to make a well-informed decision on repairing versus replacing?
“Motors fail for many reasons other than age or hours of operation,” says Mark Burgess, General Product Manager, Specialty & Definite Purpose Motors, Baldor Electric. “When a motor does fail, two things to consider are cost to repair or replace and operator downtime.”
Before repairing an existing motor it is important to know the cost of repair compared to the cost of a new motor, says Burgess. “After the EPAct law, effective in 1997 and the EISA law, effective in 2010, motor efficiency requirements have changed tremendously. Today’s motors are much more efficient than motors built in previous years. Simply rewinding a pre-EPAct motor could be less costly, but the energy savings seen on a newer premium efficient motor will offer tremendous energy savings over the life of the motor.”
Charles Dix, Account Manager, Holland Industrial, points out that the size of the motor can have a direct impact on cost. “If the motor is a standard motor (i.e., off the shelf) of 25 hp or less, we will replace bearings but will not rewind, as the cost of bearings replacement is significantly lower than that of a new motor. If the motor is greater than 25 hp and standard then we will replace bearings, and we also will rewind if the cost is less than 60% of the cost of a new premium efficiency motor.”
In Dix’s experience, the cost for a non-standard motor cost is significantly high and the lead times can be long to replace, so “if the motor is not standard (i.e., special shaft or options) then we will replace the bearings and rewind. Also, we will rewind motors over $5,000 if the cost is less than 75% of the cost of a new premium efficiency motor.”
Edward Zitney, Jr., Solution Factory Manager, SKF, says that “if it makes sense based on cost to rewind the motor, then the item to pay attention to is the stator laminations. If the laminations are not damaged then rewinding makes perfect sense.” This attention to the stator also is important to Dix, who adds his motor shop “provides test data to insure we have a good stator. There is no significant loss in efficiency if a rewind is done correctly.”
The second factor identified by Burgess the importance of downtime. “If an area of the plant is shut down due to motor failure, then what is the domino effect?” says Burgess. “The line that is down could be the feeder operation to another portion of the plant, thus contributing to a great deal of work stoppage. These hidden costs could be astronomical compared to the cost of a readily available premium efficient motor, especially a properly applied motor.”
Geoff Generalovic, retired maintenance electrician from Canada’s steel industry, echoes the importance of factoring downtime into the decision. “The major consideration is whether you can get a spare or not. Sometimes a rewind is the only option since you cannot buy a new motor to replace the existing one. Production issues are a key factor as well – how fast we can get up and running again still plays a huge factor in the decision.”
Some organizations are using a Motor Management Plan or Program (MMP) to guide decisions across all aspects of motor ownership based on a thorough knowledge of a plant’s motor inventory. Mike Williamson, Business Development Specialist, Siemens Industry, Inc., indicates that the purpose and use of a Motor Management Program is entirely driven to optimize cost of motor ownership. “The costs levers include increase in reliability, increase in up-time, reduction in inventory, maximizing maintenance costs, and increasing motor energy efficiency. A proper MMP must be tailored to a particular location, and a successful MMP at one location may not be successful at another.”
When Generalovic retired from ArcelorMittal in 2012, he says there was definitely an MMP in place. “Motor testing was done across the plant to keep tabs on these machines and how they were running, from small horsepower up to 8,000-hp main drives, both AC and DC. The plan was very effective: due to the history saved in the archives of the tester, we were able to compare like motors across the plant to see if they were performing reliably or not.”
