2018-06-07 Plant Engineering
Recent EASA research studies provide new insights on repair vs. replace.
By: Mike Howell, Electrical Apparatus and Service Association (EASA)
For many industrial plants, maintenance strategies and decisions relating to the electric motors in use are among their most critical. Without question, motors are the primary workhorses for many of these plants—driving machinery, pumps, conveyors, and other vital equipment. So when they don’t work properly or fail, the impact on regular plant operations can be enormous.
When faced with an ailing or failed motor, plant operators typically consider whether to repair or replace it. According to a 2014 study conducted by Plant Engineering magazine for the Electrical Apparatus and Service Association (EASA), just more than one-half of plants have a policy of automatically replacing failed electric motors below a certain horsepower rating. While that horsepower rating varied depending upon the plant’s installed motor population, the average rating was 30 hp (Fig. 1).
While such policies address a portion of the motors used at most plants, they do not cover what occurs with those motors. That question was addressed in a more recent research project commissioned by EASA that focused on the disposition of electric motors considered for repair. The research showed that just over three-quarters (79%) were repairable, with the remainder (21%) replaced. Within the repaired electric motor group, mechanical repairs were the most common (49%), compared with electrical rewinds (30%). Further, over the past three years, mechanical repairs are trending higher, while the electrical rewinds are declining (Fig. 2).
What are some of the reasons for these motor repair trends?
1. Availability of a suitable replacement
Any time an electric motor fails, the application should be reviewed. Over time, it’s common for processes to change, and this can alter motor loading and duty cycles. When a failure occurs, consider any such changes that have taken place and how they may have affected the suitability of the motor. If nothing has changed, this assessment will be very quick.
Other times, it could be more extensive. If the motor is deemed acceptable for the application, the decision-making process can move forward. The repair-replace flow chart (Fig. 3) can be helpful for this process.
Motors manufactured with atypical features, such as custom shafts, are more likely to be repaired than readily available stock motors. When suitable replacements are readily available, the two factors often driving the decision are time and money.
2. Cost of repair vs. replacement
Often, motor repair can be a much quicker and less expensive path than replacement. If the motor failure is catastrophic in nature, however, the merits of repair become less favorable. For example, severe damage to the rotor body, stator core, or a significant mechanical failure may decrease repair feasibility.
If an older machine fails, there may be significant opportunities to reduce operating costs by purchasing a machine with higher efficiency (e.g., NEMA Premium or IE3). Quality is an important factor, though, and higher efficiency doesn’t guarantee higher reliability. It’s also important to note that higher efficiency motors can be repaired, when repair is logical, while maintaining efficiency and reliability.
3. Repair provides opportunity to determine (and address) root cause
Depending on the importance of the application, it’s often worthwhile to perform cause analysis when failures occur. While the cost of motor repair or replacement is significant, the cost of downtime can be orders of magnitude greater. When causes are determined, and countermeasures are implemented to prevent their recurrence, the process becomes more robust.
Sometimes repairs can be tailored to address stresses seen by a specific installation, thereby improving reliability. An example of this might be insulation system modification in an installation known to experience transients. Of course, a cause analysis can be performed whether the decision is repair or replace.
4. Regular preventive and predictive maintenance practices can provide “early warning,” allowing repairs to be planned/scheduled when least disruptive to plant operations.
Use of preventive maintenance (PM) and predictive maintenance (PdM) activities has grown continually for decades and continues to grow. PM typically involves component or system maintenance performed on a periodic basis to reduce the likelihood of a failure. An example of this would be a 36-month bearing replacement cycle on a specific motor application.
PdM, which includes online or condition monitoring, assesses the condition of machines in-service to determine the need for repair. So rather than replace the bearings in the previous example on a fixed schedule, the owner may use tools like online vibration and temperature monitoring to assess bearing health and then schedule replacement when needed.
With the onset of Industrie 4.0 and the Internet of Things (IoT), PdM activities, whatever they are termed, will continue to grow. Some machines will be able to predict impending failures and schedule their own maintenance processes.
5. ANSI/EASA standard establishes motor repair best practices
ANSI/EASA AR100-2015 Recommended Practice for the Repair of Rotating Electrical Apparatus describes recordkeeping, tests, analysis, and general guidelines for the repair of induction, synchronous, and direct current rotating electrical apparatus. It is not intended to replace customer or manufacturer specifications or accepted and applicable industry standards, but can be supplemented by such other documents as needed.
It provides a sound framework of recommended practices for each step of the rewinding and rebuilding processes for rotating electrical apparatus. Many service centers use ANSI/EASA AR100 as a foundation around which to develop work instructions specific to their organization. It provides consistency in repair, which in turn yields greater reliability once the motor is returned to service.
6. EASA accreditation provides third-party assurance of motor repair practices
It has been proven that electric motor efficiency can be maintained during repair and rewind by following defined good practices. EASA has developed an international accreditation program for service centers based on the sources of these good practices, namely ANSI/EASA AR100 and the Good Practice Guide from the 2003 study The Effect of Repair/Rewinding on Motor Efficiency, by EASA and the Association of Electrical and Mechanical Trades (AEMT).
The intent of this groundbreaking accreditation program is to evaluate service centers for evidence of compliance to assure that they are using prescribed good practices to maintain motor efficiency and reliability during electrical and mechanical repairs of electric motors. The program accomplishes this by use of independent, third-party auditors. The accreditation program currently is limited to three-phase, squirrel cage induction motors.