Motor repair answers via Virtual Training Day

Motor repair advice about applications, a motor detective story, and a common misconception about motor designs are among additional answers available after a motor repair educational course.

By Thomas H. Bishop, P.E. August 27, 2020

Trends on motors and drives were covered in the CFE Media & Technology Virtual Training Day on Aug. 19, providing online training; the instructor for one of the courses answers additional questions about motor repairs. Four courses offer potential for four approved learning units with four courses:

What to know when repairing electric motors
How to specify motors for more efficient HVAC systems
Introduction to motors and drives
How to design hospital, health care facility HVAC systems.

Motor repair application, motor designs, motor detective stories

Thomas Bishop, PE, senior technical support specialist, EASA, gave 45 minutes of instruction on motor repairs followed by more than 15 minutes of questions and answers. The information will be available at www.controleng.com/online-courses until Nov. 30, 2020. Answers to unanswered questions follow.

Are there certain applications that have particular motor repair concerns?

Applications with higher than normal mechanical stress or in harsh environments may require additional repair steps. For applications with higher stress, such as a rock crusher, mechanical components such as the shaft need closer inspection to check for bending or cracks. Harsh environments such as outdoors near the ocean with its salt water can lead to extensive corrosion damage. Applying overcoating with epoxy materials would probably be called for to reduce the effects of corrosion.

Do more recent motor designs extend motor life over earlier generations? How?

The short answer is “no.” As energy efficiency of motors has increased, a popular misconception is that motor design life has also increased. However, the factors that affect motor thermal life, such as winding temperature, for the most part haven’t changed; and the bearings and mechanical systems are still the same.

Can you share a useful “detective story” about motor repairs?

An end user had a motor in a centrifugal pump application and when it required service center repairs, it was found the shaft had a crack in it. The pump application would not have caused a mechanical shock that could explain the cracked shaft, and the next suspicion was the shaft material had an inherent flaw in it. A root cause analysis (RCA) was performed and what was found was the motor had been used in a severe mechanical shock application, a shredder subject to overload, and that the crack occurred in that prior application.

Motor repairs: More questions and answers

Questions below were among those covered in the live question and answer session and included here to encourage others to listen to the presentation archive.

The repair or replace flow chart in your presentation, was very useful, thanks. Are there EASA-recommend best practices to avoid unscheduled motor breakdowns? With highest-performing critical rotating assets, should we consider installing networked sensors and monitoring analytics?

A broad-based document is EASA’s “Getting The Most From Your Electric Motors,” which is available as a free download from the EASA website. More motor manufacturers and others are getting into the Industrial Internet of Things (IIoT) and are providing real-time monitoring of motors and other rotating equipment.

Do the 1997 or 2007 efficiency laws you mentioned require motor replacements instead of repairs in any situations?

Neither regulation mandates motor replacement.

Thanks for the research showing no significant efficiency degradation on repair of premium efficiency motors. Do motor service centers need any special training or skills to work on NEMA Premium or IE-3 motors?

From a technical perspective, the repairs to premium efficiency motors are no different than their predecessors. The most significant differences are that premium efficiency motors tend to be heavier due to longer cores; and sometimes also physically longer overall. The heavier weight and longer physical size can, at times, present issues replacing a lighter and shorted motor.

Is data in the ¼ million EASA windings database being used for predictive maintenance or other automated analytics efforts? (A couple years ago, I heard an analytics software company bought a motor services company primarily for its data.)

The data is winding data, such as was shown in slide 31. I don’t think anyone would purchase a service center to gain access to this database, but it could add to the value of a service center was being sold.

Do windings naturally degrade over time following a pattern?

There is a guideline rule for thermal degradation of windings: winding life is halved for every 10 degrees C increase in temperature. As part of the inspection process, service center technicians view the condition of the windings. Even though a winding passes electrical tests, it may have evidence of overheating (discoloration) or contaminants that have adhered to the winding, creating a thermal blanket (a recent case was a winding coated with cement), and that can’t be removed without damaging the winding.

What are some common misconceptions about motor repairs?

Probably the most common misconception, and one that we have addressed in today’s session, is that motor repair degrades efficiency. Based on the factual information provided in the session it should be clear that repairs using good practices can not only maintain motor efficiency, but also maintain motor reliability.

Do you have additional advice or recommendations regarding motor repairs?

As the end user, check to be certain that the service center performing your repairs follows the good practices in ANSI/EASA AR100, and if the service center is EASA Accredited, that is further assurance they do. Although it may not be possible do so in the present pandemic, take a tour of your service center. You will gain a better appreciation of the work that goes into motor repair, and take a good look at a rewind in progress. The details and intricacies of winding insertion are rather fascinating, if not amazing. Last but not least, perform predictive and preventive maintenance on your motors to extend the mean time between failures (MTBF).

Thomas Bishop, PE, is senior technical support specialist, EASA. Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media and Technology, mhoske@cfemedia.com, who served as moderator for the session.