Electric motors are the most common machines in any manufacturing plant. They’re reliable, efficient and can drive almost any kind of load. Treat them correctly and they’ll reward you with long life. To maximize the chances of reaching that long life, you should have an effective preventive and predictive maintenance program in place that’s based on all the probable motor failure modes. Often, failure modes and effects analysis (FMEA) or reliability-centered maintenance (RCM) can serve that purpose because both technologies require intimate knowledge of how things can break down.
Let’s take a closer look at electric motor failure to examine the stresses on the motor, the most common failure modes and some of the causes of those failure modes. Then, let’s look at some ways to either mitigate the failure or to detect an incipient failure.
They can last a long time, but motor death usually has nothing to do with aging. Many questions arise before you can decide when that life is over: How do you define the end of motor life? Does bearing failure constitute the end of life? Does having to rewind the stator define it? How about having to scrap the motor because it’s beyond repair? Or, is it more sensible to replace it so you can capture the energy savings that higher-efficiency motors provide?
Fortunately, you can use standardized tables to help predict motor life (Table 1). Factors that affect motor life are numerous. They include:
- Motor design
- Materials of construction
- PM/PdM effectiveness
- Insulation class
- Bearing type and quality
- Enclosure type
- Quality of construction
- Operating environment
- Load characteristics
The motor is one part of a system that includes its power supply, mounting, coupling and driven equipment. The environment includes ambient temperature, airborne contamination in the vicinity (moisture, corrosive fumes), ambient shock and vibration, etc. It’s a combination of stresses acting on the bearings, stator, rotor, shaft and frame that causes most motor failures.
Table 2 shows the major stresses acting on the five primary motor components. A number of failure mechanisms can cause the same component to fail with a common mode and failure pattern. For example, improper voltage, overloading, blocked ventilation, excessive cycling and excessive heat all can produce the same type of winding failure. Many motor failure studies tell us that the bearings are the most common cause of failure. Next is electrical failure, followed by mechanical failure (see sidebar).
Bearing failures have many causes, but they can be tied to installation, cleanliness, lubrication or load. The first reliability rule regarding bearing installation is proper selection for the application.
Type, load rating, quality grade and shielding are some of the factors to consider. Bearing installation problems often are a question of workmanship. Proper tools, proper procedures and a clean work environment mitigate bearing failures.
Inattention to cleanliness is a major cause of bearing failure. Contamination can enter the bearing in a number of ways. A dirty installation environment is a prime contributor. Contamination also enters through poor lubrication practices such as not cleaning the grease fitting when applying grease.
Lubrication ranks with contamination as a contributor to bearing failure. Factors to monitor include using the correct grease or oil, possible lubricant mixing that causes incompatibility problems, and extended lubrication intervals. Too much grease also can be a problem. Not allowing the bearing to relieve the excess grease through the drain plug is one of the more common bad practices.
A number of issues contribute to excess bearing loading:
- Wrong coupling type or installation
- Dynamic overloading (balance, misalignment)
- Belt, sheave or insert wear
- Belt misalignment
- Incorrect belt tension
- Static loading
- Poor workmanship
- Defective bearing housings
Some common measures you can institute to mitigate bearing failures include:
- Visual inspection
- Vibration monitoring, including bearing-specific techniques (shock pulse, ultrasound and others)
- Precision shaft alignment
- Belt alignment and tensioning
- Lubrication programs that follow best practices
- Proper tools, environment and training for technicians
Because bearing failure is the most common problem for electric motors, eliminating bearing problems can prevent 25% to 80% of motor failures.
Excessive heat in the winding breaks down the insulation, which leads to failure. A commonly accepted guideline is that every 10°C increase in operating temperature corresponds to a 50% loss in motor life. Leading standardization organizations conclude that insulation failure contributed to 30% of failures and, of these, 60% were caused by overheating (see “Overheating Electric Motors: One Root Cause of Insulation Failure”, Ernesto J. Wiedenbrug, Baker Instrument Co., www.bakerinst.com/BakerWeb/PDF_Files/Overheating_Electric_Motors.pdf). It’s important to focus on eliminating every potential source of overheating, which includes: