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Facility Maintenance Decisions

The Keys to Preventing Motor Failure

Even with proper maintenance, certain components of motors degrade with time and operating stress.

Electrical insulation weakens with exposure to voltage unbalance, over- and under-voltage, voltage disturbances, and temperature. Contact between moving surfaces causes wear. Wear is affected by dirt, moisture, and corrosive fumes, and it accelerates when lubricant is misapplied, becomes overheated or contaminated, or is not replaced regularly. When any components degrade beyond the point of economical repair, the motor’s economic life ends.

For the smallest and least expensive motors, the failure of a component such as a bearing can put the motor out of service. Depending on type and replacement cost, larger motors — up to 20-50 hp — might be refurbished and get new bearings, but usually they are scrapped after a winding burnout.

Still, larger and more expensive motors might be refurbished and rewound to extend life indefinitely. Managers should complete an economic analysis before a motor fails to ensure they make the appropriate repair/replace decision.

Some manufacturers say their motors can last 30,000 hours. Others claim 40,000 hours, and still others simply say, “It depends.” The useful answer is motors probably last much longer with a conscientious maintenance plan than without one.

Motor life can range from less than two years to several decades under varying circumstances. Even in the best circumstances, degradation still proceeds, and a failure can occur if its symptoms are not detected early. Modern predictive-maintenance techniques can detect much of this progressive deterioration in time for life-extending intervention.

Even with excellent selection and care, motors still can suffer short lifetimes in unavoidably severe environments. In some facilities, motors are exposed to contaminants that are severely corrosive, abrasive, or electrically conductive. In such cases, managers can extend motor life by purchasing special motors, such as those conforming to the Institute of Electrical and Electronic Engineers 841 specifications, or other severe-duty or corrosion-resistant models.

The operating environment, conditions of use or misuse, and quality of preventive maintenance determine the rate at which motor parts degrade: Higher temperatures shorten motor life. Every rise of 10 degrees Celsius in operating temperature cuts the insulation life in half.

This fact might mislead one to think buying new motors with higher insulation temperature ratings will significantly increase motor life. This is not always true because new motors designed with higher insulation thermal ratings actually might operate at higher internal temperatures, as permitted by the higher thermal rating.

Bearing failures account for nearly one-half of all motor failures. If not detected in time, the failing bearing can cause overheating and damage insulation or can fail drastically and do irreparable mechanical damage to the motor. Vibration trending is a good way to detect bearing problems in time to intervene.

With bearings often implicated in motor failures, the rating of a bearing might be cause for concern. The L10 rating is the number of shaft revolutions until 10 percent of a large batch of bearings fails under a very specific test regimen. It does not follow that simply having a large L10 rating will extend motor bearing life significantly.

Many factors can destroy a bearing, including incorrect replacement bearings, incorrect lubricant, excessive lubricant, incorrect lubrication interval, contaminated lubricant, excessive vibration, misaligned couplings, excessive belt tension, and power-quality problems. Technicians always must follow the manufacturer’s lubrication instructions and intervals.

Finally, make sure motors are not exposed to loading or operating conditions beyond the limits defined in manufacturer specifications and NEMA Standard MG1. This standard defines limits for ambient temperature, voltage variation, voltage unbalance, and frequency of starts.

The Industrial Technologies Program (ITP) — www1.eere.energy.gov/industry — provided information for this article. ITP leads national efforts to improve industrial energy efficiency and environmental performance and is part of the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy.

posted on 9/1/2008

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