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Repair or Replace? The Complexities of Motor Failure

Key Points
  • Excessive heat, power quality anomalies, humidity and contamination can all cause motor failure.
  • Inspection and testing can help in spotting problems and deciding to repair or replace motors.
  • Payback calculations are critical to replacement economics, but there may be factors specific to your facility.

Source: http://www.doe.gov/
Motor
Motors are critical to the operation of commercial and industrial facilities and a significant part of the electric load. Once placed in service, motors can operate for years with minimal attention. At the point of failure, high horsepower motors are likely to be repaired, with many repaired three times or more during their service life. Given the investment that facilities have in motors, it makes sense to have a good understanding of the telltale signs of failure, as well as the economic implications behind the decision to repair a motor or upgrade to a newer model.

Factors causing motor failure

Motor breakdown begins to show itself in higher operating temperature, vibration, acoustic pattern changes and through various electrical tests—insulation resistance, voltage and current differences between phases and winding tests. The following issues suggest the importance of good motor maintenance:

  • Excessive operating temperature may cause insulation to deteriorate and lead to bearing failure—the cause of half of all motor failures. Infrared scans help to detect troublesome hot spots.
  • Power quality problems, such as harmonics and voltage differences, can cause overheating and reduced efficiency.
  • Moisture weakens the dielectric strength of electrical varnish and other insulating materials. It also contributes to corrosion of bearings and other mechanical components.
  • Contamination can destroy motors through abrasion, corrosion and overheating.
  • Shaft currents from motor drives can damage motor bearings. Insulated bearing sleeves, electro-conductive grease, or a shaft grounding system may help.
  • A variety of mechanical conditions can over stress bearings or cause vibration due to motor frame distortion.

Motor testing

Regular motor testing is critical to spotting potential problems, as well as understanding the economics of repair and replacement. Watching and listening to a motor can be revealing. Make a visual inspection to assess the condition of the outside of the motor; look for cracks, broken welds and missing parts. Rotate the shaft manually to check for obvious problems with the bearings or shaft, including eccentricity. If possible, run the motor at nameplate voltage on no load and check for balanced currents and vibration.

Perform an insulation resistance test to evaluate the condition of the insulation and measure the resistance between electrically isolated points. High potential and surge comparison tests are also useful. The high potential test measures insulation integrity through application of a higher than nameplate rated voltage between electrical windings or circuit elements and ground. In the surge comparison test, a specially designed instrument is used to evaluate the winding integrity and symmetry of the motor. 

Repair or replace?

In principle, the answer is simple; rewind or otherwise repair a motor when it is cheaper than buying a new one. Implementing this rule of thumb is a little more difficult because you need to look at the total cost of ownership. Consider the following:

  • The cost of repair or new purchase
  • The efficiency of the existing motor and newer models
  • Availability of a new motor
  • Possible modifications to the mounting
  • Salvage value of the existing motor
  • Cost in downtime and repairs from a possible early failure
  • Annual hours of operation and energy costs

The U.S. Department of Energy's Motor Master Tool recommends that although failed motors can usually be rewound, it is often worthwhile to replace a damaged motor with a new, premium efficiency model to save energy and improve reliability.

Calculating motor replacement economics includes factors specific to the facility, but the overall principle is simple; replace the motor if the payback period is acceptable, if the payback period is not acceptable, repair it. Use the Motor Cost Calculator to estimate operating costs based on horsepower, motor efficiency, operating hours and electric rate.

Develop a long-term maintenance and repair program

Motors are often the lifeblood of a commercial or industrial facility. It is important to develop repair specifications and work with an established repair vendor with a quality assurance program that can meet your specifications.

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