Page 347 - Improving Machinery Reliability

Chapter 6

Extending Motor Life in the

Process Plant Environment*

The extensive use of polyphase AC, squirrel-cage, induction motors in process
industries to drive pumps, fans, compressors, conveyors and other equipment
emphasizes the importance of preventing motor failures and maximizing motor life
and dependability. When the high cost of downtime can approach $100,000 an hour,
the most effective motor is not necessarily the most efficient motor; it is the one that
runs consistently, with maximum uptime.
While it is commonly assumed that motors fail because of electrical malfunction,
75% of motor failures are initiated by mechanical problems, problems which ulti-
mately cause the insulation and/or bearing systems to fail. Insulation and bearing
system failures are evident in about 90% of all motor failures.
Learning how to extend motor life requires understanding motor designs, materi-
als, and operating requirements.
Motors are designed to deliver differing speed and torque characteristics that satis-
fy different loads. Some loads may be hard to start and relatively easy to keep run-
ning once initial inertia is overcome. Other loads may be easy to start but require
more power as speed increases. Common motor designs, as defined by the National
Electrical Manufacturers Association (NEMA), with different speed and torque
curves may be selected for different loads (see Figure 6-1).
Some process industry motors such as oil well pumping motors require high slip
characteristics to withstand the cyclical load. Others require explosion-proof enclo-
sures. Most are severe-duty applications, requiring protection against the effects of
corrosives and other contaminants.
Knowing what kind of motor is presently installed and evaluating the specific
motor application are the first steps to extending motor life. A misapplied replace-
ment motor, whether designed for conventional or high efficiency, is doomed to
early failure or wasteful operation.

*Source: Jean J. Revelt, The Lincoln Electric Company, St. Louis, Missouri. Based on a presentation
at the 5th International Process Plant Reliability Conference, Houston, Texas, October 1996.

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