Page 353 - Improving Machinery Reliability

Extending Motor Life in the Process Plant Environment 319

On the other hand, a motor operating at a higher temperature does not necessarily
mean a shorter insulation (and motor) life either. A motor with a Class H insulation
system operating at 160°C will have nearly twice the life of a motor with a Class B
insulation system operating at 120°C.

Thermal Cushion

It is possible to determine if a motor is running “hot“ or “cool” and how much
“thermal cushion” it has to tolerate undervoltage or occasional overloads beyond its
service factor. The reliability professional must investigate how close the motor is
running to its designed thermal capacity. The more thermal cushion exists, the longer
the motor will last.
Temperature rise figures should be added to the ambient temperature and com-
pared to the limiting temperature, or design thermal capacity, of the insulation sys-
tem. Both insulation system classification and allowable temperature rise are nor-
mally listed on the motor nameplate.

Enclosures
A motor with the proper enclosure must be chosen, depending upon the ambient
conditions that the motor will experience. Where the ambient temperature is low and
airborne contaminants are not a problem, open motor construction can be used.
Where excessive moisture, other airborne corrosives or particulate matter are pre-
sent, totally enclosed construction is required. When ambient temperatures are
excessive, fans or other cooling provisions are needed. Motors used in explosive
atmospheres require specially sealed enclosures, thermostats to limit surface temper-
atures and other features.
Other threats to insulation life include vermin (which the enclosure may be designed
to exclude), mechanical damage, high dielectric stresses caused by voltage spikes,
bearing system failure, and application-caused external vibration. Power conditions
and mechanical concerns include voltage spikes, bearing systems and vibration.

Standard, but Different

While motor manufacturers subscribe to NEMA standards to simplify the classi-
fication and comparison of motors, there are wide latitudes within these standards
that may complicate the direct replacement of one manufacturer’s standard motor
with that of another. Although two motors may be dimensionally interchangeable,
designed to the same performance standards and full-load ratings, they can con-
sume different amounts of energy to do the same work and will expend their bank
of life at different rates.
Motors with increased efficiency, by definition, experience lower watt losses and
thus generate less heat, permitting the use of a smaller cooling fan for further