Page 354 - Improving Machinery Reliability

320 Improving Machinery Reliability

increases in efficiency. However, this reduces air flow and the cooling capability of
the ventilation system, which will result in reduced insulation life if other steps are
not taken to transfer heat away from the motor in order to limit temperature rise.
Void-free rotor construction, stagger stacking of laminations to expose more radi-
ation area, fan and rotor paddle designs, frame material, and frame configuration can
all affect the way heat is transferred away from the motor. In general, the manufac-
turer’s quality parameters determine the differences in life of standard motors.

Learning from Failures

When a polyphase, squirrel-cage, AC motor fails, inspection of the windings will
usually provide clues to the cause of the failure. A uniform overheating pattern
results when there is poor ventilation or high ambient temperatures, continuous
excess loading beyond the service factor (SF) for extended periods, or extended or
frequent starts. Secondary failures such as phase-to-phase shorts may also occur after
the insulation begins to degrade.
In extreme cases where the rotor shaft is locked, i.e., prevented from turning while
power is applied for extended periods, the rapid heat buildup will not only destroy
the winding insulation, but can even melt the rotor bars. This extreme heat buildup is
often associated with failure of the motor control or another protective device
required to prevent high input currents for extended periods. Such devices should
trip out at 15 seconds or less under conditions of locked-rotor current.
Other causes of insulation deterioration include cyclical loading where the motor
temperature regularly rises and falls, extended periods of slow acceleration, exces-
sive “jogging” or “plugging” (reversing the motor to stop the machinery), or extend-
ed downtime, which may result in moisture accumulation within the motor. Such
motors should be equipped with electrical heaters that are powered when the motor
is not running to keep condensation from forming. Continuous motor operation with-
in the load limitations of the motor will deliver the longest insulation and motor life.

More About Thermal Loading
Cases have been observed in kiln applications employing multiple fans where some
of the fans were shut down while the kilns were operating to save energy; their insula-
tion failed because the ventilation system was unable to get rid of the built-up heat. In
such cases, all the fan motors could have been slowed down with adjustable speed
drives to save the same amount of energy, while still allowing the ventilation systems
to do their job. Blocked air passages from buildup of airborne contaminants, blower
failures on TEBC enclosures and low air density at high altitudes can also lead to
failure of the ventilation system to keep temperature rises within designed limits.
Fans, pumps, and dynamic compression equipment account for a large percentage
of motors used in the process industries. Often it is desirable to run these centrifugal
loads at reduced levels to control such process parameters as flow and temperature.
This can be achieved by dampers or other restrictions to throttle the output, but the