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“Motor
Efficiency vs. Reliability -- Is That an Issue?"
With
trilingual summary
With introduction of higher efficiency a-c motor designs (under
Energy Policy Act legislation in the United States, and the "EuroDEEM"
program in Europe), some motor users feel these newer products may
be less reliable than earlier models. On the other hand, some motor
manufacturers have implied that higher efficiency motors should be
more reliable. Neither view is justified.
Reliability is usually measured by mean time between failures or
MTBF. Failure is whatever requires a motor to be removed from
service. Comparing reliability of one group of motors with another
is difficult. Comparison requires that all motors be subject to the
same operating conditions (applied voltage, environment, and
loading), and all maintained in the same way (lubrication, cleaning,
etc.).
Even then, whatever their efficiency, motors of different ages are
subject to different failure rates as built-in defects are revealed
and corrected. Equally important: industrial motor users seldom keep
complete or accurate failure history records.
For those reasons, reliability is best compared by examining those
components most subject to failure to see how a new design differs
from the old. Such studies have concentrated first on bearings,
which account for more than half of all motor breakdowns. Newer,
higher efficiency motors generally use the same bearings as in
earlier designs. Lubrication systems have not changed. Hence,
failure rates should be unaffected.
The second leading failure cause is insulation breakdown. Thermal
aging alone is seldom responsible. However, the more efficient
motors of today use Class F (155 C) insulation. Operating within the
standard Class B (130 C) limit will greatly increase the thermal
life, indicating far higher reliability. That presumes, of course,
that the user minimizes damaging conditions other than heat.
Maintenance is always a concern. Modern practices such as infrared
thermography, laser alignment, and vibration spectrum analysis may
lead some users to neglect routine surveillance, especially when
coupled with the widespread tendency to cut maintenance budgets and
staff. Any consequent increase in failure rate should not be blamed
on the motor design.
The conclusion: higher efficiency motors should be neither more nor
less reliable than earlier designs.
From “Motor
Efficiency vs. Reliability -- Is That an Issue?"
to be published in the
Electrical Apparatus May 2007 issue
. Visit our online webstore to
order
copies. © 2007
Barks Publications, Inc. All Rights Reserved.
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