Efficient Plant February 2018 - 34
As these images show, rotors from squirrel-cage
induction motors are not all alike. Differences aside,
they also can be difficult to access and evaluate in
ROTORS ARE AMONG the many variables that can
affect the reliability of squirrel-cage AC induction motors.
According to Noah Bethel of PdMA Corp. (Tampa, FL,
pdma.com), a study sponsored by the Electric Power
Research Institute (Palo Alto, CA, epri.com), and performed by General Electric in the 1980s estimated that
rotor defects were responsible for approximately 10% of
failures in such motors. Things certainly have changed
since then-including motor-testing equipment and
methods. Yet, even now, Bethel notes, one of the biggest
problems in electrically analyzing squirrel-cage induction rotors continues to be access to the rotor itself. The
solution? "We don't want to disassemble every motor just
to look at its rotor," he explained. "We have to be a little
smarter and a little more equipped with the right kind
of tools and techniques." As an example, he points to the
following six types of rotor analysis. Keep them in mind.
- Jane Alexander, Managing Editor
Noah Bethel is vice president of Product Development for
PdMA Corp., Tampa, FL. For more information on motortesting and analysis topics and solutions, visit pdma.com.
Fp (pole-pass frequency) sideband
amplitude is one of the more established methods of rotor evaluation
using the current-signature analysis
test. The slip between the rotating rotor and stator magnetic fields creates
a modulation of the stator current at
Fp presented as a peak on a spectrum
plot in the frequency domain. Differential amplitudes between the Fp
and line frequency can be trended to
identify rotor-bar defects.
Demodulating the current and
displaying it in the frequency domain
provides a look into rotor health,
as well as the electro-mechanical
machine-train components of the
motor. Research has found that the Fp
identified in the demodulated spectrum is the most sensitive indication
of developing rotor-bar anomalies for
large two-pole motors.
Broken rotor bars result in a 180-deg.
phase shift in rotor magnetic flux.
This can be seen in a rotor-evaluation
current spectrum as three peaks
separated by Fp to the left of the 5th
Inductance measurements of the
de-energized stator windings at different rotor positions can be plotted
to create a graphical representation
of the rotor magnetic flux. High resistance joints and broken rotor bars
will change the impedance reflected
back onto the stator windings creating a rotor-defect flux pattern.
High resistance connections or broken rotor bars change the reflected
impedance on the stator windings
causing a drop in the start-up current
and start-up torque. This drop in
startup torque will result in a longer
acceleration time for the motor.
Broken rotor bars result in an
increased inductance value measured
on the stator windings of a deenergized motor. Trending this value
will give ample warning of developing rotor defects. EP