Instrumentation & Measurement Magazine 24-7 - 65

are labeled, where Rc
is the
pitch radius, β is the contact
angle between the ball
and the ball bearing, and
Rb
Fig. 3. Squirrel cage rotor, examples of one broken bar, half-broken bar and incipient broken rotor bar.
is the radius of the ball.
Any physical damage present
on the outer race, inner
race, or balls bearing is considered
a bearing fault. This
fault represents 40% of the
faults present in induction
motors [3]. Bearing failure
is mainly caused by a lack
of lubricant, corrosion, excessive
load, and bearings
misalignment. These cause
an increase in vibration
and bearing temperature,
which in turn induce spurious
frequencies in both
current and vibration sigfailures.
These two types of failures have been the most studied
in the literature in recent years.
Broken Rotor Bars Fault
Fig. 3 shows the squirrel-cage rotor of an induction motor. In
this figure, one can see the short-circuited bars with the end
rings. There are also three severity levels for the same type
of fault. When one of the bars loses its continuity, it is considered
a broken bar damage, which can occur in one or more
bars. The leading causes for this failure are manufacturing
defects, excessive stress in the machine (mechanical and thermal),
asymmetry in the feeding current, and frequent starting
and stopping, among others. The damage occurs mainly between
the union of the bar and the end ring. There are other
cases where the bar is not entirely broken, which is why it is
considered partial damage (in the figure half of the bar is broken)
or incipient damage (when the damage is less than half
the thickness of the bar). These occur under the influence of
a continuous bar that is already totally damaged, because the
current that cannot circulate through the broken bar overloads
the bars adjacent to it, producing excessive stress on the
bar that eventually causes its damage. It is essential to mention
that the higher the degree of damage (one or more broken
bars), the greater the effect it produces on the current, making
it easier to detect. When the damage is incipient, the spurious
frequencies produced in the current decrease their amplitude,
complicating its detection in an early stage. To be able to detect
it, several improvements in motor instrumentation and signal
processing techniques have been proposed [4]-[6].
Bearing Fault
The bearings are supporting the rotor at its ends in order to
facilitate its rotation by reducing friction. Fig. 4 shows the internal
structure of a 6205 bearing, the outer ring race, the inner
ring race, and the rolling elements or balls. Also, the parameters
October 2021
nals. Various analysis techniques have been proposed for the
detection of bearing failures in initial stages, which are based
on advanced signal processing techniques in vibration signals
acquired by an accelerometer and recently in optical encoder
signals [7]-[9].
Mathematical Model of the Faults
Broken rotor bar faults and bearing damage are characterized
in the current and vibration frequency spectra. In both cases,
spurious frequencies are induced in the spectrum. When there
is no automatic fault detection system, an offline analysis of
the current and vibration signals is performed by a highly
qualified expert. Currently, there is much interest in early detection
based on an online automatic diagnostic technique for
use in condition-based maintenance and the Industrial Internet
of Things.
Fig. 4. Structure of bearing 6205, including its properties and main parts.
IEEE Instrumentation & Measurement Magazine
65

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