MONITORING AND FAULT
MONITORING AND FAULT
DIAGNOSIS OF INDUCTION
DIAGNOSIS OF INDUCTION
MOTORS
MOTORS
EE7000-1
EE7000-1
Oly Paz
Oly Paz
Motor Fault and Diagnosis
Motor Fault and Diagnosis
••
Safety,
Safety,
••
Reliability
Reliability,,
••
Efficiency, and
Efficiency, and
••
Performance
Performance
are some of the major concerns
are some of the major concerns
and needs for motor systems
and needs for motor systems
applications.
For a successful motor operation
the keys are:
•
Quality of the motor,
• Understanding of the application,
• Choice of the proper type of motor for
application, and
Major faults of electrical machines:
•
Stator faults resulting in the opening or shorting
of one or more of stator phase winding,
•
Abnormal connection of the stator windings,
•
Broken rotor bar or cracked rotor end-rings,
•
Static and or dynamic air-gap irregularities,
•
Bent shaft (akin to dynamic eccentricity) which
can result in a rub between the rotor and stator,
•
Shorter rotor field winding, and
Symptoms produced for one or
more of these faults:
•
Unbalanced air-gap voltages and
lines currents,
•
Increases torque pulsations,
•
Decreased average torque,
•
Increased losses and reduction in
efficiency, and
The diagnostic methods to identify
these faults can be:
•
Electromagnetic field monitoring, search coils,
coils wound around motor shafts,
•
Temperature measurements,
•
Infrared recognition,
•
Radio frequency (RF) emissions monitoring,
•
Noise and vibration monitoring,
•
Chemical analysis,
•
Acoustic noise measurements,
•
Motor current signature analysis (MCSA),
ON-LINE CONDITION
MONITORING OF MOTORS
USING ELECTRICAL
•
Electrical signature analysis is the
procedure of acquiring the motor current
and voltage signals, performing signal
conditioning and analyzing the derived
signals to identify the various faults.
•
A FFT (Fast Fourier Transform) analyzer is
required for converting the signals from
the time domain to the frequency domain.
A motor current signal is ideally a perfect sinusoidal wave at 50 Hz.
The amplitude of the peak in frequency is equal to RMS amplitude of
During actual operation, many harmonics will be present in the motor
signal. This is know as the motor’s current signature. Analyzing these
Various types of Faults and
Their Detection Techniques
•
Broken rotor bar and end ring
faults.
•
Eccentricity related faults.
•
Bearing Faults.
BROKEN ROTOR BAR AND
END RING FAULTS
Rotor bar and end ring breakage can be caused by:
•
Thermal stresses due to thermal overload and
unbalance, hot spots or excessive losses,
•
Magnetic stresses caused by electromagnetic
forces, unbalanced magnetic pull, electromagnetic
noise and vibration,
• Residual stresses due to manufacturing problems,
• Dynamic stresses arising from shaft torques,
centrifugal forces and cyclic stresses,
• Environmental stresses,
The broken bar frequencies are given by
Where,
electrical supply frequency por unit slip
=1, 2, 3, …
number of pole pairs
due to normal configuration, The amplitude of frequency
component can be
evaluated by
Where is the stator current fundamental frequency component
s
s
p
k
f
f
brb s1
sf
s
k
p ,... 13 , 11 , 7 , 5 , 1 / p k brbI
s
f
s1
2
2 2 sinp
I
I
s brb sI
p
R
b
2
The difference in amplitude between the line frequency peak and the pole
passing frequency sidebands is an indication of the rotor bar health.
Empirical research has shown that a difference of over 60 dB indicates an excellent rotor
ECCENTRICITY RELATED FAULTS
There are two types of air gap
eccentricity:
•
The static air gap eccentricity, and
•
The dynamic air gap eccentricity.
In case of the static air-gap eccentricity, the position of the minimal
radial air-gap length is fixed in space.
In case of the dynamic air-gap eccentricity, the center of the rotor is not
the center of rotation and the position of minimum air-gap rotates with
The sideband frequencies associated with an eccentricity are
where,
rotor slop number
rotating eccentricity order stator MMF harmonic order
This scheme has the advantage of separating the spectral components produced by an air-gap eccentricity from those caused by broken rotor bars, but it has the
disadvantage that it requires an intimate knowledge of the machine construction, i.e., the rotor slot number.
The second method monitors the behavior of the current at the fundamental sidebands of the supply frequency. These frequencies of interest are given by
where
This scheme provides the advantage of not requires any knowledge of the
R
dn
n
p
s
m
f
f
ecc s1
1
,... 3 , 2 , 1 m
n
p
s
n
kR
f
f
slop ecc s d1
Static eccentricity is the phenomenon of uneven stator-rotor air-gap,
typically caused due to soft foot in the foundation, cocked bearing or an improperly adjusted airDYNAMIC ECCENTRICITY
Dynamic eccentricity is the phenomenon of a variable stator-rotor air-gap,
FOUR TYPES OF ROLLING ELEMENT BEARING
MISALIGNMENT
o i s bng
f
mf
f
,
1
cos
2
,pd
bd
f
n
f
i o rwhere,
n
number of bearing balls
r
f
mechanical rotor speed in hertzbd
ball diameterpd
bearing pitch diameter
contact angle of the balls on the raceThe mechanical displacement resulting from
damaged bearing causes the machine air gap to
vary in a manner that can be described by a
combination of rotating eccentricities moving in
both directions generating stator currents at
frequencies given by
where and is one of the characteristic vibration
frequencies which are based upon the bearing
dimensions
All bearings have a set of unique defect frequencies.
The presence of high peaks at these bearing defect frequencies can identify and pinpoint
