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90 PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 90 NR 1/2014
Toomas VAIMANN1, Anouar BELAHCEN1,2, Javier MARTINEZ2, Aleksander KILK1
Tallinn University of Technology (1), Aalto University (2)
Detection of induction motor broken bars in grid and frequency
converter supply
Abstract. This paper presents the experimental study of three-phase squirrel cage induction motor broken rotor bars diagnostics. Tests have been
performed with two different machines, one through a frequency converter and the other supplied directly from grid. This comparison has been
presented, as the use of frequency converters changes the traditional current spectrum of the machine and hence the diagnostic of such machines
becomes more difficult. Necessity for further study on the behavior of the frequency converter in the weak grid is pointed out.
Streszczenie. W artykule zaprezentowano metodę diagnostyki trójfazowego silnika indukcyjnego z uszkodzonymi prętami. Zaprezentowano widmo
sygnału prądowego różnych silników. (Wykrywanie uszkodzonych prętów silnika indukcyjnego na podstawie widma częstotliwościowego
prądu)
Keywords: condition monitoring, frequency converter, electric machines, induction motors.
Słowa kluczowe: diagnostyka silnika indukcyjnego, widmo częstotliwościowe.
doi:10.12915/pe.2014.01.22
Introduction
Induction machines are by far the most widely industrial
electrical machine type in nowadays world and considering
their rugged build and cost-efficiency, their popularity can
be expected to stay the same if not increase in the near
future. In fact, in developed countries today there are more
than 3 kW of electric motors per person and most of it is
from induction motors [1]. Failures and faults in such
machines can often have dramatic results and pose danger
for the people and surroundings and cause economic
problems for the industries, whose production depends on
them.
The majority of all stator and rotor faults are caused by a
combination of various stresses, which can be thermal,
electromagnetic, residual, dynamic, mechanical or
environmental [2]. These stresses can cause a number of
different failures in electrical machines. In the case of
squirrel cage induction machines, one of the most common
faults is the cracking and eventually breaking of rotor bars.
The main concern regarding such faults is that it is often not
worth or possible to repair the rotor, if the fault has been
detected too late. However all of this can be avoided, when
the motor is supervised by an appropriate condition
monitoring or diagnostic system allowing the anticipation of
the fault and its propagation.
With the development of power electronics, many
machines are today supplied by frequency converters,
which enables good control of the torque and speed and
thus energy saving in ,e.g., pumping and ventilation
applications. However, the frequency converter supply
changes the natural and traditional current spectrum of
electrical machines. This also means that diagnostic
measures have to be changed to match the peculiarities in
the behavior of electrical machines connected to
converters. However, the main requirements for diagnostic
methods still stay the same, such as no additional changes
or disturbances to the working cycle of the machines, when
performing condition monitoring.
This paper describes an experimental study where
broken rotor bars diagnostics of induction motor is
performed on a machine that is supplied directly from grid
and also a machine that is driven though a frequency
converter. The studied parameters are the motor terminal
currents and voltages. The first part of the paper describes
the measurement set up and the following parts reviews the
Clarke’s vector equations and analyzes the measurements
based on these equations.
Measurements
Measurements described in this paper were performed
in two separate series. For the experiments, where the
induction motor is supplied straight from the grid, a motor
with a healthy rotor and a rotor with seven broken bars was
used. In the experiments with supply through a frequency
converter, a motor with a healthy rotor and a rotor with three
broken bars was used. Broken rotor bars were situated next
to each other in both cases. Schematics of the setups for
the tests are presented in Fig. 1.
Fig.1. Schematics of experimental setups used for testing. Left –
motor is fed directly from the grid through an autotransformer and
loaded with an electromagnetic brake; right – motor is fed through
frequency converters and loaded by an identical motor and
frequency converter in regeneration mode.
In case of Motor 1, the machine is supplied directly from
the grid through an autotransformer. The motor is loaded
using an electromagnetic brake. Tested induction machine
was connected in star during the tests.
Motor 2 is supplied from the grid through ABB ACSM1-
04 frequency converters. Scalar control was used for driving
the machine. Motor was loaded using an identical machine
in generator mode and the machine was also connected to
star during the testing period. The DC-links of the frequency
converters were connected in parallel and only the power
losses from both machines and frequency converters were
drawn from the grid. Table 1 presents more precise data of
the tested induction motors.
PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 90 NR 1/2014 91
Table 1. Technical data of the tested induction motors
Motor 1 Motor 2
Parameter Symbol Value Value
Rated voltage Un 177 V 380-415 V
Rated current In 14.8 A 41 A
Rated speed nn 1456 rpm 1700 rpm
Rated power Pn 3 kW 22 kW
Frequency f 50 Hz 60 Hz
Power factor cosφ 0.785 0.860
Number of poles p 4 4
Number of rotor bars Qr 44 40
Number of stator slots Qs 36 48
Analysis and discussion
Clarke’s vector approach is an easy way to decide if the
motor is healthy or not [5]. It means that the phase currents
(ia, ib, ic) are transformed to current alpha and beta
components (iα, iβ) and placed on α-axis and β-axis
respectively. In other words, a three-dimensional system is
transformed to a two-dimensional one assuming the zero
component of the space vector is not allowed to flow, which
holds true in case of star connection of the machine. The
two components of the Clarke’s current vector are then
given as:
(1)
ab
a
iii
ii
2
3
1
Its representation at steady state operation of the
machine is a circular pattern centered at the origin of the
αβ-coordinate system. This is a very simple reference
figure, which allows the detection of an abnormal condition
due to any fault of the machine by observing the deviations
of the acquired picture from the reference pattern [6].
The healthy pattern differs slightly from the expected
circular one, because of the distortion and unbalance of the
supply voltage and thus of the current space vector [7].
Clarke’s vector current pattern of the rotor with broken bars
is however more ellipse-shaped and its discrepancy from
the circular pattern could be used for fault detection [8].
The Clarke’s vector can be also used to transform the
three-phase voltages at the terminal of the machine into α-
and β-axis components:
(2)
ab
a
uuu
uu
2
3
1
where ua, ub, and uc are the phase voltages and uα, uβ are
the voltage alpha and beta components. This gives the
opportunity to monitor the stator voltage as well as current
and make the decisions upon the analyses of the obtained
graphs [6].
Using Park’s vector (and also Clarke’s vector) approach
for diagnostic purposes of electrical machines is not a novel
idea itself. It has been proposed in the end of 80’s by
Cardoso et al [9]-[11], however, the method did not become
widely used as there were significant doubts on the
automation possibilities of the process. Nowadays, when
computation technologies have advanced, the method can
be implemented far more easily, e.g., with pattern
recognition algorithms [12]. In addition, usage of Clarke’s
vector approach on stator voltage instead of current will
grant a wider segment of possible diagnostic usage as the
supply voltage of electrical machines is generally held at a
constant value and is not much affected by the scalar
control, which is by far the most used control methodology
in applications with less requirements on the dynamic
behavior of the drive system.
Grid operation (Motor 1)
In the tests with direct grid supply, the induction motor
with a healthy rotor and a rotor with seven broken bars was
used. All the broken rotor bars were situated next to each
other, as it is the most probable case in practice and the
asymmetry in such case is more severe. It should be noted
that Motor 1 has die-cast aluminum cage, which is prone to
bad casting and thus could present several broken bars not
only at operation but right away after the manufacturing
process. MATLAB software was used to analyze the data.
If one looks at the presented figures (Figs. 2-9), it can
be observed that there are some unexpected curves and
declinations from the expected vector pattern. This is
caused mainly by the supply voltage used during the tests.
As the supply was not exactly sinusoidal, the deviations
from ideal sinusoid can also be traced in the resulting
figures. However, this phenomenon can be left aside, as the
healthy and faulty conditions of the motor are clearly visible
from the figures regardless of the imperfection in the supply.
First two figures (Figs. 2 and 3) show Clarke’s vector
pattern of the stator current while the machine is working at
no load conditions.
Fig.2. Stator current Clarke’s vector pattern of healthy motor at no
load conditions (Motor 1)
Fig.3. Stator current Clarke’s vector pattern of faulty motor at no
load conditions (Motor 1)
As the figures show, the healthy motor Clarke’s vector
current pattern has indeed a more or less circular shape
and the faulty one looks more close to an ellipse as referred
to in the literature [9]. In addition, it was found that the
absolute value of current is higher in the case of faulty
motor, which was also expected prior to the testing.
Next figures (Figs. 4 and 5) are showing the machine at
rated torque conditions.
92 PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 90 NR 1/2014
Fig.4. Stator current Clarke’s vector pattern of healthy motor at full
load conditions (Motor 1)
Fig.5. Stator current Clarke’s vector pattern of faulty motor at full
load conditions (Motor 1)
Faulty case can be traced easily in the full load figures
as well, due to the major differences in the pattern shape of
healthy and faulty conditions. If one compares the figures of
no load and full load operations, it can also be clearly seen
that the current amplitude rises as more torque is applied to
the machine.
Next figures (Figs. 6 and 7) are plotted using the
Clarke’s vector approach on stator voltage at the same time
moment as the current data was gathered. Usage of stator
voltage should yield better results and no load condition is
observed first.
The figures show that the voltage pattern of the healthy
motor looks again more as a circle and that of the faulty
motor more like an ellipse. Such a dramatic change would
not be expected if the supply network was enough rigid to
withstand to effect of fault in the motor. Furthermore,
changes in scale are more drastic and better traceable in
case of the voltage graphs. Additionally, from the healthy
case graph it can be seen that deviations due to the non-
ideal sine voltage supply are not so vivid in the voltage
case.
Figs. 8 and 9 are from the full load test, again taken in
the same time moment as for the current figures.
Fig.6. Stator voltage Clarke’s vector pattern of healthy motor at no
load conditions (Motor 1)
Fig.7. Stator voltage Clarke’s vector pattern of faulty motor at no
load conditions (Motor 1)
Fig.8. Stator voltage Clarke’s vector pattern of healthy motor at full
load conditions (Motor 1)
Fig.9. Stator voltage Clarke’s vector pattern of faulty motor at full
load conditions (Motor 1)
When no load situation and full load situation are
compared, it can be seen that they match in to a very large
extend, which means that most of the changes in the
patterns are due to the voltage unbalance caused by the
seven broken rotor bars in the induction motor and not any
other deviations in the grid or other variables.
Frequency converter operation (Motor 2)
A three-phase squirrel cage induction motor with a
healthy rotor and a rotor with three broken bars situated
next to each other was used for performing these tests. The
machine was not supplied directly from grid but through a
frequency converter using scalar control instead.
It is a commonly known fact that faults such as the
broken rotor bars induce sideband harmonic components to
the stator current spectrum of the induction motor. Those
harmonics are used as fault indicators in the diagnostic
process. Frequency converter causes supply frequency to
slightly vary in time and, as a result, some additional
harmonics in the current spectrum are induced and
sidebands are reduced [13]. Depending on the type of the
frequency converter the damping of sideband frequencies
can be varying in a very large scale due to the raised
PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 90 NR 1/2014 93
amount of noise in the test signals, which makes the faults
more difficult to detect.
In the tests with frequency converters both current and
voltage changes are observed as they were in the tests with
direct grid supply. Figs. 10 and 11 present stator current
Clarke’s vector patterns of healthy and faulty cases under
no load conditions.
If the figures of different cases are compared, no large
scale changes can be detected as it was in the case with
the grid supplied machine. Main difference compared to
previous figures is the thicker line of the vector pattern,
which is caused by the slightly changing supply frequency
of the used frequency converter. As the supply frequency
changes, it causes a slight change in the trajectory of the
Clarke’s vector and thus the vector pattern line becomes
thicker.
Fig.10. Stator current Clarke’s vector pattern of healthy motor at no
load conditions (Motor 2)
Fig.11. Stator current Clarke’s vector pattern of faulty motor at no
load conditions (Motor 2)
Next figures (Figs. 12 and 13) describe stator current
Clarke’s vector pattern of both healthy and faulty cases on
nominal load operation.
Fig.12. Stator current Clarke’s vector pattern of healthy motor at full
load conditions (Motor 2)
Fig.13. Stator current Clarke’s vector pattern of faulty motor at full
load conditions (Motor 2)
Those figures again do not show as large changes as it
could be expected from the grid operation tests. Some
change in the thickness of the stator current pattern can be
observed, but change of shape is minuscule. The detection
of the fault however is very complicated if not impossible
when analyzing the stator current pattern of the machine
equipped with a frequency converter. It can be said that it is
very complicated to differentiate the faulty rotor from the
healthy rotor in the case where induction motor is used with
a frequency converter. As usage of voltage gave some
benefits in the grid operation tests, it can be expected also
when frequency converters are used. No load tests in such
case are presented in Figs. 14 and 15.
As expected, the differences are more traceable when
stator voltage pattern analysis is used. However, the signals
are very noisy due to the additional harmonic components
induced by the converter. Again the trajectory of the vector
pattern is wider, which results in a thicker pattern line, but
results are better and allow differentiation of the faulty case.
Last figures (Figs. 16 and 17) present the stator voltage
Clarke’s vector pattern under full load conditions.
Fig.14. Stator voltage Clarke’s vector pattern of healthy motor at no
load conditions (Motor 2)
Fig.15. Stator voltage Clarke’s vector pattern of faulty motor at no
load conditions (Motor 2)
94 PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 90 NR 1/2014
Fig.16. Stator voltage Clarke’s vector pattern of healthy motor at full
load conditions (Motor 2)
Fig.17. Stator voltage Clarke’s vector pattern of faulty motor at full
load conditions (Motor 2)
Both full load and no load figures are very similar to
each other. The faulty case does not look like an ellipse, as
it could be expected and this can be related to the switching
frequencies, additional harmonics and varying supply
frequency caused by the converter. It should be noted that
the filtration level during the analysis of the result has been
kept the same both in grid and frequency converter
operation figures. Nevertheless, as the changes in the
pattern shapes are clearly visible, it means that analysis of
stator voltage in case of broken bars of the frequency
converter driven induction motor can be used more
effectively for determination of the rotor fault than the
analysis of stator current [14].
Conclusion
Conducted experiments and analyses show that
although traditionally used for transformation of current
data, Clarke’s vector approach can be very effectively used
for transformation of three-phase voltage. For diagnostic
purposes this might be a better use of the method, as
supply voltage of electrical machines is generally held at a
constant value. Also using voltage as a diagnostic
parameter will gain a wider range of possible setups that
can be monitored as scalar control and load conditions do
not affect the voltage pattern. This peculiarity of voltage
patterns gives the opportunity for better and more precise
decisions as healthy and faulty case data can be compared
without any changes or recalculations of amplitudes.
Frequency converters, due to slightly time-varying
supply frequency, induce additional harmonics to the
current spectrum of the machines, which reduces or even
hinders the sideband frequencies that are used as fault
indicators. Also they create more noise in the signals, which
makes detection of the fault more complicated. As seen
from the figures, presented in this paper, current pattern
cannot be used for diagnostic purposes as the fault
indicators are not visible, which is not the case when
voltage is used.
When diagnostics via the comparison of induction motor
performance models is desired, the described method could
be used in a sufficiently effective way to decide upon the
state of the motor [13]. This can be automated using
various algorithms and thus would be an effective way of
diagnostics, which does not need vast computational
resources due to the simplicity of the mathematical model.
The model or figure of the healthy case of the motor can be
used as a master model, to which other graphs would be
compared to. It could prove to be a very good way for
performing diagnostics also in the sense that no
disturbance in the working cycle of the motor is needed in
order to perform the needed measurements and analysis.
The method could also be for testing the success of die cast
aluminum cages of low power motors.
REFERENCES
[1] B o l d e a I . , N a s a r S . , The Induction Machine Handbook.
Boca Raton: CRC Press, (2002), 950
[2] Bonnett, A., Soukup, G., Cause and Analysis of Stator
and Rotor Failures in Three-Phase Squirrel-Cage Induction
Motor, IEEE Trans. Ind. Appl., 28 (1992), No.5, 921-937
[3] Vaimann, T., Kallaste, A., Kilk, A., Using analysis of
stator current for squirrel-cage induction motor rotor faults
diagnostics, in Proc. ECT 2011, (2011), 245-251
[4] Vaimann, T., Kallaste, A., Kilk, A., Sensorless
detection of induction motor rotor faults using the Clarke vector
approach, Scientific Journal of Riga Technical University, 29
(2011), 43-48
[5] C a r d o s o , A . , S a r a i v a , E . , Computer-aided detection of
airgap eccentricity in operating three-phase induction motors by
Park’s vector approach, IEEE Trans. Ind. Appl., 29 (1993), No.
3, 897-901
[6] Vaimann, T., Kallaste, A., Kilk, A., Overview of
sensorless diagnostic possibilities of induction motors with
broken rotor bars, in Proc. EPE 2011, (2011), 183-186
[7] Benbouzid, M., A review of induction motor signature
analysis as a medium for faults detection, in Proc. IEEE IECON
1998, 4 (1998), 1908-1913
[8] M i l e t i c , A . , C e t t o l o , M . , Frequency converter influence
on induction motor rotor faults detection using motor current
signature analysis – experimental research, in Proc.
SDEMPED 2003, (2003), 124-128
[9] C a r d o s o , A . , S a r a i v a , E . , On-line diagnostics of three-
phase induction motors, by Parks Vector, in Proc. ICEM,
1 (1988), 231-234
[10] Ca r d o s o, A . , S a r a i v a , E . , On-line diagnostics of current
source inverter-fed induction machines, by Park’s Vector
Approach, in Proc. ICEM, (1990), 1000-1005
[11] Cardoso, A., Cruz, S., Carvalho, J., Saraiva, E.,
Rotor Cage Fault Diagnosis in Three-phase Induction Motors,
by Park’s Vector Approach, in Proc. IAC, 1 (1995), 642-646
[12] Amaral, T., Pires, V., Martins, J., Pires, A.,
C r i s o s t o m o , M . , Image processing to a neuro-fuzzy
classifier for detection and diagnosis of induction motor stator
fault, in Proc. IEEE IECON 2007, (2007), 2408-2413
[13] Vaimann, T., Belahcen, A., Martinez, J., Kilk, A.,
Detection of Broken Bars in Frequency Converter Fed
Induction Motor Using Park’s Vector Approach, in Proc. PQ
2012, (2012), 53-56
[14] Vaimann, T., Kallaste, A., Kilk, A., Using Clarke
Vector Approach for Stator Current and Voltage Analysis on
Induction Motors with Broken Rotor Bars, Electronics and
Electrical Engineering, 123 (2012), No. 7, 17-20
Authors: M.Sc. Toomas Vaimann, E-mail:
toomas.vaimann@ttu.ee; prof. Anouar Belahcen, E-mail:
anouar.belahcen@aalto.fi; M.Sc Javier Martinez, E-mail:
javier.martinez@aalto.fi; ass.prof. Aleksander Kilk, E-mail:
aleksander.kilk@ttu.ee; Tallinn University of Technology,
Department of Electrical Engineering, Ehitajate tee 5, 19086
Tallinn, Estonia; Aalto University, Department of Electrical
Engineering, P.O. Box 13000, FI-00076 Aalto, Finland.
