1
On the Frequency Dependence of Harmonic
Current Side-Band (HCSB) based Rotor Fault
Indicators for Three-Phase Cage Machines
C. Bruzzese, Student Member, IEEE, and E. Santini, Member, IEEE
Φ
Abstract—Inverter-fed cage induction motors can run at
variable frequency and speed, and the diagnostic techniques to be
applied for these motors must take in account frequency
variations. Especially in variable frequency railway traction
drives, bar breakage problems have been reported as a
consequence of large sixth harmonic torques produced by the
non-sinusoidal feeding. MCSA can be applied for motor
diagnostics in these cases, but suitable indicators must be used
for reliable fault severity assessment. Harmonic Current SideBand (HCSB) based Indicators (HCSBIs) for broken bars
detection present a good rejection to frequency variations, as well
as to motor load level, drive inertia, and motor parameters. In
this paper, three industrial motors with broken bars have been
tested for HCSBIs performance evaluation with different feeding
frequencies, and the results have been reported and discussed.
Index Terms—Bar Breakage, Fault Indicators, Frequency
Rejection.
T
I.
(fifth, seventh, eleventh, etc.) arise which produce harmful
sixth harmonic torques capable to excite some cage resonance
frequencies [5], [6], and to cause bar breakage. This kind of
fault mechanism is not cited in [2], which mentions, about bar
breakage causes, direct on-line starting, pulsating mechanical
loads, and constructive imperfections of the cage.
The same low-order current harmonics produce, however,
some fault-related sidebands useful for fault detection. In [7] a
theoretical demonstration was provided for HCSBIs
derivation, and we mention here the proposed bar-breakage
indicators:
I (7 − 2 s )ω
I
, Γ (7 ) = (5 + 2 s )ω ,
I 5ω
I 7ω
I
I
Γ (11) = (13−2 s )ω , Γ (13 ) = (11+ 2 s )ω ,
I11ω
I13ω
Γ (5 ) =
Φ
C. Bruzzese is with the Department of Electrical Engineering, University
of Rome “Sapienza”, via Eudossiana 18, 00184 Rome, Italy (e-mail:
claudio.bruzzese@uniroma1.it).
E. Santini is with the Department of Electrical Engineering, University of
Rome “Sapienza”, via Eudossiana 18, 00184 Rome, Italy (e-mail:
ezio.santini@uniroma1.it).
(2)
……….
INTRODUCTION
HREE-PHASE induction motors are the most widely used
electrical machines in industry, and motor condition
monitoring is nowadays a major issue in the maintenance
programs. Condition based maintenance (CBM) offers
numerous advantages, since the maintenance can be dispensed
when and where it is really needed; moreover, predictive
monitoring permits to avoid unforeseen downtimes. MCSA
(motor current signature analysis) is adapt for on-line non
invasive motor diagnostics, in all the cases in which the fault
signature can be properly individuated and evaluated for fault
severity assessment. When applied for bar breakages
diagnostics in cage motors, MCSA can suffer many disturbs
such as load vibrations [1], [2], load level variations [3], and
feeding frequency variations; moreover, drive features and
motor parameters can vary from case to case, so complicating
the problem. The authors proposed in [4] some original broken
bars indicators, which present favorable performances as
regard to disturb rejection and fault assessment affordability.
Since they are based on the sidebands of the harmonic
components of the phase current, HCSBIs can be used when a
non-sinusoidal feeding is provided for motor driving. This
kind of situation incurs, e.g., in railway drives: low-switching
frequency GTO drives are not uncommon in railway
applications, and notoriously low-order current harmonics
(1)
HCSBIs form a class of infinite elements, generally dependent
on slip and frequency. Every element actually increases with
the number of consecutive broken bars. Slip dependence has
been found almost neglectable [4], whereas frequency
dependence must be properly taken in account for variablefrequency drives. Experimental performances of (1), (2) have
been addressed in Section III.
II.
THE EXPERIMENTAL TEST-BED
Three motors (‘A’ in Fig.4, ‘B’ in Fig.6, and ‘C’ in Fig.8)
with different powers (1.5÷3kW) and pole numbers (2÷4)
were subjected to destructive tests. Motor parameters are
reported here below:
TABLE I.
MOTOR ‘A’ D ATA (MEZ-MOHELNICE).
1.5kW
380/220V
TABLE II.
3kW
380/220V
TABLE III.
1.5kW
380/220V
50Hz
3.5/6A
1410rpm
cosφ=0.82
4 poles
28 bars
MOTOR ‘B’ DATA (CAPRARI).
50Hz
6.5/11A
2800rpm
cosφ=0.84
2 poles
23 bars
MOTOR ‘C’ DATA (ELPROM).
50Hz
3.3/5.7A
2860rpm
cosφ=0.88
2 poles
19 bars
Measurements of the phase currents were done with
progressive rotor damage (increasing number of broken bars),
2
for a complete characterization of motor current spectra under
fault conditions.
The three motors was fed by a square-wave inverter, to obtain
the relevant harmonics and sidebands, Fig. 1. Then an
increasing number of consecutive bars were drilled, and the
harmonics were registered on a large load range, for every
selected feeding frequency. Fig. 2 shows a functional diagram
of the test-bed used for experimentation. Acquired data (a
sampling frequency of 20kHz was sufficient, since the motor
current Shannon frequency was found around 10kHz when the
square-wave frequency is 50Hz) were automatically processed
off-line by using a ‘script’ Matlab algorithm, that produced
Fourier transformation and harmonic discrimination on the
basis of the measured motor speed. Fault-related sidebands
can be revealed around very high harmonic order frequencies,
thanks to the square-wave feeding; however, every lowswitching frequency commutation technique can produce a
typical spectral pattern useful for fault detection.
was of concern during experimentations, Figs. 5, 7, 9. Fig.3
(obtained from motor ‘B’) well explains the better
performances of the higher-order indicators, as far as concern
the rejection to frequency and load variations, with respect to
the classical ratio between LSB (lower sideband of the first
harmonic) and the fundamental current (Γ(1)).
Measures done on motor ‘B’ (2 poles), Fig. 7, were
affected by inter-bar currents, that produced sidebands
weakening (with the lighter fault degree) so producing a
curvature [8]. For motor ‘A’ (4 poles), Fig.5, this problem was
less remarkable. Motor ‘C’ (Fig.9) initially behaved like ‘B’;
to overcome the influence of inter-bar currents, for motor ‘C’
bilateral bar interruptions were practiced, so obtaining more
linear results.
A remarkable result that rises from Figs. 5, 7, 9, is that
four-poles motors (motor ‘A’ and the motor tested in [4])
presented indicators with analogous amplitude, and the same
is true for the two two-poles motors (‘B’ and ‘C’), with
amplitudes roughly halved. This leads to the definition of a
criterion for fault severity assessment, as stated in (3):
2
N broken.bars
= Γ (ν ) ⋅
N total .bars
P
ZOOM
(3)
(where P is the polar pairs number) that can be at least used
for ν = 5, 7, in the range of the industrial frequencies and for
two/four-poles motors. Equation (3) leads in turn to define an
“electrical number of broken bars (per unit)”, ‘nel’, as in (4):
nel =
Fig. 1. Phase current spectrum for motor A. Four broken bars, 50Hz feeding
frequency, 70% of rated load. Sidebands (5+2s)f and (7-2s)f have been
evidenced.
MAIN GRID
Load
regulation
VARIABLE-VOLTAGE
AUTOTRANSFORMER
SQUARE-WAVE
VARIABLE-FREQUENCY
INVERTER
Voltage regulation
POWER
METER
voltage/power
measure
Frequency
regulation
INDUCTION
MOTOR
DYNAMOMETRIC
DC-UNIT
current
measure
OSCILLOSCOPE
speed
measure
PC
Fig. 2. Experimental test-bed (functional diagram).
III. DISCUSSION OF THE E XPERIMENTAL WORK
The frequency-dependence of the new indicators (1), (2)
N broken.bars
N bars . per . polar . pair
= 2 ⋅ Γ (ν )
(4)
that furnishes a measure of the degree of asymmetry caused by
broken bars on the electromagnetic structure along the
extension of one polar pair. This number can be retained as a
‘pure’ fault-gravity indicator itself; in fact, this can be easily
understood thinking to the higher values of sidebands in the
four poles motors with respect to two-poles motors.
IV. CONCLUSIONS
HCSBIs are a class of fault indicators for bar breakages
detection and fault gravity assessment, and they are wellsuited for converter-fed motors. Their main features are good
linear-like dependence on the number of consecutive broken
bars and relative insensibility to motor operating conditions
and drive features. Frequency dependence is not so much
preoccupant around industrial frequencies (50-60Hz), but must
be taken in account when a large variation range incurs,
especially for low frequencies (below 25Hz).
3
Fig. 3. Γ(1), Γ(5) trends for motor ‘B’, three broken bars.
V.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
REFERENCES
A. H. Bonnett, G. C. Soukup, “Cause and analysis of stator and rotor
failures in three-phase squirrel-cage induction motors“, IEEE Transact.
on Industry Applications, vol.28, No.4, July/August 1992, pp. 921-937.
W. T. Thomson, M. Fenger, "Current signature analysis to detect
induction motor faults", IEEE Industry Applications Magazine, vol.7,
pp. 26-34, July/Aug. 2001.
C. Bruzzese, O. Honorati, E. Santini, “Real behavior of induction motor
bar breakage indicators and mathematical model”, Proc. of the ICEM
2006 Conference, September 2-5, 2006, Crete Island, Greece.
C. Bruzzese, O. Honorati, E. Santini: “Harmonic current sideband-based
novel indicators of broken bars for on-line evaluation of industrial and
railway cage motor faults,” IEEE 2007 International Symposium on
Industrial Electronics, ISIE 2007, Vigo, Spain, June 4-7, 2007 (accepted
for publication).
C. Bruzzese, O. Honorati, E. Santini, “Rotor bars breakage in railway
traction squirrel cage induction motors and diagnosis by MCSA
technique. Part I: Accurate fault simulations and spectral analyses”,
IEEE SDEMPED 2005, 7-9 Sept. 2005, Vienna, Austria, pp.203-208.
C. Bruzzese, O. Honorati, E. Santini, “Minimization of harmful cage
torsional resonances in traction motors by a combined mechanicelectronic optimization,” in Proc. of 12th European Conference on
Power Electronics and Applications, EPE 2007, 2-5 Sept. 2007,
Aalborg, Denmark (approved for pubblication).
C. Bruzzese, C. Boccaletti, O. Honorati, E. Santini, “Rotor bars
breakage in railway traction squirrel cage induction motors and
diagnosis by MCSA technique. Part II: Theoretical arrangements for
fault-related current sidebands”, IEEE SDEMPED 2005, 7-9 September
2005, Vienna, Austria, pp.209-214.
R. F. Walliser and C. F. Landy, "The influence of interbar currents on
the detection of broken rotor bars,” in Proc. 1992 International
Conference on Electric Machines, ICEM’92, pp. 1246-1250.
Fig. 4. Motor ‘A’ (MEZ-MOHELNICE).
Fig. 5. Experimental results for motor ‘A’.
4
Fig. 6. Motor ‘B’ (CAPRARI).
Fig. 8. Motor ‘C’ (ELPROM).
Fig. 7. Experimental results for motor ‘B’.
Fig. 9. Experimental results for motor ‘C’
5
VI. BIOGRAPHIES
Claudio Bruzzese (S’2005) received the laurea
degree (M.S.) cum laude from the University of
Rome “Sapienza”, Italy, in 2002. He is currently
working toward the Doctoral degree at the Dep.
of
Electrical
Engineering,
University
“Sapienza”, Italy.
He is a member of the AEIT (Italian
Electrotechnique Association), of the IEEE
Industry Applications Society, and of the IEEE
Power Electronic Society. He is a Registered Professional Engineer in
Italy.
His interests cover diagnostics of induction and synchronous machines,
FEA, railway and naval power systems, electro-mechanical design, and
diagnostic computer systems.
Ezio Santini (M’91) received the Degree in
electrical engineering (cum laude) from the
University of Rome “Sapienza,” Rome, Italy, in
1982. From 1983, he was with the Department
of Electrical Engineering, University of Rome
“Sapienza,” where currently he is full professor
of Design of Electromagnetic Devices. His
research activities concern electrical machine
models, transient phenomena in transformers
and rotating machines, FEM analysis of electromagnetic fields in electrical
devices, and numerical methods in electrical engineering. Prof. Santini is a
member of the Italian Association of Electric and Electronics Engineers
and of the IEEE Power Engineering Society.