Detection of Stator-winding Turn-To-turn Faults in Induction Motors Based on Virtual Instrumentation
Transcript of Detection of Stator-winding Turn-To-turn Faults in Induction Motors Based on Virtual Instrumentation
7/29/2019 Detection of Stator-winding Turn-To-turn Faults in Induction Motors Based on Virtual Instrumentation
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Detection of stator-winding turn-to-turn faults in induction motors based
on virtual instrumentation
A. Zamarróna and M.A. Arjonab*
aInstituto Tecnológico de León, Dept. Electromechanical, 37290 León, Gto., México
bInstituto Tecnológico de la Laguna, Dept. Electric & Electronic Eng., 27268 Torreón, Coah. México.
ABSTRACT
A virtual instrumentation system for stator turn-to-turn winding fault detection is presented in this
paper. This virtual instrumentation system can be adopted for industry and academy as a useful tool to
prevent unexpected equipment downtime or severe equipment damage by detecting faults in the stator
winding of induction machines. The system uses the high-frequency carrier-signal injection technique to
obtain information about faults from the machine. Measurements of the resulting high-frequency
negative-sequence current are used for detecting turn-to-turn faults. Since the stator windings are fixed
in space, turn-to-turn fault gives rise to a stationary saliency which appears as a dc component in the
spectrum of the negative-sequence carrier-signal current in the negative-sequence carrier-signal
current reference frame. As will be illustrated in this paper, the proposed virtual instrument shows the
magnitude of this component to give the grade of failure of the stator winding.
* Correspondence author: Dr. M.A. Arjona; Instituto Tecnológico de la Laguna; Carrara 371, Col. Torreón Residencial;
27268 Torreón, Coahuila. México. Tel: +52 871 7051331 Ext 115, Fax: +52 871 7051329
Email: [email protected]
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KEYWORDS: High-frequency carrier-signal injection; induction motor; stator winding fault detection;
virtual instrument.
INTRODUCTION
Virtual instrumentation systems is extensively used in industry and teaching laboratories for
experimentation, testing, control, and performance analysis of all acquired measurement data1-3
. In this
paper, a virtual instrumentation system to detect turn-to-turn faults in the stator winding of three-phase
induction machines fed from a voltage source inverter (VSI) is presented. This virtual instrumentation
system can be helpful in industry to reduce unexpected failures and downtime, increase the time
between planned shutdowns for standard maintenance, and reduce maintenance and operational costs.
The operation of the induction motor in an unsafe condition can also be avoided. Various references in
the literature have shown that faults in the stator winding due to insulation degradation are one of the
main causes of electric machine failures.4-6
Degradation of winding insulation can lead to turn-to-turn
faults, starting a process that can progress to severe phase-to-phase or turn-to-ground faults. Although
offline methods for detecting faults in the stator winding can be used, online methods that do not
interfere with the regular operation of the machine are preferred.7-13
While some of these online
methods require additional sensors, e.g., to measure the axial flux11
, or vibration12
, methods that do not
require any additional sensors are especially attractive.
In this paper, a virtual instrument for detecting stator faults in induction motors is presented. The
virtual instrument is developed in the LabVIEW environment because of its graphical programming
style and high-quality user interface tools. The developed instrument is applied to a specially prepared
induction motor that has the ability to artificially generate turn-to-turn faults and good results are
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achieved. An inverter-fed driver, that includes sensors for current control, is used in the experiments. In
order to obtain information about stator winding faults from the induction machine the high-frequency
carrier-signal technique is implemented. This technique has the advantage that turn-to-turn faults can be
detected even when the rotor is not running and a fundamental signal voltage is not applied to the motor,
since information about fault is obtained from the dc component of the negative-sequence carrier-signal
current as a result of the high-frequency carrier-signal voltage injected to the machine.
MEASUREMENT OF SPATIAL SALIENCIES USING A HIGH-FREQUENCY CARRIER
SIGNAL
The high-frequency signal injection technique has been proven to be effective in detecting spatial
saliencies, i.e., asymmetries in ac machines14-16
, and its use for detecting turn-to-turn faults has
previously been suggested.14, 17, 18
When a balanced polyphase high frequency carrier-signal voltage, eqn
(1), is applied to a machine (see Fig. 1) a high-frequency carrier-signal current in the stator winding is
produced. When the machine contains a saliency, i.e., an unbalance, in the machine´s leakage
inductance, the produced carrier-signal current contains both positive- and negative-sequence
components, eqn (2). The positive-sequence carrier-signal current is proportional to the average stator
transient inductance and contains no saliency spatial information. The negative-sequence carrier-signal
current is proportional to the differential stator transient inductance and contains saliency spatial location
information in its phase.
t j
c
s
cqdsceV v
ω = _
(1)
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e ch t s s sqds_c qds_cp qds_cn cp cn
θ ωωc j( - ) j t i = i + i = I e + jI e
(2)
where
eθ angular position of the saliency in electrical
radians
h harmonic number of the saliency
ωc carrier frequency in radians per second
cp I ( )( )22
sssccL L LU σ σ σ
ω Δ−∑∑= magnitude of
the positive-sequence carrier-signal current
cn I ( )( )22
ssscc L L LU σ σ σ
ω Δ−∑∑= magnitude of
the negative-sequence carrier-signal current
s LσΣ = average stator transient
inductance
( + )/qs ds L Lσ σ 2
s LσΔ = differential stator transient
Inductance
( qs ds L Lσ σ− )/2
qs ds L Lσ σ , q- and d- axes stator transient inductances in
the saliency synchronous reference frame
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3. COMPUTER INTERFACE
The user interface shown in Fig. 2 was designed on the basis of having a friendly interaction between
the user and the Virtual Instrument for Detecting Turn-to-Turn Faults (DTTF). It was implemented in
the LabVIEW graphic environment because it combines the advantage of graphical programming and
high-quality user interface tools. The user interface or front panel of LabVIEW has ready-to-go controls,
such a graphs, and knobs that can be manipulated easily by the user. The front panel can be constructed
and viewed like a physical instrument, where the user can visualize the results on the computer screen.
The front panel is driven by the G-language code or block diagram, which is the actual code of the
program. This part of the VI receives data from the front panel and sends them to the main program. The
hardware interface to the computer is a National Instruments, multifunction, low-cost data acquisition
(DAQ) card. The DAQ card has 16 analog inputs in a single-ended mode or eight in a differential node
and two analog outputs. The sampling rate for an analog input is 200 kHz, and it has an accuracy of 12
bits.
The DTTF front panel has a set of buttons to modify both magnitude and frequency of the carrier and
fundamental excitation. Carrier signal excitation has a voltage range of 0-5 V and frequency range of 0-
500 Hz. In other hand, fundamental excitation has a voltage range of 0-320 V and frequency range of 0-
60 Hz. Experiments shown in this paper were developed with a fixed carrier signal frequency and
magnitude of 500 Hz and 5 V, respectively.
In the front panel top window the negative-sequence carrier-signal current spectrum of the actual
test can be seen (motor with a two- turn fault). The front panel also includes a window where can be
seen the current evolution in the time domain. A third window is included to show the negative-
sequence carrier-signal spectrum from a test data saved previously. In this case, the spectrum for a motor
with a zero (healthy) turn fault is shown. User can compare the results at different tests and analyze
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changes in the magnitude of the dc component. A database is implemented to save test data at different
dates. Data from this database can be loaded and compared with the actual test. Block diagram in Figure
3 illustrates the essential components of the front panel shown in Fig. 2.
STATOR WINDING FAULT DETECTION
An imbalance in the stator winding produced by turn-to-turn fault creates different q- and d- axes
stator transient inductances in a stationary reference frame. Since the stator windings are fixed in space,
this stator transient inductance imbalance gives rise to a stationary saliency, i.e., an asymmetry in the
machine that is fixed in space18
. This stationary saliency appears as a dc component in the spectrum of
the negative-sequence carrier-signal current in its corresponding reference frame. A modified induction
motor with shorted adjacent turns was used in the tests carried out. Figure 4 schematically shows the
stator winding design, including how turn-to-turn faults can be created. With this machine, a turn-to-turn
fault ranging from 1 to 9 turns can be created (see Table 1).
Table 1. Turn-to-turn faults
Fault number Connected terminals
0 ---1 1-22 1-33 1-44 1-55 1-66 1-7
7 1-88 1-99 1-10
LABORATORY EXPERIMENTS
The experimental setup is shown in Fig. 5, where the DTTF generates the fundamental and theqds_f v*
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carrier vectors, through the DAQ card. These signals are sent to the inverter-fed drive to move the
motor at a specific speed. The Inverter-fed driver is essentially built with a PWM voltage source inverter
and a current controller as was shown in Fig. 1. Stator current produced by the machine is measured by
hall-effect based sensors and conditioned to be sent to the DAQ. Overall stator current consists of the
fundamental current, and the positive- and negative-sequence carrier-signal currents. Two bandstop
digital filters (for fundamental and positive-sequence current) that separate the negative-sequence
current from the line current are implemented in the virtual instrument. The resultant three-phase current
time evolution and the spectrum of the negative-sequence carrier-signal current in the negative-sequence
carrier-signal reference frame have been shown in Fig. 2. It can be noted that the spectrum contains two
main harmonics. In 0 Hz is located the dc component; in healthy machines this harmonic is relatively
low compared with the same machine with stator winding faults. The magnitude of the dc component is
barely affected by the load condition of the machine.
qds_cv*
18In 8 Hz is located the harmonic produced by the
main saturation-induced saliency when the fundamental frequencye2ω e 4ω = Hz. This harmonic is load
dependent.
Fig. 6 shows the magnitude behavior of the dc component of the negative-sequence carrier-signal
current for turn-to-turn faults ranging from 0 to 9 turns. It is noted that small number of turn-to-turn
faults produce larger increments in the dc component than big numbers of turn-to-turn faults.
CONCLUSIONS
In this paper, a virtual instrumentation system for detecting turn-to-turn faults in induction machines
fed from a voltage source inverter was presented. The major advantage of the system is that it has the
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possibility to compare online the actual state of the machine respect to the state of previous tests of the
same machine, allowing to detect relatively easily a possible stator winding fault, An incipient fault
detection can reduce unexpected failures and downtime. The implementation for online detection of
stator windings in a three-phase induction machine was achieved using the high-frequency carrier-signal
injection technique. Experimental results were carried out to prove the validity of the developed virtual
instrument, where it can be concluded the state of the winding stator. A special prepared stator winding
of a squirrel-cage induction motor was used to artificially generate interturn faults. The VI can be useful
both in universities and industry as a motor diagnosis tool.
ACKNOWLEDGMENTS
The authors wish to acknowledge the financial support and motivation provided by Concyteg, Instituto
Tecnológico de León and Instituto Tecnológico de la Laguna, México.
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Figures and captions
Figure 1. Injection of a carrier-signal voltage excitation.
Figure 2. Front panel of the virtual instrument proposed for detecting turn-to-turn faults in induction
motors.
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Figure 3. Block diagram depicting the essential components of the stator winding fault detection system
Figure 4. Schematic diagram of the experimental machine
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Figure 5. Experimental setup
Figure 6. Experimentally measured cd component of the negative-sequence carrier-signal current for a
motor with different turn-to-turn faults and under the same operating conditions.
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