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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mailto:[email protected]

mailto:[email protected]



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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Detection of Stator-winding Turn-To-turn Faults in Induction Motors Based on Virtual Instrumentation