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2514 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 28, NO. 5, MAY 2013

An MRAS-Based Diagnosis of Open-Circuit Fault inPWM Voltage-Source Inverters for PM Synchronous

Motor Drive SystemsShin-Myung Jung, Student Member, IEEE, Jin-Sik Park, Member, IEEE, Hag-Wone Kim, Associate Member, IEEE,

Kwan-Yuhl Cho, Student Member, IEEE, and Myung-Joong Youn, Senior Member, IEEE

AbstractIn this paper, a simple and low-cost open-circuit faultdetection and identification method for a pulse-width modulated(PWM) voltage-source inverter (VSI)employinga permanentmag-net synchronous motor is proposed. An open-circuit fault of apower switch in thePWM VSIchanges the corresponding terminalvoltage and introduces the voltage distortions to each phase volt-age. The proposed open-circuit fault diagnosis method employs themodel reference adaptive system techniques and requires no addi-tional sensors or electrical devices to detect the fault condition and

identify the faulty switch. The proposed method has the featuresof fast diagnosis time, simple structure, and being easily insertedto the existing control algorithms as a subroutine without majormodifications. The simulations and experiments are carried outand the results show the effectiveness of the proposed method.

Index TermsFault detection, fault diagnosis, fault identifica-tion, model reference adaptive system (MRAS), open-circuit fault,pulse-width modulated voltage-source inverter (PWM VSI).

I. INTRODUCTION

T

HE permanent magnet synchronous motor (PMSM) is in-

creasingly used in powered wheelchairs, electric vehicles,

aerospace, medical and military applications, and nuclear powerplants due to its advantages such as high power density, torque

to inertia ratio, efficiency, and simple control [1]. In these appli-

cations, because an accident or fault can result in huge damages

to the human life and environments, the reliability of the ma-

chine drives is one of the most important factors to guarantee

the safe, continuous and high performance operation under even

some accidents or faults. Generally, when an accident or fault

occurs, the drive system has to be stopped for emergency or

nonprogrammed maintenance schedule. Due to the high cost of

Manuscript received April 1, 2012; revised June 5, 2012; accepted July 26,2012. Date of current version November 22, 2012. This work was supportedby a Human Resources Development grant of the Korea Institute of EnergyTechnology Evaluation and Planning funded by the Ministry of KnowledgeEconomy, Republic of Korea (2011H100100110). Recommended for publica-tion by Associate Editor J. O. Ojo.

S.-M.Jung and M.-J.Youn are with the Department of Electrical Engineering,KAIST, Yuseong-gu, Daejeon 305-701, Korea (e-mail: [email protected];[email protected]).

J.-S. Park is with the CDS Circuit Technology Group, Samsung Electro-Mechanics Company, Yeongtong-gu, Suwon, Gyeonggi-do 443-743, Korea(e-mail: [email protected]).

H.-W. Kim and K.-Y. Cho are with the Department of Control and Instru-mentation Engineering, Korea National University of Transportation, Chungju,Chungbuk 380-702, Korea (e-mail: [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPEL.2012.2212916

Fig. 1. General configuration of a PMSM drive system.

unexpected maintenance, the development of a reliable system

is the area of interest. A control system with minimum or zero

effects from the faults is called a fault-tolerant control system,

and it performs the following three tasks [2]: 1) fault detection;

2) fault identification; and 3) remedial actions. The fault detec-tion is the process to determine whether the system is healthy or

not. The fault identification is performed after the fault detection

to identify the location, type, and nature of the fault. Finally, the

remedial actions, also known as fault isolation, are the process

to remove the faulty devices and reconfigure the system for a

safe and continuous operation. Among these three tasks, the

fault detection and fault identification are considered as a prime

process for the practical implementation and are often called as

a fault diagnosis.

Typically, the motor drive systems consist of a microcon-

troller unit (MCU) for implementing the control algorithms, a

power electronic converter, i.e., pulse-width modulated voltage-

source inverter (PWM VSI), and a motor as shown in Fig. 1.In this figure, various types of faults can occur in the following

components or subsystems such as:

1) microcontroller unit (MCU);

2) motor (PMSM);

3) power converter (PWM VSI);

4) sensorsvoltage and current/position encoder;

5) connectors and wires.

The MCU, connectors, and wires have very low failure rates

compared to the remaining of the system. This is because the

MCU is very reliable and does not involve high voltages or cur-

rents. The connectors and wires are static and have low failure

rates if selected and installed properly. Some of the machine

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JUNGet al.: MRAS-BASED DIAGNOSIS OF OPEN-CIRCUIT FAULT IN PWM VOLTAGE-SOURCE INVERTERS 2515

faults caused by the winding insulation failure due to the exces-

sive voltage or current stress can practically be removed because

the line voltage surges are absorbed at the input side of the power

converter and the current stresses are limited by the overcurrent

protection of the power converter [3]. In recent years, the sen-

sor faults have been increasingly concerned in the literature

works [4][7]. The sensor faults including biased signal, loss of

signal, incorrect gain, and unresponsive signal are mainly due

to the broken or bad connections, bad communications, or some

hardware or software malfunction. Therefore, if the connectors

and wires are installed correctly, the failure rates of the sensor

faults can be lowered. For these reasons, those faults are not

considered in this paper.

On the other hand, the power converter failures can be a

critical factor to the overall drive system and cause system shut-

downs; therefore, these require a high cost of unexpected main-

tenance. It is estimated that about 38% of all the failures are

found in the power converter [8] and the most of faults are

occurred to the power switches [9]. In response to a control

algorithm, a voltage command is generated, and the VSI syn-thesizes this voltage command using the power switches, i.e.,

insulated-gate bipolar transistor (IGBT) and MOSFET, with the

techniques such as the sinusoidal PWM (SPWM) or space-

vector PWM (SVPWM). There could be a quite high chance of

failure in the switching devices due to the high electrical and

thermal stresses [10]. The failure of switching devices can take

place in the form of short circuit or open circuit. The short-

circuit fault may occur due to an improper gate signal so that

both power switches in a leg of the VSI are turned ON. This

results in a short circuit of the capacitor in the dc link that blows

out the other components particularly switching devices. There-

fore, the short-circuit fault is one of the most fatal accidents andthe most important thing in the drive system is to minimize the

time between short-circuit fault initiation and appropriate reac-

tion. Consequently, the control circuits of the switching devices

are designed to have a fast fault diagnosis characteristic to pre-

vent the abnormal overcurrent and mostly the hardware-based

protection schemes are employed. The open-circuit fault, on the

other hand, is often overlooked since it has the characteristic

of slow response and less danger to the whole drive system

compared with the short-circuit fault. The open-circuit fault

may result from the disconnection of a wire from the switch-

ing devices due to a thermic cycling or a gate driver failure.

The open-circuit fault leads to the current imbalance in both the

faulty and healthy phases and results in the pulsating currentsand torques, which highly degrades the driving performances.

The open-circuit fault is not generally harmful to the machine

drives and does not cause system shutdowns, but could lead

to the secondary faults at the other components [3]. The open-

circuit fault is one of the general faults and can be frequently

taken place in the drive system, though, and the literature has

much concerned about the fault.

Several pieces of research have been developed and pub-

lished to diagnose the open-circuit fault in motor drive sys-

tems [2], [10], [11][23], [32][38]. The comparison between

the actual and voltage command methods [2], [10], [11] is made,

which is based on an analytical model of the PWM VSI and

requires the voltage measurements or sensors at the specific

points of the drive system. In [12], the diagnosis is based on the

fact that during the fault condition the voltage across the lower

switch is about one half of the dc-link voltage. These meth-

ods [2], [10][12] have shown the characteristic of the fast fault

diagnosis performance and thus reduced the time between the

fault occurrence and its diagnosis. The major problem of these

methods, however, is the increase in system cost due to the addi-

tional hardware equipment such as voltage sensors and electric

circuits. In [13], the authors proposed two techniques, the slope

method and instantaneous frequency method. The first one uses

the analysis of the currentvector trajectory to detect the fault

condition and identify the faulty switch. The second one, how-

ever, can only determine the fault condition of the PWM VSI

from the instantaneous frequency of the current vector but can-

not identify the faulty switch. Parks vector method [14] and the

normalized dc current method [15][18] detect the fault condi-

tion and identify the faulty switch by calculating the midpoint

position of the current trajectory, which is the mean value of

the ac current space vector over one period. All these suggestedmethods have shown acceptable performances but take at least

one fundamental period between the fault occurrence and its di-

agnosis. Recently, in [19], a fault diagnosis method in the case of

the trapezoidal back electromotive force (EMF) is proposed.The

proposed scheme uses the observed phase current information

and detects the fault condition based on the operating character-

istic of the brushless dc motors. This method, however, is only

suitable for the trapezoidal back-EMF type motors. The artifi-

cial intelligence methods, such as wavelet fuzzy network [20]

and wavelet neural network [21][23], have also been proposed.

However, these digital-control-based algorithms have a major

drawback of the complicated and excessive computing process.This paper proposes a simple and low-cost open-circuit fault

diagnosis algorithm in the PWM VSI for PMSM. Such tech-

nique requires no additional sensors and is based on the ana-

lytical model of the PWM VSI. The proposed fault diagnosis

method has the fast, simple and low-cost characteristics and can

be easily embedded in the existing PMSM drive system as a sub-

routine without major modifications. This proposed method is

implemented in a digital manner using a MCU TMS320F28335

from Texas Instruments, Incorporated. The simulation and ex-

perimental results show the validity of the proposed diagnosis

method.

II. ANALYSIS OF THEOPEN-CIRCUITFAULT IN THEPWM VSI

Fig. 2 shows the basic configuration of phase A leg of a three-

phase PWMVSI. When thesystem is under thenormal condition

as can be seen in Fig. 2(a), the terminal voltage of phase A,

va0 , is determined by the phase current ias and the switching

function Sa ofQaU and QaL . If the switching function is 1,

which means that QaUis turned ON and QaL is turned OFF, the

terminal voltage of phase A is equal to Vdc/2, whereVdc is the

dc-link voltage. If the switching function is 0, which means that

QaUis turned OFF andQaL is turned ON, va0 =Vdc/2. Thepossible terminal voltages according to the switching function

and the direction of phase current under the normal condition are

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Fig. 2. Basic configuration of phase A leg of a three-phase PWM VSI.(a) Normal condition. (b) Open-circuit fault in the upper switch Qa U.

TABLE ITERMINALVOLTAGES OFPHASEA UNDER THENORMALCONDITION

TABLE IITERMINALVOLTAGES OFPHASEA UNDER THEOPEN-CIRCUIT

FAULTCONDITION

represented in Table I. Unlike the normal condition, however, anopen-circuit fault in the upper switchQaU results in changing

the corresponding terminal voltage when the phase current iasis positive and the switching function Sa is 1, since the upper

switchQaUis not working properly. In this case, the terminal

voltage of phase A is not equal to Vdc/2, but equal toVdc/2. Theequivalent circuit after the open-circuit fault occurrence to the

upper switchQaUis shown in Fig. 2(b), and the corresponding

terminal voltages are represented in Table II.

From the aforementioned analysis, it is obvious that the phase

voltages may have the voltage deviations in the steady state after

the fault occurrence from the normal condition. Based on this

fact, the proposed fault diagnosis method indirectly observes

these voltage deviations using the analytical model of the PWMVSI and the fault diagnosis can be achieved.

To take a close look at the effect of the open-circuit fault of a

switch on the phase voltages, the knowledge of the relationship

between the terminal voltagesand the phase voltagesis required.

This relationship can be represented as follows [1]: va0vb0

vc0

=

vasvbs

vcs

+ vs0 (1)

where vas , vbs , and vcs are the phase voltages and vs0 is the

neutral to center voltage, as shown in Fig. 3. In a three-phase

three-wire system, the following condition by Kirchoffs law is

Fig. 3. Relationship between terminal voltages and phase voltages.

satisfied as

ias + ibs + ics = 0. (2)

Also, the sum of each phase back EMF is equal to zero atany instant under the assumption that the air-gap magnetic flux

distribution is a sinusoid. From (2) and this assumption, the

following condition is satisfied for the PMSM as

vas + vbs + vcs =Rs (ias + ibs + ics ) + Lsd

dt(ias +ibs +ics )

+ (eas + ebs + ecs ) = 0 (3)

whereRs is a stator resistance, Ls is a stator inductance, and

eas , ebs , and ecs represent the corresponding phase back EMFs,

respectively. From (1) to (3), the neutral to center voltage vs0becomes

vs0 = 13

(va0 + vb0 + vc0 ). (4)

Therefore, the relationship between the terminal voltages and

the phase voltages can be represented as follows: vasvbs

vcs

= 1

3

2 1 11 2 11 1 2

va0vb0

vc0

. (5)

On the other hand, the effect of an open-circuit fault on one

of the switches can be represented by a deviation value from

the terminal voltages of the PWM VSI. Ifva0 is considered asrepresenting the voltage deviation due to an open-circuit fault in

the upper switchQaU , the phase voltages after the open-circuitfault occurrence are represented as follows:

vas fvbs fvcs f

= 1

3

2 1 11 2 11 1 2

va0 va0vb0

vc0

(6)

where vas f, vbs f, and vcs fare the phase voltages after the

open-circuit fault occurs to the upper switch QaU . After some

calculations, (6) can be expressed as vas fvbs f

vcs f

=

vasvbs

vcs

+ 1

3

2va0va0

va0

=

vasvbs

vcs

+

va s di stvbs di st

vcs di st

(7)

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where va s di st , vbs di st , and vcs di st are the phase voltagedeviations introduced by the open-circuit fault occurred to the

upper switch QaU . As can be seen in (7), the phase voltage after

the open-circuit fault occurrence can be divided into two parts.

The first term of (7), vks (k= {a,b,c}), are the normal phasevoltagesand thesecond term of (7), vks di st(k ={a,b,c}),arethe voltage deviations due to the open-circuit fault in the upper

switchQaU . These phase voltage deviations can be considered

as voltage distortions induced by the open-circuit fault and

observed from the machine parameters, which is discussed in

the following.

The voltage distortions in theabc frame can be transformed

to the rotor reference frame by using the relationship [1] as

vq fvd f

= T(e )

vas fvbs f

vcs f

(8)

T(e ) =

2

3

cos e cos

e

2

3

cos

e +

2

3

sin e sin

e

2

3

sin

e +

2

3

(9)

where eis the electrical angular position of the rotor. Using (7)

through (9), the voltage distortions in the rotor reference frame

in (8) can be represented as

vq fvd f

= T(e )

vas + va s di stvbs + vbs di st

vcs + vcs di st

=

vqvd

+

vq di stvd di st

(10)

where vq fand vd fare the q- and d-axis voltages after the fault

occurrence, vqand vdare the normal q- and d-axis voltages, and

vq di st andvd di st are theq- andd-axis voltage deviationsor distortions due to the open-circuit fault. The stator voltages

in the rotor reference frame after the fault occurrence can also

be represented as two parts. One is the normal q- and d-axis

voltages and the other is the voltage deviations caused by the

open-circuit fault. The analysis for the open-circuit fault in the

other switches can be made in a similar manner.

III. PROPOSEDOPEN-CIRCUITFAULTDIAGNOSIS ALGORITHM

An open-circuit fault in the PWM VSI makes the current in

that phase be zero for either the positive or negative half-cycle

depending on whether it occurs to the upper or lower switch.

If the open-circuit fault occurs to the upper switch QaU , forexample, the positive half-cycle of phase A current is always

zero. As a result, it causes a dc current offset in the faulty phase

and this offset current is equally divided into the healthy phases.

Therefore, the offset current gives the uneven current stress on

the remaining switches of the PWM VSI, which may cause

thermal defects [3].

To detect the open-circuit fault condition and identify the

faulty switch, a simple and low-cost fault diagnosis algorithm is

proposed. This proposed method is employing the model refer-

ence adaptive system (MRAS) techniques and does not require

any additional hardware circuits such as voltage sensors. The

voltage distortions caused by the open-circuit fault in the rotor

reference frame are estimated by a voltage distortion observer

which is based on the electrical model of the PMSM. And the

estimated voltage distortions in the rotor reference frame are

transformed to the abc frame for the error detection. By examin-

ing the error, the fault condition is determined by the time-based

analysis and the faulty switch is also identified immediately ac-

cording to the observed voltage distortions in the abcframe.

A. MRAS-Based Voltage Distortion Observer

The current dynamics of a PMSM including the voltage dis-

tortions caused by the open-circuit fault can be represented as

follows [1]:

diq

dtdid

dt

=

Rs

Lse

e Rs

Ls

iq

id

+

1

Ls

vq+ vq di st

vd + vd di st

+ m e

Ls0

(11)

where iqand idare the q-and d-axis currents, eis the electrical

rotor angular speed, and m is the flux linkage established by

the permanent magnet, respectively. As can be seen in (11), the

motor currents are affected by the voltage distortions caused by

the open-circuit fault.

For the reference model of the MRAS, it is assumed that the

voltage distortions due to the open-circuit fault are zero and

the system is in the healthy mode. Under this condition, the

calculated current dynamics using the nominal parameters can

also be represented as follows:

diq mdt

didm

dt

=

Rs0

Ls0e

e Rs0

Ls0

iq

id

+

1

Ls0

vq

vd

+

m 0 eLs0

0

(12)

wherevqandvd are theq-andd-axis stator voltage commands,

iq m and idm are theq-andd-axis currents of the model, respec-

tively, and the subscript 0 represents the nominal value.

From (11) and (12), the voltage distortions in the rotor refer-

ence frame caused by the open-circuit fault can be obtained asfollows:

vq di st =Ls

diq

dt

diq m

dt

vd di st =Ls

did

dt

didm

dt

(13)

where it is assumed that the nominal parameters Rs0 , Ls0 , and

m 0are identical to the real values Rs , Ls ,and m , respectively.

Also, on the assumption that the switches in the PWM VSI

are ideal, the voltage commandsvq andvd are identical to the

corresponding q- and d-axis voltages vq and vd . The average

voltage distortions over thekth PWM step can be derived from

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Fig. 4. Configuration of the proposed voltage distortion observer.

(13) as follows:

vq di st(k) =Ls

iq(k) iq m(k)

Ts

vd di st(k) =Ls id(k) idm (k)

Ts

. (14)

The modelcurrents iq m (k) and idm (k) in (14) can be obtainedfrom the discrete form of (12) and these become as follows:

iq m(k) =iq(k1) + Ts

Ls0[vq(k1) Rs0 iq(k1)

e Ls0 id(k1) em 0 ]

idm (k) =id(k1) + Ts

Ls0[vd(k1) Rs0 id(k1)

+ e Ls0 iq(k1)]. (15)

Fig. 4 shows the block diagram of the proposed voltage dis-tortion observer. The observed voltage distortions from the plant

and model currents are used for the fault diagnosis algorithm of

the open-circuit fault in the PWM VSI. In this proposed fault

diagnosis algorithm, two schemes, the error detection and fault

detection time, are used for the robustness against the false fault

diagnosis.

B. Error Detection and Fault Detection Time

Under the normal operation, the voltage distortions in (14) are

zero. However, under the fault condition, the voltage distortions

have either the positive or negative values according to the faulty

switch in the PWM VSI and are repetitively appeared while thecurrent of faulty leg is zero. Therefore, the threshold value is

employed to determine the error occurrence and given by

Vth =K (16)

where K isa positive number and carefully selected to minimize

the possibility of the false alarms mainly caused by the noises

and the nonideal characteristics of the power switches. In the

aforementioned open-circuit fault analysis, it has been assumed

that the switches are ideal; thus, the voltage commands are

the same as the corresponding voltages applied to the PMSM.

However, in a real situation, since the voltage commands are

not identical to the corresponding real applied voltages, there

Fig. 5. Ideal and practical switching pattern considering the dead-time andturn-on/off times of the switches. (a) Practical gate signal including the dead-time. (b) Actual gate signal including turn-on delay, turn-on transition time,turn-off delay, and turn-off transition time of the switches when ia s 0.

(c) Actual gate signal whenia s

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TABLE IIIVOLTAGEDIFFERENCESBETWEENVOLTAGECOMMANDS ANDREALAPPLIED

VOLTAGES TO THEPMSM ACCORDING TO THEROTORPOSITION

Fig. 6. Voltage differences between voltage commands and real applied volt-ages to the PMSM under the normal condition in theabcframe.

is selected too high, the errors may not be detected. Moreover, if

m is too small, the probability of false error detection increases.

After an open-circuit fault occurs, the voltage distortions in

(14) are observed and the following simple logic [11], [19], [26],

[27] for each phase is used to generate the Boolean errors ks(k= {a,b,c}) as

ks =

1, vks di st > Vth : error

0, |vks di st |< Vth : normal

1, vks di st

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TABLE VFAULTIDENTIFICATIONFLAGACCORDING TO THEGENERATED

BOOLEANERRORS

Fig. 7. Block diagram of the proposed fault diagnosis system.

definedin (21), and ifte Tfa u lt , thefault detectionflag FlagDis set from low to high and the open-circuit fault is detected.

The identification of the faulty switch is obtained just afterthe fault detection by the generated Boolean errors ks (k ={a,b,c}) from Table IV. According to the Boolean errors, thefault identification flagF lagIcomposed of three flags F lagA ,

FlagB , andF lagCis defined as given in Table V and used to

identify the faulty switch.

Fig. 7 shows the block diagram of the proposed fault di-

agnosis algorithm. The fault diagnosis is accomplished in the

following procedures: 1) observation of the voltage distortions;

2) generation of the Boolean errors by the error detection; 3)

determination of the fault condition by the fault detection time;

and 4) identification of the faulty switch. Fig. 8 shows the pro-

cess of the proposed fault diagnosis algorithm in the case of the

fault occurrence to the upper switch QaU . When the open-circuitfault occurs to the upper switch QaU , the voltage distortion of

phase A has a negative value and is compared with the selected

threshold value in (16) as shown in (20). Also, the error de-

tection timete is triggered to measure the time until the errors

are detected by (20). If the error detection timete continuously

elapses, and exceeds beyond the fault detection time Tf a u l tde-

fined in (21), the fault detection flag FlagD is set from low to

high. After the fault detection flag FlagD is set to high, the

fault identification is obtained from Table V and the fault iden-

tification flagF lagIis set according to the faulty switch. Fig. 9

shows the overall block diagram of the proposed open-circuit

fault diagnosis system.

Fig.8. Process of the proposedfault diagnosis algorithmwhen theopen-circuit

fault occurs to the upper switchQaU.

Fig. 9. Overall structure of the proposed open-circuit fault diagnosis.

D. Extension to the Open-Phase Fault

The open-phase fault is occurred when two switches in one

of three legs are open-circuited. The analysis can be made in asimilar manner in Section II except that the effect of the faulty

switch on the voltage distortions is appeared alternately. For

example, when two switches in phase A, QaU andQaL , have

the open-circuit faults, the voltage distortions have positive to

negative or negative to positive values by turns according to

the faulty switch. If the lower switch QaL has an open-circuit

fault first, then the phase voltages after the open-circuit fault

occurrence are represented as follows:

vas f1vbs f1

vcs f1

= 1

3

2 1 11 2 1

1 1 2

va0 + va0

vb0

vc0

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Fig. 10. Process of the open-circuit fault diagnosis when the open-circuit

faults occur to the lower switchQa L and the upper switch Qa U.

=

vasvbs

vcs

+1

3

2va0va0va0

. (23)

After the open-circuit fault occurrence to the lower switch

QaL , the upper switch QaU has an open-circuit fault and the

phase voltages are represented as follows:

vas f2vbs f2vcs f2

=

1

3

2 1 11 2 11 1 2

va0 va0vb0vc0

=

vasvbs

vcs

+1

3

2va0va0

va0

. (24)

As can be seen in (23) and (24), the voltage distortions have

the positive to negative or negative to positive values according

to the faulty switch, and the faulty switch or faulty leg can be

identified in a similar manner introduced previously. Fig. 10

shows the process of the open-phase fault diagnosis in the case

of the open-phase fault occurrence to phase A. As can be seen

in Fig. 10, the identification flag FlagIshows 011 and 100

alternately, which means that QaL and QaU have the open-

circuit faults.

E. Influence of the System Parameter Error

Generally, the accuracy of the model parameters affects the

observation of the voltage distortions in (14). In the proposed

method, the model currents in (15) are affected by the model

parameter errors. If the errors of the machine parameter are

large, the model currents can converge to the wrong values and

the observation of the voltage distortions also has wrong values.

As can be seen in (15), the model currents are affected by the

resistance, inductance, and flux linkage. In a low-speed region,

the model currents in (15) are less affected by the variations of

the machine parameters, because the currents and the electrical

Fig. 11. Simulation results of the fault diagnosis when the open-circuit faultoccurs to the upper switch Qa U. (a) Phase currents. (b) Process of the proposedfault diagnosis algorithm.

angular speed are small and the voltage commands are large.

Thus, the resistance loss, inductance loss, and flux linkage loss

in (15) can be ignored. On the other hand, in a high-speed region,

the model currents are severely affected by the variations of

the machine parameters, because the electrical angular speed

is quite large compared to the voltage commands. Therefore,

to avoid the false fault detection due to the variations in the

machine parameter errors, the threshold valueKin (16) should

be selected within a safe limit so that no false Boolean errors are

generated. In addition, to get a better performance, the online

observation algorithms of the machine parameter values could

be added in the proposed method. However, it is beyond the

scope.

IV. SIMULATION ANDEXPERIMENTALRESULTS

In this section, the simulation and experimental results are

presented to prove the effectiveness of the proposed fault diag-

nosis algorithm.

A. Simulation Result

Fig. 11(a) shows the simulation results for the phase currents

ias , ibs , andics , and the fault signal when the open-circuit fault

occurs to the upper switchQaU . As can be seen in this figure,

the negative phase current ias rapidly approaches to zero and

does not appear during the positive half-cycle after the fault

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Fig. 12. Simulation results of the fault diagnosis when the open-circuit faultoccursto phaseA. (a)Phasecurrents. (b)Process of theproposed fault diagnosisalgorithm.

TABLE VISPECIFICATIONS OF THETESTPMSM

TABLE VIISPECIFICATIONS OF THEPWM VSI

occurrence as indicated by the dotted circle in the upper trace of

Fig. 11(a). Only the negative current flows through the switch

QaL or diodeDaU . The other phase currents, ibs andics , show

the similar shapesas thenormal condition, but increase a littlebit

dueto thedc offset current causedby theopen-circuit fault. Since

the phase currents are distorted due to the open-circuit fault, the

pulsating torque may be generated and the performance of the

system is deteriorated. Fig. 11(b) shows the simulation results

Fig. 13. Experimentalresults of thefaultdiagnosiswhen theopen-circuit fault

occurs to the upper switchQa U(500 r/min). (a) Process of the proposed faultdiagnosis algorithm. (b) Fault identification.

for the process of the proposed fault diagnosis algorithm. It

compares the observed voltage distortions in the abcframe with

the threshold value in (16) and generates the Boolean errors. The

generated Boolean errors trigger the timer to measure the error

detection time teuntil the errors are detected. Ifte Tf a u l t, theopen-circuit fault is detected and the fault detection flagF lagDis set from low to high and the fault identification flag FlagIis also set to identify the faulty switch. The fault identification

is obtained from Table V and the identification flag, FlagI,

composed of three flags FlagA , FlagB , and FlagC is set thedigital word 100 which means that the open-circuit fault has

been occurred to the upper switch QaU .

The proposed method is also applicable when the open-circuit

fault occurs simultaneously to both the upper and lower switches

in one phase leg. The simulation results are given in Fig. 12 in

the case of the fault occurs to the upper and lower switches in

phase A. The results indicate that the phase current ias does

not flow through the upper and lower switches, QaU andQaL ,

which remains zero as indicated in Fig. 12(a). The observed

voltage distortion in phase A,va s di st , has both positive andnegative values by turns according to the faulty switch as shown

in Fig. 12(b). The fault condition can be detected in a similar

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Fig. 14. Experimentalresults of thefaultdiagnosiswhen theopen-circuit faultoccurs to the upper switchQ

aU (3000 r/min). (a) Process of the proposed fault

diagnosis algorithm. (b) Fault identification.

TABLE VIIIFAULTDETECTIONTIMEBETWEENPREVIOUSMETHODS

AND THEPROPOSEDMETHOD

manner to an open-circuit fault and the faulty switches can be

identified by the identification flag FlagI showing 011 and

100 by turns which means that the switches QaL and QaU are

in trouble.

Fig. 15. Experimental results of the fault diagnosis when the open-circuitfault occurs to phase A (500 r/min). (a) Process of the proposed fault diagnosisalgorithm. (b) Fault identification.

B. Experimental Result

In order to confirm the feasibility of the fault diagnosis al-

gorithm, the experiments have been realized under the same

conditions as the simulations. The parameters related to the test

motor and PWM VSI are represented in Tables VI and VII,

respectively. The three-phase PWM VSI is composed of six

IGBTs, FGH40N60SFD from Fairchild, Corp. The switching

frequency of the PWM VSI is 11 kHz. All the control laws

proposed in this paper have been realized by using a single

MCU TMS320F28335 from Texas Instruments, Incorporated.The sampling rate of phase currents is identical to the switching

frequency. The open-circuit fault condition is made by introduc-

ing no gate drive signal to the IGBT.

Fig. 13(a) shows the experimental results for the process of

the fault diagnosis algorithm. As can be seen in this figure, when

all of the switching devices are under the normal condition, the

observed voltage distortion is nearly zero. However, there is

a significant difference after the fault occurrence to the upper

switchQaU , and the error signal is generated. When the error

detection timete continuously elapses and exceeds beyond the

fault detection time Tfa u lt defined as in (21), the fault detection

flag FlagD changes from low to high. Fig. 13(b) shows the

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Fig. 16. Effects of machine parameter variations (3000 r/min). (a)Rs 0 = 0.5Rs . (b)Rs 0 = 1.5Rs . (c)L s 0 = 0.5Ls . (d)L s0 = 1.5Ls . (e) m0 = 0.8m.

(f) m0 = 1.2m.

fault identification flagF lagIcomposed of three flags F lagA ,

FlagB , andF lagCthat identify the faulty switch. In this case,

100 means that the open-circuit fault has been occurred to

the upper switchQaU . Fig. 14 shows the similar experimental

results in the case of the rated speed, 3000 r/min.

Fig. 15 shows the experimental results for the open-phase

fault diagnosis when the fault occurs to phase A. The cur-

rent of phase A remains zero after the open-phase fault oc-

currence and the observed voltage distortion has both positive

and negative values by turns as shown in Fig. 15(a). The iden-

tification flag FlagI for the open-phase fault shows two val-

ues according to the faulty switches, and in this case, the flag

shows 011 and 100, which means that the lower and up-

per switches, QaL andQaU , in phase A have the open-circuit

fault.

The comparison of the fault diagnosis time between proposed

method and previous methods is summarized in Table VIII. As

can be seen in this table, the proposed method can detect the

fault in less than 1 msec, while the previous methods take more

than 2.7 msec.

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Fig. 16 shows the effect of the machine parameter variations

on theprocess of theproposedmethod at 3000 r/min. Theparam-

eter variations affect the observation of the voltage distortions

in a high-speed region. Even under the healthy condition, since

RHS terms of (15) which are related to the machine parameters

are large due to the high electrical angular speed, the observed

voltage distortions are highly affected by the machine parameter

errors. The voltage distortions are less affected by the resistance

and inductance variation, but are severely affected by the flux

linkage variation. Fig. 16(e) and (f) show the chances of the false

alarm under 20% flux linkage variations. To avoid the falsefault detection, the threshold value Kin (16) should be selected

under the consideration of the variations in the machine param-

eter values. In this system, the threshold valueKis selected as

15. Otherwise, the online observation and compensation algo-

rithms of the machine parameter values should be added for a

better performance.

V. CONCLUSION

The fault detection and identification is becoming more and

more important for industrial applications. Therefore, it is in-

creasingly required to improve the fault diagnosis capabilities.

In this paper, a simple and low-cost open-circuit fault detection

and identification method is presented. The proposed fault diag-

nosis is achieved by the simple voltage distortion observer. Once

the voltage distortions are estimated, these are compared with

the threshold value to determine the fault condition. When the

open-circuit fault occurs, the observed voltage distortions are

bigger than the threshold value. By comparing these two values,

the fault condition is decided. Also, the fault identification is

achieved by using the observed voltage distortions, since the

voltage distortions are different according to the faulty switch.

The proposed method can be well combined with the postfault

actions which are the reconfigurations of the whole drive system

to operate safely and continuously. Two major postfault actions

are also introduced in the literature works [28][30]. However,

the postfault actions are beyond the scope of this paper. In com-

parison with the previous existing fault diagnosis [31], the pro-

posed method has simple structure and fast fault detection time.

Also, it can be implemented without any extra devices such as

voltage sensors and the computing effort is very small. The ex-

ecution of the algorithm can be easily embedded in the existing

systems without major modifications. To show the effectiveness

of the proposed method, the simulations and experiments arecarried out for the digitally controlled PMSM drive system. The

simulation and experimental results verify the validity of the

proposed method and show that the proposed method gives the

good performance and practical value.

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Shin-Myung Jung (S08) was born in Korea, in1981. He received the B.S. and M.S. degrees in elec-trical engineering from the Korea Advanced Instituteof Science and Technology, Daejeon, Korea, in 2004and 2006, respectively, where he is currently workingtoward the Ph.D. degree.

His research interests include power electronicsandcontrol,which include motor drives, faulttolerantcontrol, and high-performance switching regulators.

Mr. Jung is a Member of the Korean Institute of

Power Electronics.

Jin-Sik Park (S08M12) was born in Korea, in1981. He received the B.S., M.S., and Ph.D. degreesin electrical engineering from the Korea AdvancedInstitute of Science and Technology, Daejeon, Korea,in 2003, 2005, and 2012, respectively.

He is currently with Samsung Electro-MechanicsCompany, Gyeonggi-do, Korea. His research inter-ests include the design of motor drive systems, sen-sorless drive of ac motor, and dc/dc power convertertopology.

Dr. Park is a Member of the Korean Institute ofPower Electronics.

Hag-Wone Kim (A05) received the B.S. degree inelectrical engineering from Korea University, Seoul,Korea, in 1989, and the M.S. and Ph.D. degrees inelectrical and electronic engineering from the KoreaAdvanced Institute of Science and Technology,Daejeon, Korea, in 1991 and 2004, respectively.

He was with LG Electronics, Digital ApplianceResearch Lab., from 1991 to 2008. Since 2005, hehas been an Associate Professor and since 2008, hehas been a Professor at Korea National University ofTransportation, Chungju, Korea. His research inter-

ests include the areas of variable speed motor drives and power converters.Dr. Kim is a Member of the Korean Institute of Power Electronics.

Kwan-Yuhl Cho(S88) received the B.S. degree inelectrical engineering from Seoul National Univer-sity, Seoul, Korea, in 1986, and the M.S. and Ph.D.degrees in electrical and electronics engineering fromtheKorea Advanced Instituteof Science andTechnol-

ogy, Daejeon, Korea, in 1988 and 1993, respectively.He waswith the LG Electronics, Digital ApplianceResearch Lab., from 1993 to 2004. Since 2004, hehas been with the Department of Control and Instru-mentation Engineering, Korea National University ofTransportation, Chungbuk, Korea. He was a Visiting

Scholar at Virginia Tech., from 2010 to 2011. He is a Planning Activities Direc-tor of the Korean Institute of Power Electronics. His research interests includevariable speed motor drives and power converters.

Myung-Joong Youn(S74M78SM98) was born

in Seoul, Korea, in 1946. He received the B.S. degreefrom Seoul National University, Seoul, in 1970, andthe M.S. and Ph.D. degrees in electrical engineeringfrom the University of Missouri, Columbia, in 1974and 1978, respectively.

In 1978, he joined the Air-Craft Equipment Divi-sion, General Electric Company, Erie, PA, where hewas an Individual Contributor on Aerospace Electri-cal System Engineering. Since 1983, he has been aProfessor at the Korea Advanced Institute of Science

and Technology, Daejeon, Korea. His research activities are in the areas ofpower electronics and control, which include drive systems, rotating electricalmachine design, and high-performance switching regulators.

Dr. Youn is a Member of the Institution of Electrical Engineers, U.K., theKorean Institute of Power Electronics, the Korean Institute of Electrical Engi-neers, and the Korea Institute of Telematics and Electronics.

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