paper 2-investigation of five-phase induction motor drive under faulty inverter conditions

7/17/2019 Paper 2-Investigation of Five-Phase Induction Motor Drive Under Faulty Inverter Conditions

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978-1-4799-7800-7/15/$31.002015 IEEE 1

Investigation of Five-Phase Induction Motor Driveunder Faulty Inverter Conditions

M. I. Masoud1 and Sherif M. Dabour2

1

Sultan Qaboos University, Oman,2

Tanta University, Egypt

[email protected], [email protected]

AbstractThis paper introduces an investigation into a five-

phase induction motor drive system in cases of fault occurrence

within the inverter. This analysis deals with the different types of

failure in inverter power electronic components, namely: one

gating signal failure, one switch open, one-leg open and two-legs

open. These faults can be applied to the system before or after

steady-state operations. The study shows the effect of these faults

on the motor performance in comparison with that of healthy

conditions for no-load operation. The results showed that, for the

faults under consideration, the motor is able to continue

operation with the presence of torque pulsations as well as speed

harmonic components without modifying the control scheme or

the inverter topology. These results can be used to improving the

performance of the system by using fault tolerant control or post-

fault control design. In order to overcome these effects, two fault-

tolerant strategies for five-phase voltage source inverter (VSI)

are suggested.

Keywords five-phase; induction motor; fault-tolerant,

voltage source inverter

I. INTRODUCTION

The fault tolerant capability is one of the most distinctadvantages of the multi-phase (more than three-phase)machines [1]. The knowledge about the fault mode behavior ofvoltage source inverter based multi-phase drives are veryimportant from the viewpoint of improving the drive systemdesign and the fault tolerant control strategies. The multi-phasedrive system has to continue its operation under faultyoperating conditions. Open-circuit faults in the motor windings(phase) and VSI switches (line) are common faults [2]. In thethree-phase motor drive, if one phase is opened, the drivesystem requires a neutral line connected between the mid-pointof the VSI and the motor to allow the current in the remainingphases to be controlled to produce a rotating MMF.Comprehensive research works have been reported on fault-tolerant control of the three-phase motor drives under open-circuit fault conditions [3][7].

The five-phase induction motor is advantageous over the

three-phase induction motor for fault-tolerant operation [1].The dynamic and steady-state behavior of a five-phaseinduction motor under one and two-phase open-circuit arepresented in [2], [8]-[10]. However, these works, consider onlythe faults after the motor operates in its steady-state region.Different fault tolerant topologies were reviewed and presentedin [3], [6]-[7] for the three-phase drive system. Thesetopologies can be extended to control the five-phase VSIs.

This paper concerns with the fault mode behavior of avoltage source inverter fed a five-phase induction motor drive.In this study, the machine internal faults are not considered. Anumber of faults have been identified. From these, theperformance of the motor under a few selected fault modeshave been introduced and analyzed, and then the predictedfault performance has been verified by simulation. The drivesystem is controlled by an open-loop scalar control method.The results can be applied for the other control methods. Theresults are useful for designing the protection system,calculation of post-fault operating conditions, and for fault

tolerant control design. On the other hand, two fault-tolerantstrategies for five-phase VSI are suggested.

II. FIVE-PHASE INDUCTION DRIVE SYSTEM

Generally, there are two different types of five-phaseinduction motors. One is called symmetrical five-phase motors;it uses distributed windings that produce a sinusoidal air-gapMMF. This type requires sinusoidal excitation voltages. Theother one uses concentrated stator windings that generate atrapezoidal air-gap MMF. In this type torque production can beenhanced using stator current low order harmonic injection [9].In this paper a symmetrical five-phase induction motor isutilized. Figure 1 shows the five-phase induction motor drivesystem based on five-phase VSI.

In the following subsections the machine and invertermodels are introduced. It will be used in the simulation andmodeling process.

A.

Machine Model

The symmetrical five-phase induction motor can bemodeled referred to stationary reference frame with thefollowing voltage and flux equations [8]:

The stator voltage equations are:

(1)

where the subscript qs,ds,xsandysare the qd,xy-axis's of thestator respectively;Rsis the stator phase resistance andpis thed/dt operator. The rotor voltage equations referred to statorcircuit are:


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2

0

vdc

m

Te

a b c d eia ib ic id ie

va vb vc vd ve

n

1 3 5 7 9

2 46 8 10

Fig. 1. Schematic diagram of symmetrical five-phase Induction drive

(2)

where the subscript qr, dr,xrandyrare the qd,xy-axis's of therotor circuit respectively;Rris the rotor phase resistance and ris the motor speed in r/s. The stator flux linkages equations are:

(3)

where theLsandLlsare the self and leakage inductances of thestator circuit respectively; Lm is the magnetizing inductances.The flux linkages of the rotor circuit equations are:

(4)

where theLrandLlrare the self and leakage inductances of therotor circuit respectively. The motor electromagnetic torque, Tecan be expressed in terms of phase variables as follows:

(5)Mechanical equation of the motor is:

(6)

wherePdetermines the number of pole pairs, Jis the momentof inertia,B is the friction constant and TLis the load torque.

B.

Inverter Model

The inputs to the five-phase induction motor are the five-phase voltage supply generated from the VSI. The powercircuit topology of this inverter is shown in Fig. 1. The inverteris controlled by Space Vector PWM (SVPWM) technique toobtain a constant V/f ratio [11]. The relationship between the

five-phase induction machine phase-to-neutral voltages (van ,vbn , , vec) and inverter leg voltages (vao , vbo , , veo) isgiven with:

(7)

C.Decoupling Transformation

To develop the complete model of the drive system atransformations between the abcde to the dqxyo variables arerequired. The transformation matrix for a five-phase system isgiven by [12]:

(8)

where = 2/5. Application of (8) in conjugation with theinverter voltages yields the dqxyo-axes components of the

motor terminal voltages, i.e.,

(9)

Owing to the absence of the neutral line, the zero sequencevoltage component of the inverter must equal to zero.

III. CONVERTER FAULTS

The five-phase voltage source inverter shown in Fig. 1 candevelop various fault types that may be classified as follows:

One-leg open-circuit fault, Two-leg open-circuit fault,

Gating signals failure,

One switch open-circuit fault, and

One switch short-circuit fault,Faults may also occur inside the induction motor. These faultsare not employed in this paper. Also the possibilities ofmultiple faults occur at the same time are very rare, andtherefore are not considered.

IV. EXPERIMENTAL STUDY UNDER FAULTS CONDITIONS

To investigate system performance during faulty inverterconditions, an experimental evaluation has been developed.The experimental results are given for both normal (healthy)

and an open-circuit condition while the short-circuit casedestroys many parts in the motor drives, mainly the five-phaseinverter switches and over current protection takes action inthis case.

A.Experimental Setup

The inverter power circuit is realized by 10-powerMOSFET (IRFP460A). The system demands 6 isolatedsupplies and 10 gate-driver circuits. The employed MOSFEThas the following characteristics; voltage blocking capability is


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3

500V, current capacities is 20A, integral freewheel-diode, noneed for snubber circuit, lower switching losses, and a totalturn on and turn off times 77 and 168 ms respectively. The gatedriver circuit is based on a high speed optocoupler device(6N137) with a typical 50 and 12 ns rise and fall timerespectively. The control system is based on the DS1104controller and the motor was operated in open-loop scalarcontrol with space vector modulated five-phase VSI. Theinverter voltage transfer ratio is adjusted to 0.577 at afrequency 20 Hz. Furthermore, it has been implemented usingMatlab/Simulink and then, compiled to real time system.Measurements are obtained using a Tektronix MSO2024Bmixed signal oscilloscope and a current sensor LA25-P. Thevoltage measured by TERCO-Differential Probe MV1971 andscaled by X100. All experimental results have been obtainedwith the experimental rig shown in Fig. 2 using a switchingfrequency of 1.5 kHz and sampling time of 100sec. The five -phase IM is originally a 36 slots, 2-pole three-phase IM, whosestator has been rewound to provide a five-phase IM.

B.

Normal Operation

Fig. 3 shows the motor currents experimental waveforms atsteady-state conditions for normal/healthy operations. These

results will be taken as base results to be compared with thefaulty results.

C.

Analysis of Fault Modes

In this part, the following faults types will be considered forour paper.

1)

One-leg open-circuit fault

If the leg-aof the inverter is opened (S1 and S6 are opened)after the starting process, the inverter terminals still connectedto the remaining four phases (b, c, d, and e). The motor speedand fault-tolerant currents under these conditions are shown inFig. 4. The phase currents are initially identical to the healthycondition currents, but when the fault condition is introduced,

the stator current in the faulty phase (phase-a) is zero. Inaddition, the currents in remaining phases are increased byabout 48.5% from its steady-state value at healthy conditions.Moreover, the motor speed is reduced by a slight dip of 0.5%from its final value. When free-wheeling diodes in the faultyleg-a switches are still operating the current in phase aequalsthe free-wheeling diode conduction current as shown in Fig.5.

If the motor is accelerated with four-phase excitation (leg-aof the inverter is opened before the starting process), thevoltage is applied to the remaining four phases (b, c, d, and e).The motor starts with a long time (about 30% from startingprocess time of healthy operation) to get the steady-state speed.The fault-tolerant currents under these conditions are shown inFig. 6. The stator current in the faulty phase (phase-a) is zero.

While Fig. 7 shows the stator voltage waveform of the faultyphase, this voltage is due to the induced e.m.f in the phase-a.This voltage is proportional to the motor speed.

2) Fault on Double-leg

For a five-phase motor, there are two different cases ofdouble-phase fault. In the first case, the double-phase open-circuited are the two adjacent phases (for example phases a andb) and in the second case, the fault may occur in two non-adjacent phases (for example phases a and c).

Power Supply PC

Current transducers

DS1104

Five-Phase VSI

SpeedCurrents

Switch

Five-phase IM

Fig. 2. Test rig scheme for the case of a constant dc-voltage supply

Fig. 3.

Experimental motor currents under normal operation

Fig. 4. Experimental results under leg-a is opened after the starting process

Fig. 5. Motor current under faulty phase-a considering free-wheeling diodes

ia

ib

ic

id

ia

ib

ic

ia

ib

ic

id

Fault


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4

Fig. 6. Experimental motor currents under leg-a is opened at starting

Fig. 7.

Experimental faulty phase voltage under leg-a is opened at starting

If leg-a and leg-b are opened (S1, S6, S3 and S8 areopened) after the starting process, the voltage is applied to theremaining three phases (c, d, and e). The phase currents areinitially identical to the healthy condition currents, but whenthe fault condition is introduced, the stator current in the faultyphases (phase-a and phase-b) are zero as shown in Fig. 8. Inaddition, the currents in remaining phases are increased byabout 69% from its steady-state value at healthy conditions.Moreover, the motor speed is reduced by a slight dip of 0.58%from its final value.

If leg-a and leg-c are opened (S1, S6, S5 and S10 areopened) after the starting process, the voltage is applied to theremaining three phases (b, d, and e). The phase currents areinitially identical to the healthy condition currents, but whenthe fault condition is introduced, the stator current in the faultyphases (phase-a and phase-c) are zero as shown in Fig. 9. Inaddition, the currents in remaining phases are increased byabout 36% from its steady-state value at healthy conditions.Moreover, the motor speed is reduced by a slight dip of 0.5%from its final value.

Fig. 8. Experimental motor currents under leg-a and b are opened aftersteady state

Fig. 9. Experimental motor currents under leg-a and c are opened aftersteady state

If the motor is accelerated with three-phase excitation (forexample leg-a and leg-c of the inverter are opened before thestarting process), the voltage is applied to the remaining threephases (b, d, and e). The motor starts with a long time (aboutthree-times from starting process time of healthy operation) toget the steady-state speed. The fault-tolerant currents underthese conditions are shown in Fig. 10. The stator current in thefaulty phases (phase-a andphase-c) are zero.

3)

Transistor Gating Signal failure

The inverter switches are normally controlled by isolatedgate driver circuits. If the gate signal is lost, the correspondingtransistor is opened. Suppose, transistor Q1 is now inefficient,the phase-aof the motor is connected to the positive rail of thedc-supply through the anti-parallel diode D1. The voltage ofphase-a is then determined by the polarity of current and theswitching pattern of transistor Q4. Figure 11 shows thesimulated motor currents under these conditions

Fig. 10.

Experimental motor currents under leg-a and c are opened at starting

Fig. 11.

Simulated motor currents under gate signal failure of transistor Q1.

0 0.01 0.02 0.03 0.04-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

Time (Sec.)

(A)

ia

ib

ic

id

ia

ib

ic

id

ia

ib

ic

ia ib ic idie

van

Fault

Fault


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From these results, it can be concluded that the five-phasemotor is able to operate with a four or three-phase excitationwithout any modifications in the modulation technique with thepresence of torque pulsations as well as speed harmoniccomponents. In order to overcome these effects, two differentfault tolerant strategies are suggested in the following section.

V.

FAULT-TOLERANT STRATEGIES FOR FIVE-PHASEVOLTAGE SOURCE INVERTERS

The first step of most fault-tolerant solutions is the physicalfault isolation, especially in the case of short-circuit fault. Thefaultisolation unit (usually isolating thyristors) force damagedconverter switches or legs to be electrically isolated from thesystem to eliminate its influence over the system behavior [13].Then, the post-fault reconfiguration can be activated. Similarprinciples of three-phase VSI fault-tolerant strategies [6] can beapplied in the five-phase case. This section suggests two fault-tolerant control strategies for five-phase VSIs. These strategiesare:

a) Redundant Leg topology

b) DC-Bus Mid-point Connection topologies

The circuit topologies of Fig. 12 give the proposed fault-tolerant five-phase voltage source inverter. A conventionalfive-phase inverter consists of only five legs. In the firststrategy, the fault-tolerant inverter has one leg as redundant.The redundant leg has not been used when the conventionalfive legs are working without any fault. The isolated back-to-back thyristors are connected between the inverter outputterminals and the corresponding motor phases [14]. Thesethyristors are used as isolating switches of faulted leg.Additional five thyristors (redundant leg, inserting thyristors)are connected between the mid-point of redundant leg and themotor phases as shown in Fig. 12-a. These thyristors are usedfor inserting the redundant leg in the place of faulted phase.This strategy can be used for tolerance of all theaforementioned faults except the phase-leg short-circuited.

Similar to three-phase inverters, if one phase fails, theremaining two phases can maintain continuous operations.Two typical fault-tolerant topologies with additional switchesemployed in motor applications are presented in [6]. Thesetopologies have been proposed for the five-phase inverters as astrategies b and c as shown in Fig. 12 (b) and (c). The firstfault-tolerant topology shown in Fig. 12 (b) forces the faultyphase to connect to the mid-point of the dc-link via theadditional dc-bus mid-point inserting thyristors. After faults,the reconfigured system is similar to the structure where onlyfour switches are used to drive a three-phase machine [15]. Themaximum balanced line-to-line output voltage in post-fault

operations is reduced to half of its nominal value; this is themain drawbacks of this strategy. Moreover, this approach isonly applied in situations where the mid-point of dc-linkcapacitors can be accessed.

The last method connects the neutral point of the five-phasemotor to the dc-bus mid-point via a dc-bus inserting thyristoras shown in Fig. 12(c). Note that only one thyristor is added forthe fault tolerance.

0

vdc

m

Te

a b c d e

va

vb

vc

vd

ve

n

r

Redundant leg

Isolating

Thyristors

Redundant

leg inserting

thyristors

(a)

vdc

m

Te

a b c d e

va

vb

vc

vd

ve

n

Isolating

Thyristors

DC-Bus

Midpoint

inserting

thyristors

C

C

(b)

vdc

m

Te

a b c d e

va

vb vc vd

ve

n

Isolating

Thyristors

DC-Bus

Midpoint

inserting

thyristor

C

C

(c)

Fig. 12.

Fault tolerent strategies for five-phase voltage source inverter (a)switch redundant topology, (b) and (c) dc-bus mid-point connections


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VI. CONCLUSION

This paper introduces the effect of different types of fault ina five-phase VSI on an induction motor drive that uses anopen-loop scalar control (V/f=constant) method. The internalfaults in the machine are excluded from this paper. Someimportant faults are indicated in the beginning, then it isfollowed by a simulation study to the other faults. The aim offault performance of any drive system is very important to

determine the semiconductor devise stress, to optimize theprotection system design, and to predict the post-fault driveoperating capability. The results showed that, for the faultsunder consideration, the motor is able to continue operationwith the presence of torque pulsations as well as speedharmonic components without modifying the control schemeor the inverter topology. These simulation and experimentalresults can be used to improve the system performance byusing fault tolerant control or post-fault control design.Moreover, two strategies of fault-tolerant control for five-phaseVSIs are suggested to overcome these effects.

VII. ACKNOWLEDGEMENT

This paper was made possible by NPRP grant # NPRP 4-250-

2-080 from the Qatar National Research Fund (a member ofQatar foundation)The Statements made herein are solely theresponsibility of the authors.

VIII.

REFERENCES

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[2]

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paper 2-investigation of five-phase induction motor drive under faulty inverter conditions

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