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Ipari automatizlsi rendszerekben alkalmazott hibatr villamos

gpeksszehosonlt vizsglata

Comparative study of fault-tolerant electrical machines used in

automated industrial systems

Studiu comparative al mainilor electrice utilizate n automatizri

industriale

Ruba Mircea1, dr. Szab Lornd1, dr. Bir Kroly goston1, Kovacs Ern2

1 Kolozsvri Mszaki Egyetem, Villamosmrnki Kar, Villamosgpek Tanszk

RO-400750 Cluj, P.O. Box 358, RomniaTel.: +40-264-401-827, Fax.: +40-264-593-117

e-mail: [email protected]

web: http://users.utcluj.ro/~szabol/index.htm2 Pannon Egyetem, Mszaki Informatikai Kar, Automatizls TanszkH-8200 Veszprm, Egyetem u. 10., Magyarorszg

Tel.: +36-88-624-461, Fax.: +36-88-624-545e-mail: [email protected]

web: http://www.aut.vein.hu

Abstract: Nowadays the tendency in industrial automated systems is to implement equipmentsthat can offer a continuous operation even in the event of fault occurrence. A system representsinterconnection between feed bus bars, intelligent electronic devices and electromechanical assembles.

The fault tolerance can be implemented in all the systems levels. The result will be a safe andcontinuous operating system. The electrical machines involved in safe automated equipments are thesubject of the present study. Switched reluctance machine (SRM), and a permanent magnetsynchronous machine with two attached inverter topologies will be presented regarding them faulttolerance. The conclusions will present, according to applications, the optimal fault tolerant machines.

Rezumat:Tendina actual n domeniul automatizrilor industriale este de a oferi echipamentecare au continuitate n funcionare chiar dup eventualele apariii ale unor defecte. Sistemele sunt

formate din zone de alimentare, zone de comand i control inteligente, respectiv din ansambleelectromecanice. Tolerana fa de defecte poate fi implementat fiecrui nivel al sistemului rezultnd

n final un sistem ce ofer siguran i continuitate n funionare. Mainile electrice implicate nsistemele de automatizare devin subiectul prezentei lucrri. Diferite structuri de maini cu reluctancomutat, respectiv o main sincron cu magnei permaneni cu dou stricturi de invertor vor fi

prezentate. Concluziile vor contrura alegerea optim a mainii electrice tolerante la defecte, funcie de

aplicaie.

sszefoglal: Tendina actual n domeniul automatizrilor industriale este de a oferiechipamente care au continuitate n funcionare chiar dup eventualele apariii ale unor defecte.

Sistemele sunt formate din zone de alimentare, zone de comand i control inteligente, respectiv dinansamble electromecanice. Tolerana fa de defecte poate fi implementat fiecrui nivel al sistemuluirezultnd n final un sistem ce ofer siguran i continuitate n funionare. Mainile electriceimplicate n sistemele de automatizare devin subiectul prezentei lucrri. Diferite structuri de maini cureluctan comutat, respectiv maini sincrone cu magnei permaneni vor fi prezentate. Concluziilevor contrura alegerea optim a mainii electrice tolerante la defecte, funcie de aplicaie.


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1. INTRODUCTION

The concept of fault tolerance emerged in the field of information technology because of the demand of safety

and reliability of a system. Later on more and more fields of engineering took over the concept, and the

connection between fault tolerant equipments formed fault tolerant systems [1].

The meaning of system is the interconnection of components all reaching for the same goal, to serve an outputfor a given application. As errors are part of life, the possibility of their appearance in a system must be taken

into account.

Nevertheless fault tolerant systems are not so easy to achieve. A fault tolerant system must detect faults in its

components and also must have the ability either to correct it (for example by switching to a backup unit when

the main one fails) or to circumvent it (for example by reconfiguring the system) [2].

Nowadays the tendency in electrical engineering is to develop applications as safe as possible. The concept of

fault tolerant device became a purpose for a lot of researchers. With the help of electrical power devices that

evolved in the last years a combination between electric drives and machines pushed the limits of fault tolerance

[3]. Any attempt of study in this area that exposes new results can become of important interest for all

researchers working in the field of fault tolerance.

The switched reluctance motor (SRM) can offer at lower costs a solution for the problem of fault tolerance.

Permanent magnet synchronous machines involve higher manufacturing costs due to the permanent magnets

embedded, but these are an endless force source, hence they cannot suffer faults. The drives attached to PMSM

become widely used due to their high efficiency and power density. For example in vehicles PMSM drives can

replace traditional mechanical actuators to achieve advantages such as higher efficiency and improved dynamical

performance. It is apparent that certain functions such as electrically assisted steering and braking are of

outermost importance and that their failure cannot be tolerated [4].

Thanks to the improvements in the field of power electronics and also to digital signal processing nowadays

intelligent solutions can be provided in designing a fault tolerant electrical drive system. The separate phase

feeding and control of the machines allow an easier approach of the fault tolerant tasks, offering better results

[5].

2. THE PORPOSED MACHINES IN STUDYAchieving a fault tolerant variant of a usual electrical machine requires modified topologies. In

order to develop an efficient fault tolerant electrical machine is important to take into account also itslosses. The main idea was to shorten the flux paths in the new motor structure, hence the shorter flux

paths means lower iron losses.As a starting point in the study, a 12/8 SRM structure was considered, as that shown in Fig. 1a.The modified SRM has 14 rotor poles, as shown in Fig. 1b. A more complex structure was studied

in [6]. There the number of stator and rotor poles, hence the phase number, were increased to 24/18.


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The winding scheme is a six phase duplex type, in other words each phase is doubled. This consistsin phases placed at an angle of 180 mechanical degrees, in order to create the correct force distributionin the machine. In case of a fault on one channel the second one still will contribute to the torquegeneration and by its independence, in means of feeding and control on each channel, the faulttolerance is achieved.

Both SRMs in study have the same stator, but they have different rotors and different phase/channelconnections. Multiplication of rotor poles will increase the level of fault tolerance in means of torque

development and safe operation.Initially for the 9-phased PMSM in discussion a special, 9-branches variant of the well-known

H-bridge (full-bridge) converter (given in Fig. 2a) was proposed.In order of obtain high level of fault-tolerance for the 9-phase PMSM a special connection of its

phases will be applied (see Fig. Error: Reference source not foundb)[6].In this special scheme the winding is divided in 9 phases, grouped 3 by 3. Y connections are created

for each group of 3 windings. The 3 groups are connected to a common power supply. Obviously thisspecial connection of the fault-tolerant PMSM needs a particular converter. The starting point was the

standard three-phase voltage source inverter. To each of the three groups an extra inverter leg is added.This connection can be used because the PMSM in study has Y-connected winding groups. If a

winding fault occurs in the PMSM, the faulted phase is isolated by keeping open the correspondingtwo power switches. The supplementary inverter leg will continue to drive the currents, assuringpractically the normal current through the remained healthy phases. Since the additional inverter leg isconnected to the neutral point of the PMSM, the neutral current caries the phase currents of theremaining phases.

3. THE COUPLED SIMULATION PROGRAM

The simulations were performed using the co-simulation technique, by coupling two simulationenvironments to work together. The machines were built up using the Flux 2D finite element method(FEM) based electromagnetic field computation software.

a) b)

1. bra. The purposed SRM Structuresa)

b)

2. bra. The purposed PMSM Inverter Structures


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As the study is performed in transient with motion regime a mesh optimisation was required in orderto reduce computation times. A compromise had to be made between the imposed mesh density, the

computation accuracy and the available hardware.The simulation of the power converter is performed by using an electrical circuit build up in

Electriflux, Flux 2D's circuit editor, and attached to the FEM model of the machine. As it can be seen(Fig. 3) each channel of one phase is modelled using two electrical coils, corresponding to the "come

and go" sides of the winding. The reverse current management is handled by the diodes. In theelectric circuit model the power switches were replaced by resistors. Practically these resistors willmodel the switches, by changing their resistance from a high value, as for the OFF state of the switch,to a low one, corresponding to the ON state of the transistor. These changes are obtained using the

coupling of Flux 2D with MATLAB-Simulink. In fig. 4 the Simulink model of the entire SRM basedfault tolerant drive system is given. For the PMSM, the Simulink model is nearly the same as for the

SRM, the difference being noticed in the feed system using AC sources, instead of DC like the ones

for the SRM.The link between Simulink and Flux 2D is implemented by the Coupling Flux2D S-function type

block. The input values of the block (practically the signals to be transferred to Flux 2D) are theresistance values corresponding to each power switch.

The S-function block will receive the output signals after the field computation (the torque, thephase currents and the rotor position) and will transfer them to Simulink. Using these values theparameters of the next simulation step will computed. This way the next step of simulation, and so on,will be computed step by step till the time limit is reached.

The control strategy was implemented in SIMULINK, the most widely used platform in dynamic

simulations.The PWM technique is used only on one switch of one phase, the second one is held open for the

whole conducting period, hence practically the first modulates the current. The ON/OFF signals are

3. bra. The attached circuit to each

phase

4. bra. Flux to Simulink Coupling for the SRM study


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sent to the two switches: the resistance value of 100 k (OFF state) or 0.004 (ON state). The faultswill be simulated by imposing OFF state for both switches of one phase. The precision of the PWM isset by the hysteresis band around the imposed current.

4. RESULTS OF SIMULATIONS. CONCUSIONSFor the two SRMs, different cases were studied in order to check their fault tolerance capability:

i.) normal operating mode (reference case),

ii.) open circuit of one channel,

iii.) open circuit of one phase,

iv.) open circuit of two channels from different phases,

v.) open circuit of one phase and one channel from a different phase (worst case in study).

Different computation times were set for the two machines, in a way to be able to observe (see Fig. 5) the

effects of the faults. The 6-phase (12/14) machine was simulated 0.008 s and the other one 0.024 s.

To create an image for the behaviour of the PMSM, the study cases involved in the paper were:

i.) Normal operating mode

ii.) One phase faulty, having open circuitiii.) Two phases faulty from different groups

iv.) Three faulty phases from three different groups

v.) Two phases faulty from one group and 1-1 from two other groups.

During the transient simulation the machine is started without any faults. The first fault is imposed at 0.01 s,

the second one at 0.02 s. When more faults are simulated these are set to appear also at 0.01 s.

The characteristics for the SRM (Fig. 5a-5e) are the ones from the 12/14 structure, as [7] proved that this is

the optimal one from the two purposed topologies. These are compared to the torque characteristics from the Y

connected electronic inverter for the PSMS (Fig. 5f-5j).

The torque ripples are lower in the case of PMSM in normal operating mode, than the ripples of the SRM in

the same operational case. Against this case, in the event of fault occurrence, the PMSM will develop relatively

high torque ripples due to the missing current in the windings. As the severity of the fault is increased, modelled

by phase open circuit, the ripples are more and more obvious in the SRM torque characteristic. The increase isproportional with the severity for the PSMS too.

For the SRM the electronic converter used was a typical H bridge one. To obtain a high level of fault tolerance

a complex control system is required and high number machine's pole. The study demonstrated that increasing

the number of rotor poles, separating the phases/channels, setting new connections between the existing

windings, and using a complex control system will provide the best solution for the fault tolerant SRM based

electrical drive system.

In order to conclude and underline the best solution for an automate fault tolerant system, using a SRM is

optimal as the ripples are lower in case of fault. If, the ripples are acceptable, the PMSM can be involved too,

considering its higher manufacturing costs too. A discussion regarding costs can be held about inverters too. As

more complex the inverter, as more the costs are higher. A compromise must be taken between the applications

demand and the budget of the project.

The coupled simulation program connecting two software environment (FLUX 2D and SIMULINK) was

useful in studying the effects of different winding faults on the torque developing capacity of the SRM. The

computing power of FLUX 2D thus joined the facilities of Simulink in simply describing the different working

regimes of the machines and drives taken into study.

The main problems were regarding the computation times. In order to obtain precise results and reasonable

computation time the quality of the FEM model's mesh had to be lowered.

The coupled simulation programs allow a high-quality simulation of closed-loop real time systems.

Future ideas are regarding changes of the machine geometry, and new placing for the windings. A fault

tolerant SRM is intended to be built up, using techniques applied in other categories of electrical machines. The

resulted structure will be a combination of two already existing machines


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5. IRODALOM[1] Blanke, M., "Diagnosis and Fault-Tolerant Control," Springer Verlag, 2006.

[2] Husain, I., Radun, A., Nairus, J., Fault Analysis and Excitation Requirements for Switched Reluctance Generators, IEEETransactions on Energy Conversion, vol. 17, no. 1 (March 2002), pp. 67-72.

[3] Ertugrul, N., "LabVIEW for Electric Circuits, Machines, Drives, and Laboratories," Prentice Hall PTR, 2002.

[4] Wallmark, O., Harnefors, L., Clarson, O., "Control Algorithms for a Fault tolerant PMSM Drive," IEEE Transactions on IndustrialElectronics, vol. 54, no. 4 (August 2007), pp. 1973-1980.

[5] Heimerdinger, W., Weinstock, C., "A Conceptual Framework for System Fault Tolerance," Technical Report CMU/SEI-92-TR-033,Carnegie Mellon University, Software Engineering Institute, Pittsburgh (USA), 1992.

[6] Szabo L. Ruba M. Fodorean D.: Study on a Simplified Converter Topology for Fault Tolerant Motor Drives, OPTIM, Brasov,

Romania, 2008 ISBN 978-973-131-028-2, pp 197-202

12/14 fault tolerant SRM topology Fault tolerant PMSM topology

0 1 2 3 4 5 6 7 8

x 10-3

-50

0

50

t [s]

urrent

0 1 2 3 4 5 6 7 8

x 10-3

-50

0

50

t [s]

orque

m

. . . . . . . . .

.

0 0.005 0.01 0.015 0.02 0.025 0.03-20

0

20

40

60

Time [s]

Torque[Nm]

. . . . . .

a) normal study case f) normal operating mode

0 1 2 3 4 5 6 7 8

x 10-3

-50

0

50

t [s]

urrent

0 1 2 3 4 5 6 7 8

x 10-3

-50

0

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t [s]

orque

m

. . . . . . . . .

.

0 0. 002 0. 004 0. 006 0. 008 0. 01 0. 012 0. 014 0. 016 0.018 0. 02-20

0

20

40

60

80

Time [s]

Torque[Nm]

. . . . . . . . . .

b) one channel open circuit g) one phase faulty, having open circuit

0 1 2 3 4 5 6 7 8

x 10-3

-50

0

50

t [s]

urrent

0 1 2 3 4 5 6 7 8

x 10-3

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0

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t [s]

orque

m

. . . . . . . . .

.

0 0.002 0.004 0. 006 0.008 0.01 0.012 0.014 0.016 0. 018 0.02-20

0

20

40

60

Time [s]

Torque[Nm]

. . . . . . . . . .

c) one phase open circuit h) two phases faulty from different groups

0 1 2 3 4 5 6 7 8

x 10-3

-50

0

50

t [s]

urren

0 1 2 3 4 5 6 7 8

x 10-3

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orque

m

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.

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04-20

0

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60

Time [s]

T

orque[Nm]

. . . . . . . .

d) two channel open circuit i) three faulty phases from three different groups

0 1 2 3 4 5 6 7 8

x 10-3

-50

0

50

t [s]

urrent

0 1 2 3 4 5 6 7 8

x 10-3

-50

0

50

t [s]

orque

m

. . . . . . . . .

.

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04-20

0

20

40

60

Time [s]

Torque[Nm]

. . . . . . . .

e) one phase and one channel open circuit j) two phases faulty from one group and 1-1 from two other groups

5. bra. Torque plots of SRM & PMSM in study under faulty conditions


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[7] Ruba M., Szab L., Strete, Larisa, Viorel, I.-A., "Study on Fault Tolerant Switched Reluctance Machines,"Proceedings of the 2008 International Conference on Electrical Machines (ICEM '2008), Vilamoura(Portuglia), CD-n 1200.pdf.

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