XIX International Conference on Electrical Machines - ICEM 2010, Rome

Novel Modular Switched Reluctance Machine for Safety-Critical ApplicationsMircea Ruba, Ioana Benia, Lornd Szab Abstract -- Electrical machines and drives used in various safety-critical applications are of special design in order to achieve the required fault tolerance level. In the paper a novel modular fault tolerant switched reluctance machine is proposed and studied. It is proved by means of dynamic simulations that the proposed machine is able to have continuous operation also despite of five severe winding fault conditions.

Index Terms -- Fault tolerance, reluctance machines, reluctance motor drives, machine windings, simulation.

ault tolerance is the ability of a system to continue performing its intended function in spite of different faults, expectedly occurring in any system. As the complexity of a system increases its reliability drastically gets worse, unless compensatory measures are taken [1]. An advanced fault tolerant electrical system has to be capable of detecting its faults and able to adequately compensate the failures [2]. Fault tolerance is an obligatory feature in almost all the safety-critical applications, where loss of life or environmental disasters must be avoided (aeronautical, aerospace, medical and military applications, power plants, etc.) [3]. Hence the electrical machines and drives used in such systems must be of high fault tolerance. Switched reluctance machines (SRM) seems to be the best solutions for such applications [4]. The SRM is inherently more fault tolerant than other electrical machines, because it can continue operating and producing torque in certain limits also with one or more faulty phases [5]. This is due in a large part to its independent concentrated windings. It's brushless and permanent magnet free simple construction enables a maintenance free utilization also in high temperature, dusty, dirty and vibrations exposed harsh environments [6]. Several methods exist for improving the fault tolerance of a SRM. As a first step the stator poles and phase numbers can be increased [7], [8], [9]. Another usual solution is the division of the phases into individual coils, called channels [10]. This way a fault of a channel will not influence the operation of the other channels of the same phase or of other phases. The drawback of this solution is that a more complex power converter is required, having as many converter legs as channels [11]. Combining the fault tolerance increasing solutions with the modular construction concept a novel SRM was developed, which is high reliable and quickly repairable.Mircea Ruba, Ioana Benia and Lornd Szab are with the Department of Electrical Machines, Technical University of Cluj, Cluj, Romania (e-mails: Mircea.Ruba@mae.utcluj.ro, ioana.bentia@yahoo.com, and Lorand.Szabo@mae.utcluj.ro). Work partially supported from the Romanian PNCDI 2 Partnership Research Grant "Fault-Tolerant Equipment Controlled By Bio-Inspired Electronic Architectures (ElBioArch)", no. 12-121 / 01.10.2008 (http://elbioarch.utcluj.ro).

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I.

INTRODUCTION

Its fault tolerance is studied by means of dynamic simulations performed by a program set up in the MATLAB/Simulink environment. In the simulation program the magnetic flux and force computations are substituted with data taken out from two look-up tables containing the flux and force characteristics of the machine obtained via finite elements method (FEM) based numeric field computations [12]. All the obtained results emphasize the high fault tolerance of the proposed SRM. II. THE MODULAR FAULT TOLERANT SRMStator module

The proposed modular SRM is given in Fig 1.Spacer

Rotor

Winding

Hole for fixing rod

Fig. 1. The modular SRM in study

The machine has four phases, each divided into two channels. Each channel is wound on one of the eight module's yoke. The modules of a phase are placed diametrically opposed. The detailed construction of the SRM is shown in Fig. 2.Stator module Rotor Module spacer End shield

Shaft

Frontal spacer Hole for fixing rod

Fig. 2. The construction of the SRM

978-1-4244-4175-4/10/$25.00 2010 IEEE

The distance between two neighbored modules is set by nonmagnetic spacers. One module without its winding and a spacer are given in Fig. 3.

Fig. 3. The iron core of the module and a spacer

The entire modular construction is tightened by totally 16 rods, 2 through each module. The modular stator construction is placed between two end shields. Nonmagnetic spacers keep the stator in the middle of the motor. The rotor has a usual construction built of laminations. The flux lines in the motor for unaligned and aligned positions obtained via field computations are shown in Fig. 4.

As it can be seen the magnetic flux is closed between the two poles of a single module, hence there are not passing through the central part of the rotor. Therefore due to the shorter flux paths the losses in the machine are less than in a classical 4-phase SRM. Also due to this design the forces are better balanced in the machine. The modular construction allows both easy manufacturing and fast replacement of the damaged modules in case of a winding failure. Only a single end shield and the two fixing rods of the faulted module have to be detached and the module can be easily pulled out and replaced. This way there is no need of decoupling the machine from its load, a major advantage in industrial environment [13]. The machine's power converter given in Fig. 5 has a separate half H-bridge for every channel for independent control [14]. During the conducting period only one switch of an inverter branch is commanded via a hysteresis current controller. The other one is held open for the entire period.+1 1'

2

2'

3

3'

4

4'

Fig. 5. The power converter

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a) unaligned poles

The control system of the machine must have the intelligence to detect the fault, to isolate and remedy it, all to ensure that the machine's behavior is influenced as less as possible by the faults [13], [15]. A first answer of the control system to an open channel is the increase of the currents in the healthy remained coils up to a predefined value, which was taken into account when the windings were designed. The four phase sample machine to be simulated has 350 W. Its rated voltage and current are 300 V and 6 A. It is capable to develop a rated torque of 5 Nm. The main data of the sample machine in study are given in Table I.TABLE I THE MAIN DATA OF THE MACHINE

b) aligned poles Fig. 4. The flux lines in the SRM in study obtained by means of numeric field computations

Module height Rotor pole height Air-gap Stator and rotor yoke height Rotor and stator pole width Winding height Outer diameter Number of turns of a channel

35 mm 26 mm 0.5 mm 11 mm 13 mm 19 mm 210 mm 220

III. THE SIMULATION PROGRAM To be able to study the fault tolerance of the proposed machine dynamic simulations in different conditions should be performed [16]. The simulations of the speed control system with the fault tolerant SRM in study were carried out by a program set up in the MATLAB/Simulink environment. Preliminarily two main characteristics of the machine were set up based on the results obtained by means of field computations performed by the Flux 2D program [17]. These are the torque of the machine and the magnetic flux thru the energized coil versus the rotor position and current. These two characteristics are given in Fig. 6.6

In the modularly built up program the main units of the speed control system (the speed controller, the power converter and the modular SRM) can be easily distinguished. The speed controlled is of PI type. The machine's model with the two 2-D Look-up Table-type blocks integrating the two characteristics mentioned above is given in Fig. 8 [18].v 1 TL 2 K Ts z-1 Flux Flux Torque Rs Rs Angle Pos_sensorangle w I (A) Te Teta

Mechanic SystemTL w w (rad/s) teta (rad)

Sum of Elements

Scope4

Flux (V*s)

1

4

CurrentA.mat To File

w (rad/s)

m1

2 Torque [N m]CurrentB.mat

0

To File1

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Fig. 8. The Simulink model of the SRM

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-6 8

7

6

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4

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Current [A]

1

0

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20

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80 70 60 50 40 Angular displacement [degrees]

90

The model of the power converter is built up of SimPowerSystems blocks. The open circuit faults of the windings were simulated by imposing OFF state for both switches of the corresponding converter leg. IV. THE RESULTS OF SIMULATIONS In order to emphasize the machine's behavior both in normal and faulty operation mode several simulations were performed for the following conditions [19]: a) healthy condition; b) one channel open; c) two open channels; d) three open channels; e) four open channels; f) one completely faulty (open) phase. All the simulations were performed in identical conditions. The simulation time was 0.5 s. The trapezoidal profiles of the imposed speed and of the resistant torque are shown in Fig. 9.6 600

a) the developed torque0.25

0.2

Magnetic flux [Wb]

0.15

0.1

0.05

0 8

7

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Current [A]

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80 70 60 50 40 Angular displacement [degrees]

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b) the magnetic flux thru the energized coil Fig. 6. The main characteristics of the SRM computed via numeric field analysis for different rotor positions and currentsn* [r/min]

SPEED CONTROLERv I*

CONVERTERv1 G v2 v3 V+ v4 v11 v21 Vv31 v41

M ODULAR SRMv m1 TL

To File1 TORQUE.MAT

200

2

300 V

0

0

0.1

0.2 t [s]

0.3

0.4

0 0.5

R_TORQUE.MAT RESISTAT TORQUEw sig alfa beta

To File3

-K-

Scope1

Fig. 9. The imposed speed and the resistant torque vs. time

4 36

To File2 SPEED.MAT

POSITION SENSOR

The obtained results are given in Fig. 10. In all the six cases in study two sets of currents (those in the first, respectively second channels of all the phases), the speed and the developed torque were plotted versus time.

Fig. 7. The main window of the simulation program

T R [N m]

The main window of the simulation program is given in Fig. 7.

400

4

8 6 I [A]

8 6 I [A]0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5

4 2 0

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8 6 I [A]

8 6 I [A]0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5

4 2 0

4 2 0 0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5

600 n [r/min] 400 200 0

600 n [r/min]0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5

400 200 0

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15 10 5 0 0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5

T [N m]

a) healthy condition8 6 I [A] I [A] 4 2 0 0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5 8 6 4 2 0 0 0.05 0.1 0.15

T [N m]

d) three open channels

0.2

0.25 t [s]

0.3

0.35

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0.5

8 6 I [A] I [A] 0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5 4 2 0

8 6 4 2 0 0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5

600 n [r/min] 400 200 0 n [r/min] 0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5

600 400 200 0

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T [N m]

b) one open channel8 6 I [A] I [A] 4 2 0 0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5 8 6 4 2 0 0 0.05 0.1 0.15

T [N m]

e) four open channels

0.2

0.25 t [s]

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8 6 I [A] I [A] 0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5 4 2 0

8 6 4 2 0 0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5

600 n [r/min] 400 200 0 n [r/min] 0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5

600 400 200 0

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15 10 5 0 0 0.05 0.1 0.15 0.2 0.25 t [s] 0.3 0.35 0.4 0.45 0.5

T [N m]

c) two open channels

T [N m]

f) one open phase

Fig. 10. Results of simulation. The currents, speed and torque vs. time in different motor conditions

In Fig. 10a the results for the healthy condition of the machine are presented. For achieving the imposed speed the rotor has to be accelerated by a torque greater than its rated value. During the constant speed operation of the machine also the developed torque is almost constant, with low ripples. The maximum value of the current pulses are around 5 A, the rated current of the machine. When a single channel is opened (see Fig. 10b) the imposed speed profile is still closely fulfilled. To overcome

the missing channels contribution to the torque generation the currents in the healthy channels were raised by the control system. Due to higher currents also the torque ripples become greater. The SRM was designed (also from the thermal point of view) to work in continuous regime at a maximum 8 A phase current. In the case of two open channels from two different phases (Fig. 10c) the imposed speed profile is also nearly

fulfilled. The currents in the healthy channels of the faulted phases are increased to the maximum value of 8 A in order to compensate the winding faults. In this case the torque ripples are less than in the previous case, as it can be seen in Table II. The reason is that two adjacent channels are opened in this case. Hence the machine has two consecutive phases fed at the rated current and two at higher current. When the rotor is aligned with a pole having its winding fed with different current than that from the previous pole higher torque ripples occur. The mean developed torque in this condition is around 5 Nm, the rated torque of the machine (see Table II). When three channels from different phases are opened (Fig. 10d) the machine still can run at the imposed constant speed. The developed torque ripples are greater in comparison with the previous case because the opened channels are not consecutives and the command current's maximum value is varying from the rated to the maximum value. As it can be seen in Table II also in this case the machine is able to develop the rated torque despite of three winding faults. If four channels are faulted (Fig. 10e) the machine will practically operate with only half of its structure. The machine cannot accelerate to its constant speed as rapidly as it is imposed, but after about 0.15 s the imposed speed is reached and the machine is able to maintain this speed at the imposed load. As the four channels are from different phases in all the

healthy remained coils the currents will be maintained at their maximum value (8 A). It is interesting that in this case, when the SRM have four open channels, the torque ripples are the lowest in comparison with other faulty conditions. The reason is, as it was already pointed out, that in all the healthy remained channels the currents are of the same magnitude. The last condition in study is the most severe one, when an entire phase is opened (see Fig. 10d). The starting of the motor is quite heavy. This can be also impossible if the rotor is in an initial position where the first phase to feed is exactly the faulted one. If the motor starts, or if the fault occurs during its movement the inertia of the motor and load can move further the rotor from a position where torque is not developed due to the missing phase. In this case the currents in the rest of the phases are increased, especially in the adjacent phases to the faulted one. This way the rotor will develop more torque to overrun the pole with the faulted phase. Unfortunately in this case the torque ripples are the greatest ones from all the conditions in study. It is important to mention that in all the conditions taken into study the mean torque developed by the machine is near the rated one, which means that even in the case of several winding faults the machine is able to continue its movement at the imposed speed and torque, the main task in case of a safety-critical application.

TABLE II THE MINIMUM, MAXIMUM AND MEAN TORQUE, RESPECTIVELY THE TORQUE RIPPLE FOR ALL THE CONDITIONS IN STUDY

Condition Healthy One open channel Two open channels Three open channels Four open channels One open phase

Torque [Nm] Minimum 4.2467 -0.5021 0.1870 -0.1212 1.2386 -2.0694 Maximum 6.0396 13.6080 11.3618 12.9501 8.1011 14.8225 Mean 5.0814 5.0937 5.0661 5.0952 5.0936 5.1058 Ripple 1.7929 14.1102 11.1748 13.0714 6.8624 16.8919

V.

CONCLUSIONS

The novel fault tolerant modular SRM topology completes the few structures cited in literature and can be interesting both for researchers working in the field of SRMs and in fault tolerant systems. The design of the proposed structure was performed accurately and taking into account all electromagnetic phenomena of such a complex structure [20]. The number of poles was increased and the phase windings were split into two separately fed channels. As it was proved in a previous paper, the FEM based numeric computations were in accordance with the analytical ones, which proved the correctness of the design [13]. The MATLAB/Simulink model of the machine in study was demonstrated to be a useful tool in dynamic simulations of diverse conditions of the machine. The integration in the Simulink model of the two tables computed via FEM based numeric field analysis much increased the precision of the simulations. Hence in short time numerous simulations

could be performed for various machine and load conditions [21]. All the obtained results proved the fault tolerant capability of the proposed modular SRM. Also in very severe conditions (with up to the half of the SRM's coils opened) the main task of a fault tolerant machine was fulfilled: to continue its movement at the given load. Of course in such conditions the torque ripples are higher and the speed can be reduced. Beside its fault tolerance another advantage of the machine is its simplicity. The modules can be manufactured separately and the stator can be easily assembled. The passive rotor is very simple. The proposed SRM can be quickly repaired if winding faults occur without removing the machine from the load. The main drawback of the proposed SRM is the complexity of its power converter [22]. By splitting into two channels each phase the number of the power converters branches were also doubled. The machine can be used in safety-critical applications where the reliability is a key issue: advanced factory

automation systems, automotive and aerospace applications, military, energy and medical equipment, etc. In the future more faulty conditions should be studied (short circuits, power converter faults, etc.). Special attention has to be given to the torque ripple minimization. This can be improved by increasing the number of poles (and inherently the number of converter legs) or by applying more advanced control techniques [23]. The laboratory model of the proposed SRM is in construction. The measurements to be performed hopefully will confirm the correctness of the simulated results. VI. REFERENCES[1] [2] [3] [4] R. Isermann, Fault-Diagnosis Systems: An Introduction from Fault Detection to Fault Tolerance, Springer-Verlag, Berlin, 2006. G.J.J. Ducard, Fault-tolerant Flight Control and Guidance Systems. Practical Methods for Small Unmanned Aerial Vehicles, Advances in Industrial Control Series, Springer-Verlag, London, 2009. E. Dubrova, Fault-Tolerant Design: An Introduction, KTH Royal Institute of Technology, Stockholm (Sweden), 2008. URL: http://web.it.kth.se/~dubrova/draft.pdf. J.A. Sanchez, P. Andrada, B. Blanque, M. Torrent, J.I. Perat, "Postfault performance of a fault-tolerant switched reluctance motor drive," in Proceedings of the 11th European Conference on Power Electronics and Applications (EPE '2005), Dresden (Germany), pp. 1-8, 2005. G. Henneberger and I.A. Viorel, Variable Reluctance Electrical Machines, Shaker Verlag, Aachen (Germany), 2001. S. Gopalakrishnan, A.M. Omekanda and B. Lequesne, "Classification and remediation of electrical faults in the switched reluctance drive," IEEE Transactions on Industry Applications, vol. 42, no. 2 (MarchApril 2006), pp. 479-486. M. Ruba, L. Szab, L., Strete and I.A. Viorel, "Study on Fault Tolerant Switched Reluctance Machines", in Proceedings of the 18th International Conference on Electrical Machines (ICEM '2008), Vilamoura (Portugal), on CD: Fullpaper_comm_id01200.pdf. P.C. Desai, M. Krishnamurthy, N. Schofield and A. Emadi, "Novel Switched Reluctance Machine Configuration With Higher Number of Rotor Poles Than Stator Poles: Concept to Implementation," IEEE Transactions on Industrial Electronics, vol. 57, no. 2 (February 2010), pp. 649-659. F. Soares and P.J. Costa Branco, "Simulation of a 6/4 Switched Reluctance Motor Based on Matlab/Simulink Environment," IEEE Transactions on Aerospace and Electronic Systems, vol. 37, no. 3 (July 2001), pp. 989-1009. I. Husain, A. Radun and J. Nairus, "Fault Analysis and Excitation Requirements for Switched Reluctance Generators," IEEE Transactions on Energy Conversion, vol. 17, no. 1 (March 2002), pp. 6772. E. Martnez et al., "Environmental and Life Cycle Cost Analysis of a Switched Reluctance Motor," in Proceedings of the 18th International Conference on Electrical Machines (ICEM '2008), Vilamoura (Portugal), on CD: Fullpaper_comm_id01221.pdf. M. Kuczmann and A. Ivnyi, The Finite Element Method in Magnetics, Akadmiai Kiad, Budapest (Hungary), 2008. M. Ruba, I. Benia and L. Szab, "Novel Modular Fault Tolerant Switched Reluctance Machine for Reliable Factory Automation Systems," in Proceedings of the IEEE-TTTC International Conference

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on Automation, Quality and Testing, Robotics (AQTR '2010) THETA 17, Cluj (Romania), Tome III, pp. 47-52, 2010. N.S. Gameiro and A.J. Marques Cardoso, "Fault Tolerant Power Converter for Switched Reluctance Drives", in Proceedings of the 18th International Conference on Electrical Machines (ICEM '2008), Vilamoura (Portugal), on CD: Fullpaper_comm_id00840.pdf. K.-J. Lee, N.-J. Park, K.-H. Kim and D.-S. Hyun, "Simple Fault Detection and Tolerant Scheme in VSI-fed Switched Reluctance Motor," in Proceedings of the 37th IEEE Power Electronics Specialists Conference (PESC '06), Jeju (Korea), 2006, pp. 1-6. I., Husain and S.A. Hossain, "Modeling, Simulation, and control of switched reluctance motor drives," IEEE Transactions on Industrial Electronics, vol. 52, no. 6 (December 2005), pp. 1625-1634. L. Szab and J.B. Dobai, "Combined FEM and SIMULINK Model of a Modular Surface Motor," in Proceedings of the IEEE-TTTC International Conference on Automation, Quality and Testing, Robotics A&QT-R '2002 (THETA 13), Cluj (Romania), tome I., pp. 277-282. I.A. Viorel, L. Strete and I.F. Soran, "Analytical flux linkage model of switched reluctance motor", Revue Roumaine des Sciences Techniques - Serie lectrotechnique et nergtique, vol. 54, no. 2, pp. 139-146, 2009. B. Schinnerl and D. Gerling, "Analysis of Winding Failure of Switched Reluctance Motors," in Proceedings of the IEEE International Conference on Electrical Machines and Drives (IEMDC '2009), Miami (USA), 2009, pp. 880-885. A. Radun, "Design considerations for the switched reluctance motor", IEEE Transactions on Industrial Applications, vol. 31, no. 5, 1995, pp. 1079-1087. K. Hameyer and R. Belmans, Numerical Modelling and Design of Electrical Machines and Devices, Advances in Electrical and Electronic Engineering Series, vol. 1, Witt Press, Southampton (UK), 1999. M. Ruba, L. Szab, L. Strete and I.A. Viorel, "Study on Fault Tolerant Switched Reluctance Machines", in Proceedings of the 18th International Conference on Electrical Machines (ICEM '2008), Vilamoura (Portugal), 2008, on CD: Fullpaper_comm_id01200.pdf. S. Mir, M.S. Islam, T. Sebastian and I. Husain, "Fault-tolerant switched reluctance motor drive using adaptive fuzzy logic controller," in Proceedings of the IEEE International Electric Machines and Drives Conference (IEMD '03), Madison (WI, USA); vol. 2, pp. 835-841, 2003.

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VII.

BIOGRAPHIES

[10]

Mircea Ruba received the B.Sc. and M.S. degree from Technical University of Cluj (Romania) in electrical engineering in 2007, respectively in 2008. He is a full time Ph.D. student working in the field of fault tolerant switched reluctance machines. Ioana Benia received the B.Sc. degree from Technical University of Cluj (Romania) in electrical engineering in 2009. She is a full time Ph.D. student working in the field of modular variable reluctance machines. Lornd Szab (M '04) received the B.Sc. and Ph.D. degree from Technical University of Cluj (Romania) in electrical engineering in 1985, respectively in 1995. Currently, he is a Professor in the Department of Electrical Machines of the same university. His research interests are in the areas of variable reluctance machines, fault detection, etc. He published over 170 scientific papers and books in these fields.

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[12] [13]