Diagnosis Of Advanced Fault Tolerant Switched Reluctance Machines Used In Safety Automated Industrial Systems

DIAGNOSIS OF ADVANCED FAULT TOLERANT SWITCHED RELUCTANCE MACHINES USED IN SAFETY AUTOMATED INDUSTRIAL SYSTEMS Mircea RUBA1, Loránd SZABÓ1, Viktor FÜVESI2, Ernő KOVÁCS2 1

Department of Electrical Machines, Technical University of Cluj RO-400750 Cluj, P.O. Box 358, Romania; e-mail: [email protected] 2 Department of Electrical and Electronic Engineering, University of Miskolc H-3515 Miskolc-Egyetemváros, Hungary; e-mail: [email protected] ABSTRACT Industrial systems are in a process of continuous expansion in means of increased safety and fault free operation. More and more high tech automated equipments are included in systems to supervise, control and offer best quality of the final product of the process. In addition sensors and traducers are placed along the process line, coupled with fault tolerant components. These are usually electromechanical actuators with built in motion controlled electrical machines. The paper’s aim is to present the development, testing and analysis of a fault tolerant switched reluctance machine, which can be used in automated systems that demand high operating skills even in case of electrical defects. Different fault scenarios are studied to prove that the proposed machine will meet the system’s demand. To be able to set up a good simulation environment, two software were coupled, MATLAB/Simulink and Cedrat's Flux2D, allowing to perform both analytical and FEM computations. 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 word system can have multiple meanings, regarding the environment that is included in. Usually, a system can be considered as a network that couples different hardware or software components, all achieving the same goal, the good operation of the process. As the number of components rise, the possibility of fault occurrence is higher. Usually, a system is divided in stages, each with an appropriate responsibility in the global process. Fault occurrence can be seen similar to a computer virus, which can spread from a component to other, from the same or from different stages. Thus, in general, the result of the fault tree synthesis process is a network of interconnected fault trees which record logical relationships between component and system failures. The top events of these fault trees represent system failures. Leaf nodes represent component failure modes while the body of intermediate events (and

intervening logic) records the propagation of failure in the system and the progressive transformation of component malfunctions to system failures. The malfunction of the actuator will provide a total control loss of the process and full system shut down will be required. If the actuator is built in the manner of being able to operate in faulty conditions, it will be a fault tolerant one. As financial issues must be considered, especially in building series of industrial equipments, low cost and high efficiency actuators are needed. In this paper such a fault tolerant switched reluctance machine (SRM) will be presented. The electrical machine is studied by means of simulation. The model was built using specific software packages. One, Flux 2D, was used for modelling the machine via finite element method (FEM). The second, MATLAB-Simulink environment was applied for modelling the inverter's control system and to generate the faults for the studied cases. These two programs were coupled and worked together using the Flux-to-Simulink Technology. That rate it was possible to study in details all the typical faulty states of the motor [3]. THE PROPOSED SRM STRUCTURE Achieving a fault tolerant variant of a usual electrical machine requires modified topologies. While developing efficient fault tolerant electrical machines it is important to take into account also its losses. The main idea was to shorten the flux paths in the motor, hence the shorter flux paths means lower iron losses [3] (Fig. 1). Upon this criterion a 12/14 structure was designed.

Fig. 1. Proposed fault tolerant SRM The machine has six phases, and each of them is divided into two separate fed and commanded channels. Hence the power converter of the machine has 12 H-bridge branches, as it is shown in Fig. 2. This way, in case of an open or short circuit of a channel, the converter isolates by its command the faulty zone and operates the rest of channels to cover the loss provided by the defect. As each phase is divided into two channels, if one channel is shut down due to a fault, the second one will still operate and contribute to the torque generation. Hence, continuous operation despite faults is achieved by the joint of special SRM structure and the complex converter.

Fig. 2. The proposed power converter POSSIBLE FAULTS OF THE SRM AND THEIR DETECTION The block diagram of a SRM’s control system, with fault detector and current correction feedback is given in Fig. 3.

Fig. 3. Block diagram of a SRM drive system Several components may become faulty: the converter, the machine or the control circuits [4]. The paper is dealing mainly with the failures that can occur in the electrical machine. Early studies already gathered information about the faults that can appear in a SRM [5], [6]. The faults taken into account are: turn to turn, or phase to phase short circuits, etc. Other issues are the open circuits due to mechanical damage of the wiring or corrosion of the junctions. Detection of fault is an extremely important problem and also a critical one. The lack of the full channel current is simple to detect. The problem is the detection of small changes in the machine’s behaviour. Two detection methods are mainly used. Firstly an overcurrent detector can be applied, using the sensed current signal, setting a comparator having a threshold above the normal operating range of the phase currents (Fig. 4.a).

a) Current differential detector b) Flux differential detector Fig. 4. Fault detection methods

The second one, the flux differential detector (Fig. 4.b) uses search coils wrapped around the stator poles. When a fault occurs the imbalance in the pole fluxes induces a voltage in one search coil that is greater than the voltage in the other coil, producing a net voltage that can be detected with a bidirectional comparator [7]. THE COUPLED SIMULATION PROGRAM As already mentioned, the best solution to provide correct and considerable results to emphasize the behaviour of the machine both in normal and faulty operation is to use coupled software simulation (Fig. 5).

Fig. 5. The Simulink application for the command stage Matlab/Simulink offers the possibility to embed the Flux2D FEM simulation program for synchronous computation of the electronic converter linked to the FEM model of the SRM. Each phase of the converter is driven by a module that computes the turn ON/OFF angles for the power switches, regarding the rotor position, speed, and imposed current value. These signals are sent to Flux2D via the Flux-To-Simulink Coupling Module. One time step is computed and feedback signals (the rotor position, currents and speed) are sent back through the same module to Simulink in order to compute the next set of commands for the power switches. RESULTS OF THE SIMULATION To study the machine’s operation in faulty regime, different scenarios had to be set up. All these are compared with the normal operating mode (Fig 6a), considered reference. The studied faults are: an open channel (Fig 6b), one open phase (Fig 6c), double channel opened (Fig 6d) and a phase and a channel opened (Fig 6e). The reference torque is considered to be around 20 Nm.

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Fig. 7. Results of simulations. Currents and torque vs. time at different healthy and faulty regimes By analytical calculations, increased current values can be set to establish in all cases the mean torque equal with the reference value. The ripples are naturally increased because the lack of current in the damaged channel/phase. Although a single channel is opened, the most probably faulty case, practically the SRM will develop at unchanged currents near the same mean torque as in the case of the healthy machine. The single effect of the fault is in the increase of the torque ripples, but by less than 4 Nm. If two channels of different phases are opened the currents must be increased by 10% to achieve the rated mean torque. If an entire phase is opened the current has to be increased by over 30% to achieve the rated

mean torque, and the torque ripples in this case are greater by 2.5 times. The proposed fault tolerant SRM is able to develop near its rated mean torque also in the worst case in study. In this case the phase current must be increased by 1.5 times. The torque ripples are not much greater as in the case of a single open phase. CONCLUSIONS It was proved by simulations that the proposed fault tolerant SRM and its converter both offer a high level of tolerance to defects. This property added to its low manufacturing costs provides a considerable reason to be used as industrial actuator where safety is critical. Future work will include the increase of both stator and rotor poles, of the number of phases and channels in order to raise the tolerance level of the drive system. ACKNOWLEDGEMENTS The work was possible due to the support given by the Romanian Ministry of Education and Research, National Authority for Scientific Research (ANCŞ) and the Hungarian National Office for Research and Technology (NKTH) in the framework of the "Romanian-Hungarian Intergovernmental S & T Cooperation Programme for 2008-2009". The authors should like to sincerely thank this way for the financial support. REFERENCES [1] Blanke, M.: Diagnosis and Fault-Tolerant Control. Springer Verlag, 2006. [2] Papadopoulos, Y. Parker, D. Grante, C.: Automating the Failure Modes and Effects Analysis of Safety Critical Systems. Proceedings of the Eighth IEEE International Symposium on High Assurance Systems Engineering (HASE '04), pp. 310-311. [3] Ruba, M. Szabó, L. Strete, L. Viorel, I.A.: Study on Fault Tolerant Switched Reluctance Machines. Proceedings of the 18th International Conference on Electrical Machines (ICEM '2008), Vilamoura (Portugalia), on CD: Fullpaper_comm_id01200.pdf. [4] Husain, I. Anwa, M.N.: Fault analysis of switched reluctance motor drives. Proceedings of the International Conference on Electric Machines and Drives (IEMD '99), pp. 41-43. [5] Suresh, G. Omekanda, A.: Classification and remediation of electrical faults in the switched reluctance machine. IEEE Transactions on Industry Applications, vol. 42, no. 2, pp. 479-486, 2006. [6] Fodorean, D. Ruba, M. Szabó, L. Miraoui A.: Comparison of the Main Types of Fault-Tolerant Electrical Drives used in Automobile Applications. Proceedings of the 19th International Symposium on Power Electronics, Electrical Drives, Automation and Motion (SPEEDAM '2008), Ischia (Italy), 2008, pp. 895-900. [7] Szabó, L. Ruba, M.: On Fault Tolerance Increase of Switched Reluctance Machines, Proceedings of the 2009 EUROCON Conference, St. Petersburg (Russia), 2009. In print.