Control and performance analysis of permanent magnet switched reluctance machine

Switched Reluctance Machines (SRMs) comes to the fore with its simplicity and robustness. Their manufacturing cost are relatively lower than their counterparts, induction and synchronous machines. SRMs can show high performance within a wide speed range and thus very suitable for variable speed applications. Owing to absence of winding on rotor, their efficiencies can be higher than Induction and Synchronous Machines with same power range. The other advantage of winding-less rotor is that rotor inertia is low. Thus, the SRM responses to instant speed change very fast and can be run at maximum speed levels that its counterparts never reach. Like synchronous machines, SRMs runs at synchronous speed. The continuous rotation can be achieved by synchronizing excitation current pulses with rotor position. Although their basic characteristics are quite similar to the separately excited DC machines, they do not possesses a brush-commutator assembly. SRMs are highly nonlinear machines due to salient structures of both stator and rotor. Therefore, obtaining their analytical model for all operation range is extremely hard. Today, their design and analysis are carried out by using finite element based computer aided tools. A large amount of researches have been done on the design and control strategies of SRMs. However, most of them could not find a chance to be commercialized. Although an SRM itself has lower cost than its counterparts, a power electronic drive circuit usually required to be run. The problem is that possible drive circuit topologies are not serially produced as package. Therefore, the user should individually construct the required drive circuit, which inevitably increases the drive circuit cost. As the researches on SRMs increase, some possible methods may be developed for reducing the overall machine cost so that the SRMs can be more preferable, especially for variable speed applications. Recently, most of the researches on SRMs focus on improving the performance of the SRM in the control stage rather than improvement in the machine design stage. Therefore, investigations on SRM design are still limited and there is a need for filling this gap. The motivation of this thesis relies on the idea that may allow to increase power capability of SRMs without serious increase in manufacturing cost. The increase in power as well as in torque can be obtained by inserting permanent magnets on stator of a classical SRM. Placing permanent magnets on stator does not affect rotor inertia. Thus, constructed permanent magnet machine still shows the feature of low inertia. The other advantages of using permanent magnets are to improve speed response and reduce torque ripples. Additionally, these magnets can significantly improve the developed torque in the machine during a possible failure in a phase and let the machine continuously rotate and thus increase reliability of the SRM.Although various type permanent magnets are available in the market, NdFeB magnets, which are rare-earth type magnets, are predominantly used in the design of permanent magnet machines. This is because NdFeB magnets have high residual flux as well as high resistance to demagnetization. However, NdFeB magnets are very expensive and thus inevitable increase the design cost of permanent magnet machines. Therefore, alternative magnets should be investigated for SRM application. Analysis in this research demonstrates that a permanent magnet switched reluctance machine (PMSRM) can possess the mentioned advantages by using cheap permanent magnets rather than using expensive magnets such NdFeB magnets. Among the alternative magnets, Alnico magnets, which are non-rare earth type magnets, have potential to be used for performance increase since their residual flux is as high as those of NdFeB magnets and their price are much lower than NdFeB magnets. The main drawback of Alnico magnets is that they can be easily demagnetized with a reverse magnetic field. Reverse magnetic field in an electric machine often occurs when there are bidirectional current flows through windings. However, SRMs are often controlled in a way that phase windings carry unidirectional current. An Asymmetric Half Bridge (AHB) converter, which is accepted to be classical converter SRMs, is suitable for PMSRM as well. It provides natural protection for Alnico magnets while allowing an independent phase control. In order to investigate static characteristics of PMSRM, a 6/4 version has been designed and analyzed in FEMM. Current and rotor position has been taken as reference parameters to obtain flux and torque tables. These data are then employed to form a model to be used for further analysis of designed PMSRM in Matlab/Simulink. A control unit, drive circuit as well as machine model has been constructed for 6/4 PMSRM in order to investigate its dynamic behavior at various operation conditions. The PMSRM has been controlled through a AHB converter whose gate signals are generated by considering current and speed control. A current control loop is built on the basis of hysteresis current control algorithm. On the other hand, a PWM algorithm has been used to achieve speed control. Both classical SRM and PMSRM are simulated with the same conditions to verify the effectiveness of using permanent magnets. The simulation results demonstrate that PMSRM has superiorities on the classical SRM. Besides a power increase is observed, the torques ripples due to nature of the machine has been remarkably reduced. Another advantage of using permanent magnets has been observed at transient speed response, which means PMSRM has better acceleration and deceleration behavior. The works in this thesis has been organized as 6 sections: In the first section, history of SRM has been firstly summarized and then previous researches on SRMs has been briefly mentioned and criticized. Some important industrial applications of SRMs has been given. The current researches and design trends on SRMs have been discussed. The need of designing a new type SRM machine is well described and motivation of this thesis has been highlighted. The second section has been reserved for theoretical explanation of SRMs. The general structure as well as operating principle of SRMs have been introduced and related equivalent circuit and mathematical expressions have been given. The possible converter topologies that are required to operate SRMs have been also mention in this section. Third section gives the design and analysis stages of a 6/4 SRM and PMSRM via finite element based software called FEMM. The technical and magnetic specifications of both machines have been also given in this section. The conditions of static magnetic analysis have been well described and methods of obtaining flux and torque tables have been explained. Additionally, the magnetic behavior of both machines have been examined and comparatively given to make further discussion. In the fourth section, overall Matlab/Simulink model of SRM/PMSRM simulation has been given. Particularly, constructing a dynamic model of PMSRM together with AHB converter and applied control algorithms have been introduced. Each part of modeling employed in Matlab/Simulink has been thoroughly illustrated and discussed. The way of obtaining proper gate signals for converter switches have been well described. In the fifth section, simulation results for various conditions have been discussed in detail. The effects of using permanent magnets in the machine has been thoroughly investigated. The reasons of improved performance in the case of PMSRM have been well addressed. Additionally, the fault tolerant analysis results of both machines have been given and they have been comparatively discussed in detail. The last section has been reserved for conclusion. Superiorities and design constraints of PMSRM have been discussed. Comments on possible improvements on the results and future work on this thesis has been clearly stated.