THE THEORETICAL AND EXPERIMENTAL STUDY OF CLAW POLE ALTERNATORS

Transcription

1 FACULTY OF ELECTRICAL ENGINEERING Cristian Petru BARZ THE THEORETICAL AND EXPERIMENTAL STUDY OF CLAW POLE ALTERNATORS -PHD THESIS- (abstract) Scientific advisor, Prof.dr. Vasile IANCU 2010

2 The INTRODUCTION presents the general context in which this thesis is elaborated in, and then it presents briefly the chapters of this work. A new approach of the claw pole machine will be presented here. Each chapter of the thesis deals with a stage in the process of building and testing the machine used in the power conversion system, each one being closed with conclusions. Chapter 1 is a general presentation of the claw pole alternator s constructive particularities, of their advantages and disadvantages in producing electrical energy. Various possible topologies are presented in order to build such an alternator (Figure 1.4) and their functioning principle is presented. Figure 1.4 Representation of the alternator s rotor The present stage and the innovations brought to the claw pole alternator are also presented, and various analytical (Figure 1.22) or numerical (Figure 1.29) models used for the calculation and dimensioning of the claw pole alternator are also shown. Figure 1.22 Network of magnetic reluctances Figure 1.29 The model of the 7 phase alternator Chapter 2 presents the dimensioning algorithm developed for such a generator. The two major aspects of this procedure are pursued: establishing the geometric dimensions of the rotor core, of the stator, the winding excitation as well as choosing the type of stator winding. Because this type of machine normally has the rotor formed of two flanges on which rotor poles are found and between them the excitation winding is found, determining the configuration of this type of pole is hard to realize and choosing some dimensions based on previous experiences presented in literature is necessary. After establishing the main formulas for the determination of the claw pole synchronous generator s geometrical dimensions, the next step can be the verification of the data obtained in the first two stages with the help of numerical analyze, using a programme for the numerical analyze of the electromagnetic field. Generally, the mutual magnetic flux and the mutual inductance are proportional with the ratio between the effective surface and the effective length of the air-gap, considering the poles geometry and the excitation winding s position. Pag. 1

3 The Magnet 6.11 programme was used for the numeric modelling, aiming at verifying the results obtained in the dimensioning calculation and also at visualizing the induction maps on different components of the generator, pursuing the presentation of the magnetic induction distribution in the air-gap and in the stator teeth. Starting from the studied model of claw pole alternator and verifying in the same time its geometric dimensions based on the design, the three-dimensional model of the claw pole alternator was realized. An interface for the introduction of the initial designing data (Figure 2.7) was realized with the help of VB Script programming language, for the three-dimensional construction of the claw pole alternator s calculation model. In the same time, with the help of the programme, the link to the soft of threedimensional analyze of the magnetic field with Magnet 6.11 finite elements is realized. Figure 2.7 Inputs date Figure 2.10 The 3D model of the alternator s rotor Because of the complex and un-symmetrical form from the median plan of the machine (Figure 2.10), we cannot perform 2D analysis of the alternator, the only analyses of the electromagnetic field that can performed are the three-dimensional ones. Figure 2.11 Models of the claw poles Due to the importance of the teeth shape, as well as to the various influences of the teeth shape on the magnetic field, the possibility of modifying the size of the rotor teeth was required within the programme, as well the base part of the claws as their top part (Figure 2.11), according to the wanted configuration. The author realized a programme to ease as much as possible the work of building the geometric model of the claw alternator, its construction being laborious due to the constructive particularities of the claw pole alternator. By interpreting the results obtained from the field analyze (Figure 2.20), the possibility of automatically modifying the entrance data in order to obtain a different constructive model was offered. This model will present the new constructive data, as a result of pursuing the optimization of the claw pole alternator s shape and functioning characteristics, being reanalyzed until the requirements imposed by the analyst are accomplished. Pag. 2

4 Figure 2.20 The magnetic induction in the alternator, as a result of the analyze The author tried to gain a relatively precious time with the help of the programme for automatic construction of the alternator s numeric model, without being necessary to reconstruct the entire model for the smallest modification of one of its geometric dimensions. Chapter 3 presents the methods used to calculate the electromagnetic field from the numeric analyse, these standing at the base of the software which allows the numeric analyse of the claw pole alternator. The analysing of an electromagnetic problem is at present a problem of solving Maxwell s equations system, respecting the given boundary conditions. These equations of Maxwell represent the very concise and complete expression, with a single set of equations, of the laws governing the electromagnetic field for immobile and no discontinuity environments. Starting from the experimental determinations of the classic claw pole alternator, the numeric model realized in the previous chapter is being analyzed. The influences of the alternator s geometry are verified, as well as the various materials that enter in its composition, the data obtained for various functioning conditions imposed and verified from experimental data, will be used as reference data for ulterior comparisons of the prototypes studied in the thesis. Due to the fact that the numeric analyze with finite elements (Figure 3.7) is based on the energy stored by the complete construction of the alternator s model, the no load running inductance of the model is calculated according to this, the influence of the air volume surrounding the model being important (Figure 3.5). Figure 3.5 Air volume Figure 3.7 The distribution of magnetic inductance values for the alternator s stator Pag. 3

5 The three-dimensional magneto-static analyze was done in order to observe the behaviour of the alternator studied analytically and measured experimentally, in the analyzing environment. Figure 3.12 presents the distributions of the magnetic induction s amplitude on the surface of the stator and rotor for various excitation currents (1, 2, 3 and 3,5 A). I e = 3 A I e = 3 A Figure 3.12 Representation of the magnetic induction on the stator and rotor The behaviour of the alternator for the experimental values was followed, and we are going to pursue its reaction for various chosen configurations. The importance of the magnetic induction s values within the alternator model is given by the fact that the future models proposed in the thesis will be compared to the values of the classic construction, their analyse leading to the viability of the proposed models or to their elimination from the stage of the prototype s execution. In the analyses, certain routes were chosen along the axial length of the rotor, on which the magnetic induction s distribution on circumference was verified, for the ulterior comparison with other constructive variants. Figure 3.30 shows a graphic representation of the magnetic induction in the claw pole along its axis, comparing the values obtained for the same electrical data, but for different pole tops. Figure 3.30 The representation of the magnetic induction according to claws dimensions Chapter 4 puts into practice the methods of determining the synchronous machine s parameters from the no load running test in permanent regime, by analysing the excitation current and the tension of the no load running phase, the test of symmetrical three-phased permanent short-circuit, as well as the load characteristic, according to the excitation current s variation. Their representation can be seen in Figure 4.4, where the various forms of wave for rpm speeds can be observed. Pag. 4

6 Figure 4.4 The no load running characteristic of the claw pole alternator Figure 4.16 The distribution of Aluminium between the claw poles of the alternator The performed numerical analyse aims at three constructive variants of the rotor subset, pursuing the decrease of saturation in claw poles, in order to decrease losses and increase the alternator s efficiency. The first analyzed variant implies a distribution of the aluminium between the rotor claws, replacing the air in the air-gap with aluminium, but only on the portion between the claws tops, as shown in Figure The comparison of the values obtained for the magnetic field in the classic case and the proposed case (Figure 4.23) shows an increase of the field s (magnetic induction) saturation in the claw pole, but it does not overcome the saturation value of the material, which leads to an improving of the alternator s efficiency. Figure 4.23 The magnetic induction in the alternator s rotor Chapter 4 includes the experimental data of the alternator prototype measurements realized in practice, these containing Aluminium and Copper rings, introduced in the construction of the rotor (Figure 4.24). This chapter analyses the models realized in practice, as well as the models that are harder to realize constructively, but, based on simulations and numerical analysis of the field, they are compared to the classic model. Pag. 5

7 This construction is easier to be realized constructively, thus it was realized in practice and measurements were carried out on the testing stand, the experimental data being compared with that of the classic variant (Figure 4.28). Two cases of rings were analysed, one with rings made of Aluminium (Figure 4.29) and the other with rings made of Copper (Figure 4.35). The influence introduced by materials in the magnetic field of the alternator was followed and also the influence on the magnetic saturation in the claw poles. Figure 4.24 The model with channels of the alternator s rotor Figure 4.28 The comparison of the magnetic induction for constructions with Al rings and Al between claws On the base of the results obtained in the dimensioning stage, as well as of those obtained as a result of the numerical analyse, the prototype of a claw pole alternator was realised in practice, having a single stator but with two different configurations of the rotor, the two configurations having rings of Aluminium and Copper. Figure 4.29 Rotor with Aluminium rings Figure 4.35 Rotor with Copper rings Figure 4.30 The experimental stand of the claw pole alternator The experimental tests of the alternator (Figure 4.30), (no load running, three-phased symmetrical short-circuit) pursued the data measured experimentally for the classical case of the alternator. Figure 4.36 presents the characteristics measured for the three constructive variants, which are the classic variant, the one with Aluminium rings and the one with Copper rings. The major influence in the case of performances is represented by the cutting of channels for rings, in the claw poles. Pag. 6

8 Figure 4.36 The comparative representation of the data for the no load running tension The third variant proposed for analyse consists in a construction of rings in the peripheral part of the rotor in order to decrease the saturation of poles, it is only realised in the free space between the base of the claws (figure 4.40.a.), the placing of rings is done the same way as in the case of ring construction (the Al and Cu cases). Figure 4.40 The constructive variants with Al and Cu segments placed on the rotor crown The construction couldn t be realized in practice due to the problems raised by the fixation of Aluminium and Copper segments on the claw poles and the implications of the centrifugal force, due to the high speeds at which the alternator runs. The influence introduced by materials into the magnetic field of the alternator was pursued. Figure 4.43 The representation of the magnetic induction along the pole for the three variants As a result of these numerical analyses, a comparison is made to show if the prototypes data bring improvements to the classical case, presenting the main advantages and disadvantages of the proposed prototypes. The distribution of the magnetic induction is obtained in all the analyzed situations, in the air-gap, in the rotor teeth, in the stator s structure which is compared to those of the classical alternator, for the same main components of the alternator and for the conditions imposed in chapter 3. Pag. 7

9 As it can be observed (Figure 4.43), the values obtained on the same contours and electrical conditions imposed to the classical variant, show us the fact that the values in the case of Aluminium rings in the rotor are clearly inferior to those in the other two variants. An increase of the saturation can be observed along the claw pole axis in the first case, with aluminium between the claws, compared to the case of standard alternator. But in the second case studied, it is important to decrease saturation with up to 15%, when the aluminium is found as segments on the rotor crown. An important decrease of the magnetic saturation is observed in the area of the aluminium, but the decrease of saturation is also significantly present (up to 5%) towards the top of the claw pole, where the saturation is initially lower. Figure 4.49 The magnetic induction along the claw pole axis From the experimental data, for the variant with the Aluminium rings compared to the classical one (Figure 4.49), we can observe that the variant with the increase of magnetic induction in the claw poles of the alternator is recommended, but without overcoming the saturation value of the materials they are made of. Regarding the author s scientific contribution in the approached field, it is seen that the work he has done mainly aimed three aspects: a dimensioning of the classical variant of claw pole alternator; a numerical field modelling of the claw pole alternator with the MagNet 6.11 soft, which constitutes the base for future analyses of the models proposed within the thesis; the third aspect is the part of constructing the alternator s prototypes variants with the two rings variants introduced in the rotor s configuration, carrying out experimental measurements and comparing the numeric models for various proposed variants. Presenting the influence of the aluminium and copper spacing in the air-gap between the alternator claws, at the rotor s periphery, on the specific functioning characteristics of the studied machine. The deviations of the values calculated with the help of numerical modelling compared to the values determined experimentally, can be explained by the geometric simplifications of the numerical model compared to the real geometry and by the refinement of the finite element network used in analysing the model, but also due to the differences between the material characteristics used in the numerical calculation and the real material characteristics. The numerical analyse that has been done aims two constructive variants of the rotor subset, pursuing the decrease of saturation in claw poles in order to minimize losses and to increase the alternator s efficiency. I consider that it is necessary to also do research in variable electromagnetic field where the introduction of diamagnetic rings reduces the eddy currents, which implicitly reduces the temperature during functioning. Pag. 8