Design optimization of permanent-magnet axial-flux eddy current brakes

In this study, electromagnet coils, which are used a source of magnetic field, are removed from conventional magnetic brake and permanent magnets replaced with these coil. Therefore, the design is simplified by eliminating coil and electrical source which feeds the coils. Then, total braking torque is calculated while changing main design parameters and the effect of these parameters over magnetic brake torque are examined. Those calculations are performed numerically in computer using software packages. Conventional magnetic brakes employ a set of electromagnet coils to generate magnetic field. The value of the applied current determines the value of magnetic field produced by these coils. As a result, the value of the applied coil current determines the total braking moment. In this study, the moment of permanent magnet brake is controlled by adjusting the airgap between the permanent magnets and the conductive disk. If the airgap is increased, the effect of magnetic field over conductive disk decreases. Therefore, total brake moment decreases, and vice versa. One of the problems which must be dealt with is the heat generation within the disk while the magnetic brake is in braking action. The mechanical energy is converted into heat energy through the medium of eddy currents. This heat energy must be transferred to the surrounding atmosphere directly or by means of forced cooling system in order not to overheat components of magnetic brake. The problem can be solved with radial air channels which can be placed on conductive disk because of fact that these radial channels provide air circulation from center to edge of disk. Thus, the circulating air transports heat from disk to surrounding atmosphere. The design issues regarding the cooling system are not, however, studied in this thesis. One of the most important parameters which affect the moment capacity of a magnetic brake is the construction of conductive disk. The radius of the conductive disk, the effective area in which eddy-currents occur, the thickness of the disk are the constructional parameters which have direct effect on moment capacity. Nonetheless, choosing excessively large radius and thickness values in order to increase the moment capacity cause geometric integration problems. Furthermore, heavy disk have an impact on vehicle dynamic owing to the fact that conductive disk is directly connected to shaft. In the light of this circumstance, disk is designed based on commercial available brake systems. The material properties of conductive disk are important factors which affect design and the torque characteristic of permanent magnet brake. For instance, the self-conductivity of the disk influences amount of eddy currents which occur inside of the disk. Thus, the torque capacity of permanent magnet brake is affected by self-conductivity. Furthermore, the thermal properties of conductive disk have an effect on disk design owing to the fact that thermal properties determine the amount of heat energy which transfers from disk to air. Additionally, strength of material is another conductive disk feature which must be considered in order to provide durable magnetic brake. Then, the density of the disk material is an important factor in terms of vehicle dynamic and fuel economy due to the fact that conductive disk is directly installed to prop shaft. Also, the material cost is a significant factor for project feasibility. In light of this information, the conductive disk material is selected from among coper, steel and aluminum (the materials that are commonly used in the industry) by making optimization with respect to material properties. The key parameter of permanent magnet brake is the magnetic properties of the magnets. Because the eddy current and magnetic torque are generated owing to magnetic field and relative angular velocity between magnets array and the disk. The angular velocity of prop shaft is related to vehicle speed. In addition, magnetic flux density occurs with respect to magnetic coercivity of magnet; therefore, choosing magnetic material for permanent magnet is so significant. Hence, NdFeB magnet is selected for this study on account of fact that NdFeB magnets are used commonly, magnetized easily and have high magnetic coercivity. Their disadvantage are, however, low curie temperature. Therefore, heat generation problem must be solved with cooling system. In order to generate eddy currents in a rotating conductive disk, the disk must be exposed to a time-changing magnetic field according to Faraday's Law. This time-changing magnetic field is obtained by a number of repeated N-S pole pairs, but different arrays such as Halbach array can also be used alternatively. Since it is assumed that the number of magnetic poles influences total magnetic brake moment, therefore, the effect of the pole number on the moment is investigated. Referencing to commercially available heavy vehicle brake system with electromagnet coils, first 8-pole permanent magnet brake is designed and then analyzed for different operating speed values. The pole number is doubled and the analyses with 16 poles are performed with different speed values to draw the moment-speed characteristic curve. In addition, analyses of another design with 24 magnets are performed for different speed values to understand comprehensively the effect of number of poles. The magnetic brake torque is controlled by adjusting airgap between the permanent magnets and conductive disk. Hence, numerical torque calculations are analyzed with different air gap lengths. 1 mm air gap is defined for first calculation with different rpm values. Then, the analyses are performed with 3 mm airgap and 5 mm airgap at 600, 900, 1200 revolutions per minute for designs with 8, 16 and 24 poles. The results of these analyses are compared according to airgap lengths. Finally, the effect of air gap between magnets and conductive disk is monitored. Another way is to place an iron in the rear of magnets in order to increase magnetic field intensity inside the conductive disk. In other words, total magnetic resistance is reduced while adding a component whose magnetic permeability is extremely high compared with the magnetic permeability of air. Additionally, the magnetic brake moments are calculated for the designs having backiron at 600 rpm and the effect of backiron is evaluated by comparing the designs which do not have a backiron. One of the parameters which affect the magnetic brake torque is the thickness of permanent magnet. Provided that the thickness of permanent magnet is increased, the magnetic flux density which is produced by permanent magnet rises inside the conductive disk. Nonetheless, this magnetic flux density increase is not linear proportional to magnet thickness increase. The enhancement of magnetic flux density decreases with increasing permanent magnet thickness. Furthermore, the magnetic brake moments for 8-pole design are analyzed while changing the thickness of magnets at 900 rpm and the curve of magnetic brake torque is plotted with to respect to magnet's thickness. Four main factors which are affecting the magnetic brake moment capacity are mentioned in this study. These are airgap between permanent magnets and conductive disk, the presence of backiron whose magnetic permeability is high, the number of magnets, and the permanent magnet thickness. These factors are evaluated and discussed in order to understand their influence on magnetic torque characteristic. For example, backiron increases dramatically the torque capacity of magnetic brake for 8-poles design whereas backiron is not able to provide the same expectation about torque increase for 24-pole design. In additional, the increase of airgap diminishes extremely brake torque of the design with 24 poles. However, this torque reduction is low 8-pole design compare to 24-poledesign. The brake torque capacity enhances with enlargement of magnet thickness, however, the increase of thickness is not feasible after a peak value. The number of magnets generally increases torque capacity of magnetic brake, but torque capacity decreases after a pole number because of eddy current skin effect.