نشریه الکترومغناطیس کاربردی
سال دوم شماره 4 (پیاپی 5، زمستان 1393)

In this paper a method for simulation of the magnetic signature of vessels is presented. This method includes using a special magnet array - called Halbach magnet array - for producing a uniform magnetic field as a source that models Earth’s magnetic field. Simplicity of this array beside its proper uniformity makes it possible to perform simulation with acceptable precision. For validating the proposed method and determination of its error, two simple models of spherical shell and prolate spheroidal shell that can be analytically handled are used and a comparison has been done between simulation results of these models and their analytic signatures. Good adaptability between these results confirms the accuracy of the proposed method and determines its precision level. After being ensured that this method works well, two real ship models with the dimensions of 20 m and 130 m are used to perform realistic simulation and their magnetic signatures are presented in 10 m depth.

Today, the permanent magnet motors are widely used in various industries especially military applications due to such characteristics as high efficiency, low weight and high energy density. Torque ripple is an inherent characteristic of this type of motors. Time and space harmonics produced by the motor drive and the motor structure itself give rise to torque ripples in these motors. Geometric structure of the magnets and stator slots lead to cogging torque (one of the causes of torque ripple) in these motors. This unwanted torque component causes noise, vibration and motor starting issues. It also creates bubbles in the application of these motors as the submarine's propulsion. It should be noted that bubbles could lead to submarine detection which is undesirable in military application. This paper provides analytical formulations and validation by threedimensional finite element analysis to reduce this unwanted torque. This is an important step in achieving the goals of the country's marine industries organization.

Regarding the necessity of low noise and low vibration operation of the BLDC motors in some especial applications, a novel method is proposed in this paper in order to shift the natural frequencies of a BLDC motor’s stator to higher values. Therefore the noise and vibration of the structural resonances can be reduced since the electromagnetic excitation forces in the BLDC motor have lower magnitudes at higher frequency components. The proposed method is based on placing a single hole, with definite diameter and location, on definite regions of the stator cross sectional area (each region contains one stator tooth and its upper parts in the stator’s yoke). The frequency analyses have been done in the SolidWorks software environment and the best diameter and location of each hole is extracted by the Response Surface Methodology (RSM).Preventing the magnetic saturation of the stator structure and the motor cogging torque enhancement prevention are considered as constraints for the problem. The new method also can be used for other types of electrical machines’ stator.

In this paper, the segmentation method is used to reduce cogging torque in the surface-mounted permanent magnet machines. In this method the magnet pole is divided into several magnet blocks. The permanent magnet is segmented in two ways, equal- and unequal- size permanent magnet blocks. In the both methods the half-wave symmetry of the magnetic poles is applied. The dimension of the permanent magnet s is optimized to reduce the machine cogging torque using an analytical model combined with the genetic algorithm. The effect of the slotted armature is taken into account in the analytical model. The cogging torque is obtained from the air gap magnetic flux components and the Maxwell’s stress tensor. The model is obtained by solving the Poisson’s equation. This model is used as a fast tool to compute the objective function in the genetic algorithm. In addition, the validity of the proposed model is verified with finite element analysis.

In this study, effect of Carbon Nanotubes (CNT) ferrite samples of Cu0.2Ni0.4Zn0.4Fe2O4, Mg0.95 Zn0.05TiO3 and Ca0.6La0.8TiO3 examined for the reflection loss investigation. In first step the mentioned ferrites were prepared by solid state reaction and thermal shock techniques. Then, samples with a mixture weight ratios of ferrite and carbon nanotubes of 98 to 2 percent were placed in an ultrasonic bath. In order to identify the phases of the synthesized samples, X-ray diffraction (XRD) was used. Ferrite grain sizes were characterized by field emission scanning electron microscopy (FESEM). To investigate the microwave electromagnetic region properties, the prepared nanocomposites were mixed with xylene solution and the reflection losses curves were plotted. The maximum reflection losses with appropriate broadband was obtained for Cu0.2Ni0.4Zn0.4Fe2O4 sample. Also, field emission scanning electron microscopy images reveals that the synthesized ferrite materials have nanoscale grain sizes.

In this paper, a
 /2-phase shifted distributed feedback Raman fiber (DFB-RF) laser above threshold condition is analyzed theoretically. The cross phase modulation (XPM) and self phase modulation (SPM) nonlinear effects are considered in the presented model. Numerical results show that the XPM and SPM increase the output power of DFBRaman fiber laser. Variation of wavelength of structure for different values of pump power has been evaluated in the presence and absence of nonlinear effects. It is found that in the absence of nonlinear effects wavelength of laser is constant. However, in the presence of nonlinear effects a red shift about 5×10-2nm with increasing pump power is observed. Simulation is performed by using transfer matrix method to solve three coupled nonlinear wave equations which describe the propagation of pump, forward and backward Stokes waves. The transfer matrix used for DFB Raman fiber laser, is introduced for the first time in this paper.