Miyoshi Takahashi

Submicron domain observation and write-read characteristics of magneto-optical TbFeCo discs

Submicron or micron size magnetic domains on magneto-optical TbFeCo 5 inch discs recorded at high speed disc rotation rates (1800-3000 rpm) were obserbed to investigate the relation between write-read characteristics and magnetic properties of films. The domain size was calculated on the basis of the temperature dependence of coersive force Hc and saturation magnetization Ms. Carrier to noise ratio (C/N) characteristics were explained through the domain size.

Analysis of saturated synchronous reactances of large turbine generator by considering cross-magnetizing reactances using finite elements

This paper describes a method to calculate saturated reactances of synchronous machines at various loads under steady-state operation by considering influence of cross-magnetizing phenomenon between d- and q-axis circuits. In the method, mutual reactances x/sub dq/ due to the cross-magnetizing phenomenon can be calculated on the basis of two-dimensional magnetic field analysis. The validity of analysis is shown by good agreement between measured and calculated field current values of a 592 MVA turbine generator at various loads. It is clear that quantitative accuracy requires the magnetic coupling between d- and q-axis circuits be considered. Subsequently, steady-state reactances at various loads are analyzed by considering mutual reactances x/sub dq/ due to cross-magnetizing phenomenon. The relation between the load angle /spl delta/ and mutual reactance x/sub dq/ is clarified, and the difference between equivalent synchronous reactances that are obtained by considering x/sub dq/ and conventional defined synchronous reactances that do not consider x/sub dq/ is shown numerically.

Dynamic harmonic field analysis of a cage type induction motor when magnetic slot wedges are applied

Applying magnetic slot wedges to stator slot openings is an effective means to reduce the slot ripple harmonic components in the magnetic field. To investigate the effect of the wedges in detail, this paper proposes a new harmonic field analysis method that considers the rotor movement dynamically and the harmonic components of the secondary current as induced current by a two-dimensional finite element method (2D-FEM) using a time stepping technique. The differences in harmonic characteristics due to the use of plural wedges (a nonmagnetic wedge and magnetic slot wedges in different dimensions) for the stator slot opening of a three-phase cage induction motor are evaluated using the method. Suitable wedge dimensions to reduce the slot ripple harmonics are determined. From the comparison of calculated and experimental results of secondary current, the proposed method is shown to be appropriate and useful for the quantitative estimation of harmonic fields for induction motors.

Calculation of leakage inductances of a salient-pole synchronous machine using finite elements

An analysis method for calculating three components of leakage inductances, i.e. the stator, rotor and winding distribution differential leakage inductances, which properly represent the amount of each leakage flux in a salient-pole synchronous machine is presented. The winding distribution differential leakage inductance has a positive value mainly arising from the difference between the armature and field winding distributions. The value of the turns ratio must be changed as the saturation condition changes in order to transform leakage inductances into the proper armature base values. Calculated leakage inductances are shown to validate the method. Further investigations using the presented analysis may generate useful knowledge for pole designs.

Observation and simulation of 0.3/spl mu/m lenght domain on a high speed rotating magneto-optical disk

The magnetooptical recording characteristics of a 0.2-0.3- mu m-length domain were studied using a magnetic field modulation method. The ultimate goal of the investigation, performed on an extremely high-density magnetooptical disk, was to realize high-speed rotating 5.25-in disks. Through optical observation, it was found that domains of 0.3- mu m length could be recorded separately under the condition of 15.1 m/s linear velocity and 25.2 MHz. However, the domain shape became more irregular with increasing recording power and linear velocity. This could be explained by calculated results showing that the domain wall temperature when the magnetic field is reversed becomes higher as the power or the velocity increases. It was observed that the domain shape of a quadri-layer structure disk with a reflective layer having a higher thermal conductivity, such as Al film, is more irregular. This is explained by the difference in temperature profiles obtained from simulation. >

Steady-state magnetic circuit analysis of salient-pole synchronous machines considering cross-magnetization

A simple magnetic circuit of salient-pole synchronous machines is presented. A method for calculating steady-state electrical performances using the circuit is described. Magnetic saturation including the cross-magnetizing phenomenon is considered in the method. The saturated reactances, field currents, and load angles are calculated easily from the method without much computational effort as compared with the finite element method. Despite the simplicity of the magnetic circuit, the calculated reactances at various load conditions using the method are close to the measured and FEM values.

Relaxation of Trapped High Magnetic Field in 100 m-long Class

This paper reports on the relaxation of trapped magnetic field in a MgB2 solenoid coil in PC mode operation. For PC mode operation, we fabricated a closed loop circuit with a MgB2 coil, a PCS and some superconducting joints. The MgB2 coil was fabricated employing a 100 m-long PIT processed MgB2 wire using a wind & react method. The critical current Ic of the coil reached 166 A and the coil was able to generate a maximum magnetic field of about 2.5 T at 4.2 K without an external field. The PCS was fabricated employing a long NbTi wire stabilized Cu-Ni alloy. The joints were fabricated between MgB2 and NbTi conductor, between NbTi (PCS) and NbTi conductor. The resistance of all fabricated joints was estimated to be less than 1.0times10-13 Omega. In PC mode operation with the closed loop circuit, a magnetic field of 1.46 T was trapped for about 50000 s (above 12 hr.) without any detectable decay. These results demonstrate the possible use of the MgB 2 superconductor for applications such as MRI superconducting magnet

rm MgB_2

A simple analysis method is proposed for output voltage waveforms and magnetic flux waveforms passing through the main stator parts of synchronous generators. The method considers the rotor movement in order to analyze the higher harmonic sequences and some techniques for simplifying the calculation using the symmetry of armature windings and stator slot arrangements. The effectiveness of the higher harmonics analysis method under saturated conditions, is verified by 50-MV A-class generator analysis. It is useful for quantitative evaluation of the voltage and magnetic flux waveforms and no-load saturation characteristics of synchronous generators. >

Solenoid Coil in Persistent Current Mode Operation

Since 1989 development of a practical superconducting generator has been carried out in Japan. The authors have developed a rotor having a superconducting field winding and a stator having an air gap winding composed of a double transposed copper coil. A series of tests were completed at the end of 1997. The output of 78.7 MW was recorded which is the highest value yet obtained world wide. Test results of the shop test in 1996 and the site test in 1997 and analysis are described.

Higher harmonics calculation of synchronous generators on the basis of magnetic field analysis considering rotor movement

It is important to understand the relationship between iron core configurations and leakage flux distributions to get a better design of synchronous machines. This paper presents a method for calculating the steadyand transient-state leakage flux distributions in salient-pole synchronous machines. The method provides information on leakage flux distributions that cannot be obtained by terminal quantities. The method analyzes the leakage inductances that properly represent the corresponding leakage fluxes. They are calculated using the gap flux distributions by finite elements with magnetic saturation. The calculated leakage inductances of a synchronous machine under various loads are also shown and the relationship between the inductances and magnetic saturation is discussed.