Noriaki Hino

A study on optimal coil configurations in a split-type superconducting MRI magnet

Optimal coil configurations in a split-type superconducting MRI magnet have been investigated, and the feasibility of the compact and high performance magnet is presented. A highly homogeneous magnetic field of about 10 ppm/50 cm diameter spherical volume (DSV) is attainable by eliminating error fields of higher orders and, moreover, the radius of the outermost coil can be reduced to about 70 percent of that of the magnet with positive current simply by alternately arranging coils with positive and negative currents.

Three-dimensional optimization of correction iron pieces for open high field MRI system

We have developed a new magnet for open high field (0.7 T) MRI systems. In order to homogenize magnetic fields in the field of view, an optimization method to determine initial configurations of large correction iron pieces was developed. In our method, we combined linear programming (LP) and 3D magnetic FEA and, using analytic formulae for LP, we computed field harmonics due to the volume of magnetized iron.

A direct search shape optimization based on complex expressions of 2-dimensional magnetic fields and forces

A 2-dimensional shape optimization method for magnetic materials and conductors such as those of induction machines has been developed. The method combines a steady state AC field analysis based on complex expressions of magnetic fields, a method to compute Lorentz force in complex form and the direct search method. As an example, shape optimization of a single sided linear induction motor is reported.

Shielding stray magnetic fields of open high field MRI magnets

We have developed a new magnet for open high field (0.7 T) MRI systems. Since MRI systems are installed in hospital sites, it is important to reduce stray magnetic fields of the magnets. We studied passive, active and hybrid shielding methods and compared them. In order to study feasibility of active shielding, we considered higher order multipole stray fields. We present details of the calculation method and the results.

A new concept of gas turbine system: Motor-assisted gas turbine with high-speed motor

This paper proposes a new gas turbine power generation system called the “motor-assisted gas turbine” (MAGT). The MAGT is composed of a dual-shaft gas turbine and an inverter-fed motor. The conventional dual-shaft gas turbine has two shafts that rotate at different speeds: a power turbine shaft and a compressor shaft. The power turbine shaft is connected to a synchronous generator as usual. In the MAGT, the new idea is that the compressor shaft is coupled to an inverter-fed motor. By assisting the compressor with this motor, the MAGT is able to increase the power output of the gas turbine when the intake air temperature is high. In addition, the MAGT can be used as an energy storage device like a flywheel. The inertia energy of the compressor can be charged and discharged by varying the rotation speed with the motor directly connected to the compressor. This paper also presents a method for designing motors that are suitable for MAGTs. The interior permanent-magnet synchronous motors were studied by using a joint optimization method of stress and electromagnetic analysis for megawatt-class high-speed applications.

A New Concept for Power Grid Stabilization Using a Motor-Assisted Variable Speed Gas Turbine System

As the penetration ratio of renewable energy sources becomes larger, the fluctuations of grid load also become larger and larger because of the intermittent generation of wind power and photovoltaic power. These fluctuations cause instability of voltage and frequency in the power grid.Recently, there has been considerable research into solving these challenges, leading to development such as batteries, flywheels, and improved flexibility of thermal power plants. The batteries and the flywheels are confronted with the challenge of high initial cost for the Mega-Watt class. Improving flexibility for the thermal power plants is effective, but this improvement has several limitations such as load-follow operation capability under mechanical constraints and frequency regulation within governor-free regulating capacity.To overcome these problems, we propose a new gas turbine system named Motor-assisted Gas Turbine (MAGT). MAGT is composed of a two-shaft gas turbine: one free turbine shaft is connected to a synchronous generator rotating at a constant speed, and the other compressor shaft is coupled to an inverter-fed motor controlled at variable speed. The motor and inverter capacity is appropriate: about 5–10 % that of the gas turbine. MAGT improved the reaction rate corresponding to the load fluctuation by changing the speed of the compressor. Since the motor’s shaft, which has a compressor and a high pressure turbine, rotates at high speed and those masses are considerable, it has rotational energy of about several kWh. This energy could be charged and discharged through the converter that controls the motor speed, the same as for flywheels. This response could be much faster than conventional gas turbines, which contain huge amounts of working gas. MAGT controls its rotational energy in seconds and controls gas turbine power in minutes; thereby it improves response totally. Moreover, by assisting the compressor by using motor power, MAGT can increase gas turbine power output. Since the density of air decreases with as temperature increase, the mass of working gas is reduced. Thus, the fuel input must accordingly be reduced to suppress the combustion temperature without damaging turbine blades. As a result, power output is reduced. In such cases, a motor-assisted compressor can increase working gas. That allows more fuel input.The proposed system was evaluated using numerical simulations. The results showed that frequency variations were within ±0.1Hz and the output power was recovered under high ambient temperature.Copyright

Calculation method of circulating current in parallel armature windings in consideration of magnetic circuit

Recently, the capacity of turbo generators used in power plants is increasing in order to keep up with the growth of electric power consumption in the world. Turbo generators are consequently experiencing problems, including increasing electromagnetic force, temperature rise of armature coils, etc., as we try to increase the armature current to keep pace with the capacity increase. One way of avoiding these problems is to increase the number of parallel armature windings for decreasing the armature current per coil. However, the circulating current in the parallel windings is generated by the difference of the linkage flux of each winding, when the number of parallel windings is not a divisor of pole numbers. In this paper, we propose a simple method to calculate the circulating current by using a magnetic circuit in the design phase. We confirmed the proposed method has a similar accuracy and faster performance in comparison with the finite element method (FEM) analysis. And then we applied the proposed method to a calculation of the circulating current in 2- and 4-pole generators and considered the factors affecting the circulating current.

Motor design approach utilizing regularity of a two-dimensional magnetic field

An approach for finding and optimizing two-dimensional (2-D) motor geometries, based on the regularity of 2-D magnetic fields, is reported. In particular, solving optimization problems such as cogging torque minimization and maximization of time-averaged motor torque, the effectiveness of our approach in cutting computational time is verified. The elapsed time to find the optimal solution is less than 1/10 of the time by a conventional approach whose objective function is calculated from the toque waveform.

A complex representation approach for the optimal design of PM devices

A 2-dimensional shape optimization method for permanent magnet configurations, which is based on a combination of nonlinear programming and field analysis with a complex representation technique of the 2-dimensional magnetic fields, has been developed. As a typical problem example, computational results for a segmented dipole magnet are presented. The results show that the objective function values are reduced significantly and a very uniform dipole field is obtained at the magnet center. The validity of the method is also discussed by comparison with the theoretically presumed solution.

Frequency control of small power system with wind generators installed by using DFIG based diesel generator

In recent years, environmental problems are becoming serious and renewable energy systems, especially wind power generation, have attracted much attention and been introduced into power systems. However, wind power generator output is intermittent, resulting frequency fluctuations in the connected power system. A doubly fed induction generator (DFIG) based diesel generation system is proposed in this paper to suppress the frequency fluctuations of the small power system with the squirrel cage induction generator (SCIG) based wind turbines installed. Simulation analyses have been performed using PSCAD/EMTDC software. The simulation results show that DIFG based diesel generation system is very effective to suppress the frequency fluctuations of the power system compared to conventional synchronous generator (SG) based diesel generation system.