Kohji Maki

Motor modeling for EMC simulation by 3-D electromagnetic field analysis

A building technique of high-frequency motor models by 3-D electromagnetic field analysis has been developed to establish the simulation technology for EMC estimation in the design stage of motor drive systems. Stray capacitance of each part of a motor was calculated by static electric field analysis, while inductance and resistance of its stator windings were calculated by magnetic field analysis. A motor equivalent circuit was then built with these circuit constants to compute frequency characteristics of motor impedance. In order to take eddy current in laminated iron core into account precisely, we analyzed a 3-D model of half thickness of one laminated steel sheet, and a simplified 3-D model of coil end of a motor. We also took the difference between magnetic field distribution of common mode and that of differential mode into account by setting different coil current and boundary conditions in the analyses. The frequency dependence of calculated inductance and resistance was introduced into circuit simulations. As a result, we have obtained frequency characteristics of motor impedance, whose basic features are similar to those of measured data. Possible causes of the discrepancy have also been discussed.

Development of Superconducting Split Magnets for NMR Spectrometer

We have developed a high-resolution NMR spectrometer with a superconducting split magnet and a cross-shaped bore. Because of the spatial limitations of the magnets structural design, the split magnet requires many pairs of coils to generate a high magnetic field. For this reason, it is difficult to compare the high stability and homogeneity of its magnetic field to conventional NMR magnets. Therefore, we have developed advanced superconducting magnet technologies and successfully fabricated two types of self-shielded superconducting split-magnet systems: a 7 T (300 MHz) and a 14 T (600 MHz) magnet system. The 300 MHz system consists of 12 NbTi coils with 15 joints and a stored energy of 0.7 MJ; it achieved a field drift of about 0.1 Hz/h in persistent current mode operation. The 600 MHz magnet system consists of 12 Nb3Sn coils and 8 NbTi coils with 53 joints and a stored energy of 10 MJ. Its field drift was less than 1 Hz/h.

An optimal design of coaxial coils with constraints on inner and outer multipole magnetic fields

An optimal design method for magnets with double-layered coaxial coils to generate a uniform central magnetic field and a small stray magnetic field has been proposed. To find the optimal current density distribution, we used linear programming with constraints on the multipole field strengths, computed with analytical formulae, of the central and stray fields. The final coil positions were optimized with a quasi-Newton method, again taking into account the inner and outer multipole field strengths.

Development of Shimming System for a Highly Sensitive NMR Spectrometer With a Superconducting Split Magnet

A highly sensitive NMR spectrometer with a superconducting split magnet and a solenoid-type probe coil is being developed. One of the most challenging tasks is to achieve high homogeneity of magnetic field in a measuring volume with this configuration, because inevitable structural asymmetry and limited production accuracy of the magnet cause larger inhomogeneity of magnetic field compared with a conventional solenoid-type NMR spectrometer. Numerical estimation revealed that field correction capability of conventional shim coils, i.e., superconducting solenoid and saddle coils placed outside main coils, is insufficient for the split NMR magnet. Therefore, we have designed and constructed superconducting shim coils with sufficiently high capability by adopting the following three novel ideas: (1) superconducting shim coils are placed not only outside main coils but also inside them in the vicinity of a measuring volume; (2) superconducting shim coils with ldquoperiodically wavyrdquo shape are utilized to correct non-axial inhomogeneous magnetic field; (3) the current of each superconducting shim coil is independently controlled to correct plural modes of magnetic field simultaneously.

Air-gap remeshing technique for rotating machines in 3D finite element modeling

We present a 3D air-gap remeshing technique for rotating machines, which permits independent meshing of rotor and stator to facilitate machine modeling. Comparison of torque and eddy current analysis results for a brushless motor with the results obtained by the conventional lock-step technique validates the air-gap remeshing technique.

Integrated simulation and analysis of resolver sub-system for HEV electric drive

Resolver is widely used in electric drive system especially for hybrid electric vehicles (HEV). A resolver system includes not only a variable-reluctance resolver but also an excitation circuit, shielding cables and a resolver-to-digital (R/D) IC as a multi-physics system with electrical, magnetic, mechanical and control characteristics. This paper studies a resolver for HEV electric motor, and proposes an integrated systematic approach using co-simulation between several commercial simulation tools to simulate the resolverpsilas electromagnetism using transient solver of the finite element method, the excitation and termination circuits, and the behavior model of Type II tracking closed loop algorithm used by the R/D IC. This systematic approach paves way for a high accuracy system level simulation, fault analysis and trouble shooting, etc., which can be used to develop component design, DFMEAs and system integration that match performance in the field and not just a simplified model. A bench test of the presented resolver system is completed to make correlation to validate the model, and the analysis of some fault conditions is provided to make concept proof of this method.

Equivalent circuit simulation of stator coil end of high-voltage electrical machine at high dV/dt

We have been developing insulation design techniques based on an equivalent circuit to predict the voltage distribution of the stator coil end of high-voltage inverter-fed electrical machines. A new equivalent circuit was proposed by applying additional modeling of cover insulation, the corona protection shield, and measurement circuits to the conventional equivalent circuit for mains-fed electrical machines. The proposed equivalent circuit was used to simulate voltage distribution at dV/dt of inverter surge equal to 5 and 11.5 kV/μs, and its applicability was confirmed by experiment results. From the comparison of these simulation results, less voltage drop at the end of the corona protection shield was observed when the dV/dt was 11.5 kV/μs. This voltage drop at the end of the corona protection shield causes voltage drop at the end of stator core.