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Permanent magnet synchronous motors for electric vehicles should meet three criteria: high maximum torque, high efficiency at low torque, and constant output in a wide speed range. However, meeting these criteria has come with a trade-off in terms of the magnitude of the field magnetic flux. We propose a consequent-pole-PM-type axial-gap motor with ferrite PMs and field windings, which has structure variable field magnetic flux. In order to examine the operational characteristics of the proposed motor, we produced and tested a prototype. Experimental results show that the proposed structure meets all of the three criteria above.

This research proposes a novel low-voltage inverter-fed motor drive with variable parameters achieved through the independent armature winding for the automobile traction motor. The independent armature winding with low-voltage inverters can improve the total efficiency by reducing the converter losses. The inverters are able to change their connections to different coils groups of the armature winding in order to achieve the wide operating range. The voltage equation of each mode can be shown by using the transformation matrix when the winding connection is changed. Thus, the proposed motor is called the MATRIX motor. In this paper, the downsized model was designed to verify the principle. The principle model is able to achieve four motor parameters. It was confirmed that the principle model could extend the operating range and could achieve the high-efficiency driving in wide speed range. Furthermore, the novel winding reconfiguration method is proposed that not only avoids the surge voltage, but also compensates the torque reduction during the winding reconfigurations.

A new high-temperature superconducting (HTS) application project supported by the New Energy and Industrial Technology Development Organization (NEDO) started in July 2016. One of the targets of this project is to demonstrate the half-size of whole-body magnetic resonance imaging (MRI) system using HTS magnets with REBCO conductors. In the first phase of this project (FY2016–FY2018), a three-tesla magnet with an active shield will be manufactured. The room-temperature bore of the magnet is 480 mm in diameter. The magnet will be cooled down to lower than 20 K with a G-M refrigerator and the MR imaging will be performed to evaluate the magnetic field uniformity and stability. The design of the half-size 3-T coil has been finished: it consists of 220 single-pancake coils, and it has an inner diameter of 560 mm, an outer diameter of 1200 mm, and an axial length of 1000 mm. The rated current is 148 A and the stored energy at the rated excitation is 1.58 MJ. The designed magnetic field homogeneity is 1.7 ppm (250 mm DSV) which is adequate for MR imaging. Furthermore, the fringe field is suppressed by active shielding technology. The number of 53 single-pancake coils was manufactured in FY2016, and the yield rate was about 95%.

This paper used a finite-element magnetic field analysis (FEA) to investigate the instantaneous and average power in a single phase transformer and a three-phase rotating machine. We examined the influence of the hysteretic properties of the magnetic materials on the instantaneous power in FEA using the isotropic vector play model. We applied post-correction to estimate the accuracy of the average power from their equivalent circuits.

This paper proposes a homogenization method for a laminated iron core taking into account hysteretic property. The proposed method is applicable to a static or low-frequency problem neglecting eddy currents induced by the magnetic flux parallel to the lamination. Furthermore, we apply the proposed homogenization method to finite-element magnetic field analyses to demonstrate its effectiveness. As a consequence, it is verified that the proposed homogenization method can accurately estimate a hysteresis loss of a laminated iron core and decrease computational costs.

The superconducting magnet is effective to get a high stable and high magnetic field for magnetic resonance imaging (MRI). The current MRI superconducting magnet needed cooling in the liquid helium (4.2 K) to use NbTi superconducting wire. In the past few years, price increase and low availability of liquid helium has become a serious problem. Under such circumstances, the development of a high-temperature superconducting (HTS) coil dispensing with liquid helium cooling is greatly desired. The research and development project of the high stable magnetic field ReBCO coil system fundamental technology that started from the latter half of 2013 develops a ReBCO coil for 3 T MRI superconducting magnets. It gets a prospect of the practical use as the final aim. In this project, we will produce an HTS test coil of 300 mm bore experimentally and evaluate the magnetic field. This coil is cooled in less than 20 K by a GM refrigerator. We are going to make MRI used by the ReBCO coil field to evaluate the uniformity and stability of the magnetic field.

Permanent magnet synchronous motors (PMSM) for electric vehicles (EV) requires three properties; high torque, high efficiency at low torque region, and constant output in wide speed range. However, these have a relation of a trade-off at magnitude of field magnetic flux. In this paper, consequent pole PM type axial-gap motor with the ferrite PMs and the field windings, which has structure variable field magnetic flux, is introduced. Also, in order to examine operation characteristics of the proposed motor, a prototype is produced and tested. Experimental results show that the proposed structure is effective in realizing all of three properties; high torque, high efficiency at low torque region, and constant output in wide speed range.

This paper investigates a method to increase fuel economy of motors for an electric vehicle (EV) traction system. The low-torque region of an EV motor is frequently used in everyday urban driving and in fuel economy measurement. Pulsewidth modulation (PWM) carrier harmonic iron loss accounts for a high percentage of total loss for EV motors in this region. This paper reveals the relationship between phase current and carrier harmonic iron loss for a permanent-magnet synchronous motor (PMSM) in the low-torque region. Thus, a novel carrier harmonic loss reduction technique is proposed and applied to two sets of three-phase windings for a concentrated winding PMSM. The loss is successfully reduced in a 70-kW EV motor as well as a two-dimensional finite-element method (2-D-FEM) calculation. The efficiency of the developed motor is calculated and compared with that of a conventional motor in the JC08 mode driving cycle.

This paper presents an examination of increasing operation speed of consequent pole type axial gap motor to achieve higher output density. Our research group has been developing consequent pole type axial gap motor with field windings for traction motor of electric vehicles. From the point of fuel economy and layout, smaller and lighter traction motor is required. It is profitable to increase operation speed for reducing the size of motor. It is necessary to suppress line to line voltage to achieve high speed operation, and we examined pole-slot combination. Moreover, as rotor’s outer diameter of axial gap motor is larger than that of radial gap motor, the strength of rotor should be considered. We studied the adoption of non- magnetic high tensile strength steel for rotor supporting component. Motor design and the result of spin burst test are presented and we confirmed the operation of target maximum speed and output power density can be possible.

High efficiency electrical machines such as rotating machines and transformers are expected to be realized for solving the environmental problem. It is well known that the iron loss of stator core for rotating machines increases by a mechanical stress caused by the shrink fitting of the stator housing. In order to clarify the influence of the mechanical stress to the characteristics of rotating machines, several papers [1]– [6] describe the simulation technic such as the combined analysis of a mecahnical stress and an electromagnetic field. In this paper, we develops the variable applying stress system with the hydraulic unit to clarify the influence of the shrink fitting to the motor characteristics. Fig. 1 shows the schematic of the variable applying stress system. We designed these system following three requirement. (a) The circumferential compressive stress of the stator core caused by these system is more than 100 MPa. (b) These system can adjusts the circumferential compressive stress of the stator core at an interval of 1 MPa. (c) These system uniformly applies the circumferential compressive stress of the stator core in the axial and circumferential direction. In order to realize the concept of these three points, these system is adopted the hydraulic mechanism. When the hydraulic unit applies the oil pressure to the oil room, the pressure bulk head is deformed by the oil pressure and consequently the circumferential compressive stress is generated in the stator core. These system can adjusts the circumferential compressive stress of the stator core by the oil pressure of the hydraulic unit. The circumferential compressive stress, which is measured by eight biaxial strain gages installed on the surface of the back-iron for the stator core, is changed linearly with respect to the applied oil pressure by the hydraulic unit, and the maximum circumferential compressive stress is generated more than 100 MPa under the oil pressure 15 MPa. Fig. 2 shows the measurement results of the iron loss in no-load with respect to the applied oil pressure by the hydraulic unit. The load motor rotates the test motor, which is adopted the interior permanent magnet motor with concentrated winding, at a constant rotating speed. Then, the loss of the test motor with the unmagnetized and magnetized permanent magnet rotor in no-load is measured by the torque detector. The iron loss of the test motor is calculated by a difference between the loss of the test motor with the magnetized permanent magnet rotor ant it with the unmagnetized permanent magnet rotor, which is included only the mechanical loss without the iron loss. As the applied oil pressure increases, the iron loss is gradually increased and the iron loss under the circumferential compressive stress of 100 MPa (applied oil pressure 15 MPa) in the stator core is increased by 2 times compared with the non-stress. As explained above, the proposed system can measures the iron loss of the actual motor under mechanical stress with various operating point. The more measurement results will be included in the full paper.

This paper presents the examination of consequent pole type ferrite PM axial gap motor with field winding. Especially the configuration of magnet and rotor core is analyzed with equivalent magnetic circuit and 3D FEA. The prototype was assembled and the comparison between test result and analysis is also provided. It is confirmed that high torque and efficiency characteristic is achieved when the width of magnet and rotor core are 21 deg. and 15 deg. (mechanical angle) respectively.

This paper proposes a homogenization method for laminated iron core taking account of hysteretic property by using the isotropic vector play model. Furthermore, we apply the proposed homogenization method to finite element magnetic field analyses to demonstrate its effectiveness.

This paper describes the results of an experimental investigation on the effects of the damper winding of a four-pole synchronous generator at a synchronous generator transient. It is known from simulations that the damper winding acts effectively under synchronous generator transient conditions. However, experimental proof has not been obtained yet. Thus, experiments on damper effects were conducted using a laboratory-scale power system. The damper winding of the tested generator consisted of five damper bars for each pole; the number of working damper bars could be changed manually. The damper currents at each bar were measured by a Rogowski coil. FFT analysis was applied to both the damper currents and armature currents under different operating conditions. The relationships between damper currents in the rotor and armature currents in the stator were made clearer than before. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 180(2): 33–42, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21278

SUMMARY It is well known in the numerical simulations of a synchronous generator that the damper winding contributes to suppressing three-phase circuit unbalance in power systems. However, an experimental study has not been performed yet. In this paper, we verify experimentally the suppression of three-phase unbalance by the damper windings. In order to simulate the three-phase unbalance, a 470-W dispersed generator of single-phase two-line type was connected to a three-phase laboratory-scale power system that included a 6-kVA synchronous generator. We measured and analyzed line voltages and currents as well as damper bar currents both with and without the dispersed generator. The influence of damper windings on the unbalance of the three-phase circuit was also investigated. The results show that the damper winding contributes to improving the three-phase unbalance. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 186(1): 43–50, 2014; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22305

This paper examines the cogging torque of a permanent magnet (PM) motor resulting from the asymmetry property of magnetic poles, which comes from the performance variation between magnets. A PM motor with 32 poles (= 16 pole pairs) and 36 slots is selected for verification, because the motor whose pole/slot ratio is 8/9 is sensitive to the performance variation between permanent magnets. Assuming that two different magnetization levels of magnets are mixed together in one rotor, the amplitude of the 2.25th and 4.5th components of cogging torque, which show 36 (= 2.25 × 16) and 72 (= 4.5 × 16) times of pulsation per rotation respectively and both of which result from the asymmetry property of the magnetic poles, are evaluated. As a result, it is clarified that the cogging torque characteristics depend on the alignment pattern of the two kinds of magnets. The amplitudes of the 2.25th and 4.5th components of cogging torque are proportional respectively to the amplitude of the 36th and 72nd order harmonics of the squared magnetic flux density around the rotor which is set in the space without stator. Using the proportional constants found from the finite element analyses in some alignment patterns, the cogging torque amplitudes of the motors with other alignment patterns can be predicted by calculating the squared magnetic flux density around the rotor only. The predicted cogging torque amplitudes correspond to the actually calculated results. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 163(3): 57– 67, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20669

This research proposes a novel low-voltage inverter-fed motor drive with variable characteristics achieved through the individual winding current control for the automobile traction motor. The independent armature winding with low-voltage inverters can improve the total efficiency by reducing the converter losses. Furthermore, the motor can expand the operating range since the inverters are able to change their connections to different coils groups of the armature winding. In this paper, the down size model was designed to verify the principle. It was confirmed that the principle model can extend the operating range and can expand the high efficiency area by FEA and experimental results.

This paper is investigated to increase fuel economy of motor for electric vehicles (EVs) traction system. Low torque region of EV motor is frequently-used point in usual urban driving and in fuel economy measurement. Pulse-width modulation (PWM) carrier harmonic iron loss takes up high percentage for EV motors in this region. This paper reveals the relationship between phase current and carrier harmonic iron loss for permanent magnet synchronous motor (PMSM) in low torque region. Then, a novel carrier harmonic loss reduction technique is proposed, which is applied two set of three phase windings for concentrated winding PMSM. The loss reduction effect is tested successfully on a 70 kW EV motor as well as two dimensional finite element method (2D-FEM) calculation. The efficiency of developed motor is calculated and compared with that of conventional motor in JC08 mode driving cycle.

A traction motor system of electric vehicles (EVs) requires wide torque-speed operation range, high efficiency, and low torque ripple according to its driving point. To achieve the requirements without increasing motor size, a novel variable characteristics motor system is proposed in this paper. A winding structure is studied to reduce unprofitable conductor which makes conduction loss. The developed system is applied 48-slot 8-pole permanent magnet synchronous motor (PMSM). The motor has structural full-pitch distributed winding with 12 open-end windings connected to insulated 12 full-bridge inverter. The developed motor simulates 4 types of winding mode, and each characteristic are calculated based on static two dimensional finite element analysis (2D-FEA). The calculated results show increase torque-speed operation range, higher efficiency, and lower torque ripple by selecting appropriate winding mode.

This paper discusses an accurate magnetic field analysis that addresses stress distribution in the magnetic core by using the finite-element method (FEM) to estimate motor characteristics. This analysis method consists of the following steps: 1) measurement of the magnetic characteristics of electrical steel sheets in a state where stress is added; 2) calculation of deformation and stress distribution using structural analysis; 3) preprocessing of the magnetic field analysis, which generates the FEM mesh taking account of the deformation by stress and the changes of the magnetic properties in each of the core elements corresponding to stress distribution; and 4) magnetic field analysis using the data measured in step 1) and the FEM mesh generated in step 3). As an example, a result of a permanent magnet motor's cogging torque calculation is shown, focusing on stress in the stator core resulting from shrink fitting into the frame. As a result, the cogging torque waveform was shown to differ depending on the shape and the materials of the frame.

The stress that acts on the core by installing a stator in a frame influences the performance of a motor. To reflect this influence in the equipment design, the stress dependence of the magnetic property was examined for non-oriented electrical steels, which are the core materials. This time, the stress dependence of the magnetic power loss in the range of ±225 MPa was evaluated for high-grade non-oriented electrical steel sheets of 0.5 mm thickness, and the following results were obtained. (1) The influence on the magnetic power loss is larger under compression than under tensile stress. The loss increases rapidly with an increase in the compressive stress. However, when the stress exceeds -50 MPa, the saturation tendency of the loss gradually becomes strong. (2) The influence of the stress on the magnetic power loss is different in the rolling direction and the traverse direction. (3) A greater magnetic power loss that occurs with compressive stress is mainly due to an increase in hysteresis loss, and the eddy current loss also increases slightly. It is thought that this slight increase in the eddy current loss that occurs with compressive stress depends on the change in the anomalous loss.

This paper describes the cogging torque of the permanent magnet synchronous (PM) motors due to the magnetic anisotropy of motor core. The cogging torque due to the magnetic anisotropy is calculated by the finite element method using two kinds of modeling methods: one is the 2D magnetization property method, and the other is the conventional method. As a result, the PM motors with parallel laminated core show different cogging torque waveform from the PM motors with the rotational laminated core due to the influence of the magnetic anisotropy. The amplitudes of the cogging torque are different depending on the modeling methods in the region of high flux density.

The market of the electrical power steering is expanding rapidly because of its fuel efficiency. The DC motor is generally used in this system, whose noise reduction is demanded. The DC motor has some dispersion in contact number of segments or contact resistance between the brush and the commutator. When these conditions are different with plural brushes, currents in armature coils are in asymmetrical distribution and the unbalanced magnetic pull occurs in radial direction.We measured the radial force and currents in armature coils with the developed experimental apparatus that can also set up the eccentricity of rotor to stator. In this paper, we investigated the factor and the process of the radial force generated in the DC motor. Then we propose short circuit wires for symmetrical position that makes winding current symmetrical and realizes the noise reduction.

Permanent magnet synchronous motors (PMSMs) with rare-earth permanent magnets (PMs) are most popular for automotive applications in which high torque density and high efficiency are required. Because the rare-earth materials have risks of lacking stable supply, rare-earth-free motors such as claw pole motors are widely studied. This paper presents the driving characteristics of a claw pole motor with no use of heavy rare-earth materials. In this motor the magnetic flux density in the air gap can be controlled arbitrarily by controlling field current. This paper shows that the variable field excitation can contribute to enlarge high efficiency range compared with fixed field excitation. Moreover, the electricity consumption driven in the Japan's fuel economy criteria (JC08 cycle) as EV and HEV applications are discussed.

This paper reviews recent technical trends in variable flux motors. Permanent magnet motor is a major component to realize high torque density with keeping high efficiency. However, when high speed revolution is required for further miniaturization, as well as the large torque in the low speed region, the flux yielded by the permanent magnet encounters a serious trade-off, i.e. the former needs less flux and the latter requires much. The solution for this trade-off is varying the flux. In general, the flux of the permanent magnet is constant, and many efforts are under development to overcome this as to be shown in this paper.

This paper presents a specification of a claw pole motor that has a field excitation coil for hybrid electric vehicles. The characteristic of the claw pole motor is no use of Dy and Tb which can increase torque capability. The filed excitation which can be generated by controlling filed current in a rotor provides us controllable efficiency. Additional ferrite magnets are installed between the rotor poles for the purpose of increasing torque capability due to moderating magnetic saturation of each claw. FE analysis and experimental results are shown in this paper.

In this paper we show a novel motor design method, considering stress distribution in the motor core. This method consists of several steps as follows: (a) measurement of magnetic characteristics of the electrical steel sheets in the state where the stress is added, (b) calculation of deformation and stress distribution using structural analysis, (c) pre-process of the magnetic field analysis, which makes FEM mesh deformed by the stress and changes the properties of each elements of the core corresponding to the distribution of the stress, and (d) magnetic field analysis using the measured data by step-(a) and the FEM mesh generated by step-(c). As an example, we show a result of cogging torque calculation of permanent magnet motors, considering the stress in the stator core resulting from shrink fitting in the frame. As a result, we show that the cogging torque waveform is different depending on the shape of the frame