Kazunobu Oi

Prototype and characteristics measurement of bearingless motor with wide air gap structure

Nowadays, more and more applications have been developed along with continuous research on magnetic suspension. In these applications, wide air gap structure always plays an important role. Therefore, bearingless motor with such structure which can provide large magnetic suspension force was proposed. This paper shows the experimental results of such bearingless motor based on a prototype model and high efficiency and stabilized transitional characteristic were specified.

Operational characteristics of an IPM-type bearingless motor with 2-pole motor windings and 4-pole suspension windings

A bearingless motor (BelM) integrates the characteristics of an electric motor and a magnetic bearing. Typically, a surface-mounted permanent magnet (SPM)-type BelM with a ring magnet magnetized in parallel is employed for high-speed operation. It is noteworthy that a carbon fiber bandage is needed to fix the ring magnet. However, when an SPM-type BelM is applied to a high-output motor, it becomes difficult to generate sufficient suspension force because of the wide magnetic gap arising from the carbon fiber bandage and the ring magnet. In addition, the manufacturing cost for the large-scale ring magnet and the carbon fiber bandage is high. Therefore, to realize a high-output and high-speed BelM at a low cost, we have focused on an interior permanent magnet (IPM)-type BelM with 2-pole motor windings and 4-pole suspension windings. Moreover, a rotor structure for suppressing suspension force ripple arising in the IPM-type BelM has been proposed based on three-dimensional finite element analysis. In this study, a prototype of the IPM-type BelM with the proposed rotor structure is produced. Experiments with this prototype show that the proposed rotor structure is effective in reducing suspension-force ripple. Additionally, the operational characteristics of the prototype are shown in detail.

A high efficiency hybrid 7-level inverter with single DC source

A high efficiency 3-phase hybrid 7-level inverter is presented in this paper. Compared with traditional hybrid 7-level converters, the proposed topology and its T-type cell have lower power loss and simple configuration. It can be easily implemented and applied in renewable energy areas. In the paper, power loss of proposed T-type cell is analyzed, by which conduction loss of T-type cell is proved to be lower than commonly used H-bridge. Therefore, it can be considered to replace H-bridge as a higher efficiency approach in many multi-level topologies. Due to single DC source proposed, DC voltage control scheme for auxiliary inverters is also discussed and presented. A comparison study is carried out by MATLAB + PLECS simulation platform in a grid-connected system.

Novel Structure of Three-Axis Active-Control-Type Magnetic Bearing for Reducing Rotor Iron Loss

Magnetic bearings (MBs) are useful for suspending the rotor shaft of a high-speed, high-output motor because they can do so without mechanical contact. In [2], we have proposed a three-axis active-control-type MB (3-axis AMB) with a cylindrical rotor whose diameter is equal to that of the rotor of a radial MB (RMB) or a motor. The proposed 3-axis AMB integrates an RMB and a thrust MB in one unit. However, the assembly of the conventional 3-axis AMB is complex, and the rotor iron loss is large. Thus, in order to simplify the assembly of the 3-axis AMB and to reduce rotor iron loss, this paper discusses a novel structure for the 3-axis AMB in which small auxiliary permanent magnets (PMs) are inserted in the radial winding slots. The suspension force and the rotor iron loss of the proposed device are evaluated by 3-D finite-element analysis in comparison with a 3-axis AMB without small auxiliary PMs.

Operational Characteristics of an IPM-Type Bearingless Motor With 2-Pole Motor Windings and 4-Pole Suspension Windings

A bearingless motor (BelM) integrates the characteristics of an electric motor and a magnetic bearing. Typically, a surface-mounted permanent magnet (SPM)-type BelM with a ring magnet magnetized in parallel is employed for high-speed operation. It is noteworthy that a carbon fiber bandage is needed to fix the ring magnet. However, when an SPM-type BelM is applied to a high-output motor, it becomes difficult to generate sufficient suspension force because of the wide magnetic gap arising from the carbon fiber bandage and the ring magnet. In addition, the manufacturing cost for the large-scale ring magnet and the carbon fiber bandage is high. Therefore, to realize a high-output and high-speed BelM at a low cost, we have focused on an interior permanent magnet (IPM)-type BelM with 2-pole motor windings and 4-pole suspension windings. Moreover, a rotor structure for suppressing suspension force ripple arising in the IPM-type BelM has been proposed based on a three-dimensional finite element analysis. In this study, a prototype of the IPM-type BelM with the proposed rotor structure is produced. Experiments with this prototype show that the proposed rotor structure is effective in reducing suspension-force ripple. Additionally, the operational characteristics of the prototype are shown in detail.

A ZCT soft switching approach for T-type 3-level converter — Analysis, design guideline, experiment and issues

A zero current transition (ZCT) approach for T-type 3-level converter is presented in the paper. Through State-Plane analysis of its voltage and current trajectory in 4 operation zones, a design guideline is developed. The proposal is verified in a 10KVA converter, which is a mini-model of 500KVA UPS product. Preliminary test result shows that diode reverse recovery is a big concern for such ZCT soft switching techniques. Countermeasures for reducing the impact of diode reverse recovery are tested and discussed.