A inductive voltage divider(IVD) is widely used as a ratio arm of precise impedance measurement bridges at low frequencies of audio frequency range and a well fabricated IVD has ratio error of $10^{-7}$ order without any calibration. Recently, the order of $10^{-8}$ of the best measurement uncertainty is needed for calibration and maintenance of impedance standards as national standards. In order to achieve that uncertainty, the IVD which is used for a ratio arm of precise impedance measurement bridge should be calibrated within the uncertain of order of $10^{-8}$ For this purpose, a ratio calibration bridge for IVDs has been developed. The measurement uncertainties of both inphase and quadrature of the bridge are analyzed less then $3{\times}10^{-8}$ respectively at 1 kHz and 25 V.

This paper deals with the reduction of cogging torque of a outer-rotor type BLDC motor mainly used for washing machines. The motor comprises permanent magnet outer-rotor and stator with coils and core. This structure inherently produces vibration and cogging torque because of uneven reluctance according to rotation of the rotor. Up to now, adopted a type of 24 magnet pole and 36 slot-stator. This generates high main torque but accompanies comparatively large cogging torque. This paper proposes a 32-pole 36-slot type motor which reduces cogging torque remarkably. The influence of cogging torque is varied according to pole-slot combinations. The characteristic of the motor was obtained by a two-dimensional finite element method coupled with a drive circuit. The performance of the proposed model is superior to that of the existing model because of the reduction of torque ripple and the improvement of back ernf wave form.

Many conventional multi-level inverters have detected switching faults by using the over voltage and current. However, fault detection of the switching elements is very difficult because the voltage and current due to each switching fault decrease more than the normal operation. Moreover, the dc-link unbalancing voltage causes a serious problem in the safety and reliability of system when the 3-level inverter faults occur Therefore, this paper proposes the simple fault diagnose method and the neutral-point-voltage control method that can protect the 3-level inverter system from the unbalancing voltage of the do-link capacitors when the faults of switching elements occur in the 3-level inverter that is very efficient in ac motor drives of the high voltage and high power applications. Through experiment results, the validity of the proposed method is demonstrated.

This paper proposes a new configuration of 36-pulse voltage source converter which consists of two 6-pulse bridges and a pulse-interleaving auxiliary circuit. The system topology of proposed converter was derived to increase the pulse number of converter output voltage without increasing the number of 6-pulse bridges. The gate pulse generation was analyzed using the theoretical approach of multi-pulse switching converter, The operational feasibility of proposed system was verified by computer simulations with PSCAD/EMTDC software and experimental works with 2kVA hardware prototype. The proposed converter can be widely used for the uninterruptible power supply, the power quality compensator, and the distributed power generation, such as solar and fuel cell power system.

The fuel cell systems are one of very useful energy sources. The systems have advantages as renewable and environmental sources. To obtain AC components from fuel cells, it needs inverters. A multilevel converter is used as a power conversion system for a high power fuel cell system. Through harmonic analysis, it is shown that the harmonic components and THD increase while a fundamental component of output decreases as voltage droop increases. To solve the voltage disturbance problems, three different approaches are investigated in this paper; installation of a boost converter at the fuel cell output, control of pulse widths, and use of ultracapacitors. The proposed three approaches are analyzed and compared through simulation and experimental results.

Magnetic encoder with relatively low pulse per rotation is generally used for detecting speed of the high-speed rotating machine. It is due to the fact of the mechanical problems of vibration and bearing stiffness and also the limit of maximum output pulse of the mounted encoder. The magnetic encoder is divided into two types, that is, toothed gear-wheel method and magnetic wheel method according to the shape of the rotation disk. In case of detecting speed by the tooth gear-wheel, the encoder itself can be acted as the additional inertia where the number of tooth determining the output pulse and the width of the wheel detecting the change of the magnetic flux density are relatively enough large considering the volume of the rotating machine. While the magnetic wheel method has the limit of the magnetizing number of the ring magnet, there is relatively few, if nv, the influence of inertia on the machine. In this paper, it is proposed a simple magnetic wheel encoder suited for the high speed rotating machine and the method of signal processing and the output characteristics are examined through the V/F operation of max 48,000(rpm) and 2.4(KW) spindle motor.