Evaluation of superposition technique for calculating cogging torque in permanent-magnet brushless machines

We evaluate the superposition method for calculating the cogging torque in permanent-magnet brushless machines, the resultant torque being synthesized from cogging torque components associated with a pair of magnets. Although finite-element analyses and measurements show that the resultant cogging torque cannot be synthesized directly from the torque components due to a single magnet, we use the concept of a fictitious single magnet to analytically establish the relationship between the cogging torque and key design parameters. The method is particularly useful in assessing the influence of the slot number and pole number combination.     

Phase formation and modification of corrosion property of nitrogen implanted Ti-Al-V alloy

In the present investigation, polished samples were implanted with nitrogen ion at an energy of 60 keV and implantation doses were 1×10¹⁶, 5×10¹⁶, 1×10¹⁷ and 6×10¹⁷ ions/cm². Glancing incidence X-ray diffraction was employed on the implanted specimens to understand the phases formed with increasing dose. The valence states of nitrogen, titanium and carbon on the sample surfaces were analyzed by X-ray photoemission spectroscopy. The corrosion resistance was examined by the electrochemical methods in a solution with pH=10 at room temperature in order to determine the optimum dose that can give good corrosion resistance in a simulated nuclear reactor condition. Scanning electron microscopy was used to observe the topographies of nitrogen-implanted Ti-Al-Zr after potentiodynamic measurement. It was found that implanted nitrogen dissolved in titanium matrix with increasing dose and the resultant nitrides such as TiN and Ti₂N precipitated. Implantation of nitrogen ions into the surface of Ti-Al-V alloy improves its corrosion resistance, and the increase of the corrosion resistance depends on the nitrogen dose employed; the maximum improvement of the corrosion resistance was observed at a dose of 1×10¹⁷ N⁺/cm².     

Secondary electron emission and surface charging evaluation of alumina ceramics and sapphire

The breakdown of alumina rf windows is mostly caused by multipactor, as well as by material defects and contamination. Since multipator induces localized surface heating, leading to surface melting, it is necessary to observe secondary electron emission (SEE) coefficients of alumina ceramics under high temperature conditions. The SEE coefficients of commercial alumina ceramics and sapphire were measured by a scanning electron microscopy (SEM) with a single short-pulsed electron beam (100 pA, 1 ms) at room temperature and at 650degC. Additive materials used for sintering alumina, such as SiO ₂ and MgO, were also investigated. Surface charging evaluations have also become important because the accumulated charges are discharged at the threshold Held, resulting in surface discharge. The surface charging evaluations were carried out by multi-pulse measurements with the injection of successive pulses on the sample. As a result, reductions in the SEE coefficients with temperature were confirmed, except for sapphire. The multi-pulse measurement results indicated that surface charging of the sapphire was higher than that of other samples. This may be one of the factors that causes sapphire not to be durable for rf window applications, compared with alumina ceramics. Although there are few exceptions, it was found that the SEE coefficients of alumina ceramics increased with the purity and the average grain size     

Generalization of active filters for EMI reduction and harmonics compensation

This paper provides generalized analysis of active filters used as electromagnetic interference (EMI) filters and active-power filters. Insertion loss and impedance increase of various types of active-filter topologies are described with applicable requirements and limitations as well as the rationale for selecting active-filter topology according to different applications.     

A novel frequency-offset monitoring technique for direct-detection DPSK systems

We propose and demonstrate a novel technique to monitor the frequency offset between the optical source and delay interferometer (DI) for direct-detection differential phase-shift-keying (DPSK) systems. In this scheme, a phase-modulated tone is applied to DPSK signals at the transmitter and then detected after being converted into an amplitude-modulated tone at the DI to be used for the monitoring signal. Our experimental demonstration shows that the monitoring range and sensitivity of the proposed scheme are measured to be /spl plusmn/2 GHz and /spl sim/10 MHz, respectively, which we believe are good enough to be used either to generate alarm signals for the frequency offset monitoring or to control the feedback loop of the DI.     

Voltages induced on an overhead wire by lightning strikes to a nearby tall grounded object

The aim of this study was to identify conditions under which the presence of tall strike object can serve to increase or decrease lightning-induced voltages on a nearby overhead wire. We examined the ratios of magnitudes of lightning-induced voltages on the overhead wire for the cases of strikes to a tall object and to flat ground as a function of distance from the lightning channel d, current reflection coefficients at the top of the strike object /spl rho//sub top/ and at the bottom of the strike object /spl rho//sub bot/, the current reflection coefficient at the channel base (in the case of strikes to flat ground) /spl rho//sub gr/, and the return stroke speed v. Lightning-induced voltages were computed using the finite-difference time-domain (FDTD) method. The transmission line (TL) model was used to find the distribution of current along the lightning channel and the strike object. The ratio of magnitudes of lightning-induced voltages for tall-object and flat-ground cases increases with increasing d (ranging from 40--200 m), decreasing /spl rho//sub bot/(<1), decreasing /spl rho//sub top/ (<0, except for the case of /spl rho//sub bot/=0), and decreasing v (相似文献