Verification of Iron Loss Calculation Method Using a High‐Precision Iron Loss Analyzer

Because of the improved performance of power devices, the volume of the ac filter inductors used in high‐frequency PWM inverters has been reduced. However, the temperature rise in the filter inductor due to this miniaturization has become more pronounced. Therefore, we have proposed an iron loss calculation method for the ac filter inductor. However, the accuracy of the value calculated via the loss map method cannot be verified, because the iron loss arising during each switching period cannot be measured with conventional power measuring instruments. In order to resolve this problem, we developed an inductor loss analyzer (ILA), which allows precise measurement of the iron loss in the inductor during each switching period. The accuracy of the calculation of iron loss in the filter inductor by the loss map method was verified with the ILA. We found that the value calculated by the loss map method differed slightly from the value measured with the ILA. However, these differences can be reduced if we take into account the accurate flux density calculation and the effect of the duty ratio of PWM pulses on the loss. Finally, we verified that the loss map method can provide accurate iron loss calculations.     

锂离子电池交流阻抗热失控变化规律研究

利用自主设计的交流阻抗监测锂离子电池热失控平台，对锂离子电池热滥用过程中阻抗变化规律进行研究。基于交流阻抗技术与锂离子电池热失控机理，分析了锂离子电池热滥用过程中阻抗、相位角、欧姆内阻、极化内阻及电压随电池表面温度的变化。结果表明：锂离子电池在热滥用过程中，阻抗会先减小并在电池断路前几分钟快速增长，电池电压保持稳定直至电池断路；相位角在加热中会减小然后保持稳定。18650型锂离子电池阻抗主要受欧姆内阻影响。锂离子电池极化内阻能在电池断路前保持稳定，说明正极材料在此阶段没有发生化学反应。     

宽带无线接入网技术的应用

宽带无线接入网技术作为一种有效的宽带解决方案而受到人们高度重视。本文主要介绍几种宽带无线接入方式及其特点、应用范围、优缺点等,并作了对比,供相关业者参考。     

智能交流接触器动态电感测量及合闸位移预测

动态电感和合闸位移是衡量智能交流接触器动态特性的两项重要参数,为对动态电感和位移进行预测,引入空气间隙与等效磁阻之间的数学关系,建立了一种智能交流接触器等效磁路模型。通过在线检测线圈输入电压和电流,实现对动态电感和合闸位移在线实时监测的目的,可有效掌握和评估智能交流接触器的实时动态特性。采用激光位移传感器对接触器动态特性进行同步对比实验,实验结果显示预测位移与位移传感器实际测量位移较为吻合,表明在无需位移传感器的条件下,可有效预测接触器的动态特性。     

Goodput and throughput comparison of single‐hop and multi‐hop routing for IEEE 802.11 DCF‐based wireless networks under hidden terminal existence

We investigate how multi‐hop routing affects the goodput and throughput performances of IEEE 802.11 distributed coordination function‐based wireless networks compared with direct transmission (single hopping), when medium access control dynamics such as carrier sensing, collisions, retransmissions, and exponential backoff are taken into account under hidden terminal presence. We propose a semi‐Markov chain‐based goodput and throughput model for IEEE 802.11‐based wireless networks, which works accurately with both multi‐hopping and single hopping for different network topologies and over a large range of traffic loads. Results show that, under light traffic, there is little benefit of parallel transmissions and both single‐hop and multi‐hop routing achieve the same end‐to‐end goodput. Under moderate traffic, concurrent transmissions are favorable as multi‐hopping improves the goodput up to 730% with respect to single hopping for dense networks. At heavy traffic, multi‐hopping becomes unstable because of increased packet collisions and network congestion, and single‐hopping achieves higher network layer goodput compared with multi‐hop routing. As for the link layer throughput is concerned, multi‐hopping increases throughput 75 times for large networks, whereas single hopping may become advantageous for small networks. The results point out that the end‐to‐end goodput can be improved by adaptively switching between single hopping and multi‐hopping according to the traffic load and topology. Copyright © 2015 John Wiley & Sons, Ltd.     

IEEE 802.11b DCF三维Markov非饱和链路模型

为了准确评估实际网络中IEEE 802.11b分布式协调功能(Distributed Coordination Function,DCF)的系统性能,提出了一种三维Markov链路模型,该模型综合考虑了DCF协议退避计数器冻结状态和有限重传次数,并结合M/M/1/K排队模型给出了在负载有限的情况下DCF系统性能的理论模型。仿真结果表明,该模型能准确地预测IEEE 802.11b DCF协议的系统性能。     

Impact of device unavailability on the reliability of multicast transport in IEEE 802.11 networks

Multicast transport is an efficient solution to deliver the same content simultaneously to many receivers. This transport mode is mainly used these days to deliver real-time video streams. However, multicast transmissions support over IEEE 802.11 networks does not provide any feedback policies, which implies a definite loss of missing packets. This impacts the reliability of the multicast transport and the application employing it. An alternative to improve the reliability of multicast streaming over 802.11 networks is to prevent packet losses. In this perspective, it is necessary to identify the loss causes and to perform the required prevention actions. It is well known that collisions and path loss are two fundamental sources of transmission failures. Their impact can be eliminated by means of collision prevention and data rate adaptation. However, several works show that the loss rate of multicast packets may be considerable even in collisions-free environments and using an appropriate transmission rate. Particularly they show that losses may have a bursty nature which does not correspond to the bit error rate model of the PHY layer as defined by the chipset manufacturers. Therefore, in this paper, we carry out a thorough investigation of the loss causes in wireless networks. We show that device unavailability may be the principal cause of the significant packet losses that occur and their bursty nature. Particularly, our results show that the CPU overload may incur a loss rate of 100%, and that the delivery ratio may be limited to 35% when the device is in the power save mode.     

Fast Directional Handoff and lightweight retransmission protocol for enhancing multimedia quality in indoor WLANs

More and more mobile devices such as smartphones are being used with IEEE 802.11 wireless LANs (WLANs or Wi-Fi). However, mobile users are still experiencing poor service quality on the move due to the large handoff delay and packet loss problem. In order to reduce the delay, a new handoff scheme using the geomagnetic sensor embedded in mobile devices is proposed in this paper. The proposed scheme predicts the movement direction of a Mobile Station (MS) from the currently associated Access Point (AP) and performs active scanning with a reduced number of channels. In terms of the packet loss, a lightweight retransmission protocol is also proposed to minimize lost packets on Wi-Fi without producing a lot of acknowledgement packets. The proposed approaches are implemented on Android smartphones, and their performance is evaluated in a real indoor WLAN environment. The evaluation results demonstrate that the proposed schemes maintain seamless quality for real-time video even in an environment with frequent handoffs. Note that the proposed schemes are a client-only solution and do not require modification of the existing APs, which renders them very practical.     

Mathematical model of five-level ac/dc converter in abc reference frame

This paper proposes a mathematical model for three-phase multilevel diode clamped ac/dc converter (DCC) in abc-reference frame, in order to study dynamic responses and stability of the system, where multilevel converters are employed. An attempt is made to model the converter adopting 5-level DCC and can be easily extended to DCC with any number of levels. The converter output and dc capacitor voltages are expressed in terms of input source currents and switching functions; which are basic inputs provided to a converter. Connection and capacitor currents of the converter have been analyzed. The effectiveness of the proposed mathematical model is verified by the computer simulations and experimental results. The obtained mathematical model has been validated with the physical converter model in MATLAB/Simulink environment and the prototype model using dSPACE DS under closed loop control system. The developed mathematical model equations are able to represent the physical converter with similar characteristics.