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1IntroductionThe most popular multi-level topology is the three-level inverter[1-4],first proposed as“neutral-point-clamped(NPC)inverter”,selectingrelativelylarge ca-pacitors for the capacitor bank.The performance ofoutput voltage of inverter is mainly determined by thepulse width modulation strategy.Space vector pulsewidth modulation(SVPWM)is widely applied becauseof such advantages as convenient realization and fullDC-link voltage utilization.The schematic of a three-level inverter is … 相似文献
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ZVS（zero voltage switching） PWM（pulse width modulation） inverters have attracted much attention recently. The basic idea is to use ZVS circuit as the main circuit of inverter resonance at the beginning of every carrier period so that each power device can commutate when voltage of the main circuit＇s DC bus line is zero. To ensure the resonant circuit to operate properly, sawtooth with alternate slope （positive or negative） is used as carrier. But the time of zero voltage vectors with such PWM pattern is greatly different from traditional hard-switching PWM pattern. This paper discusses the locus of flux linkage under soft-switching PWM pattern by using space voltage vector. It is pointed out that, under the hard-switching PWM pattern, speed of flux linkage is adjusted by zero space voltage vector. When soft-switching PWM pattern is used, however, effective time of space voltage vector varies considerably, sometimes even without zero space voltage vector. Therefore non-zero space voltage vector has been used to make the speed of flux linkage locus equal to that of hard-switching PWM pattern. The cause of current distortion in soft-switching PWM inverters is also discussed. Based on the flux linkage locus circle, corresponding compensation methods are proposed. Experimental results show that the described method can effectively improve output current waveform of inverter. 相似文献
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筹办文秘专业计算机速记专门化 总被引：1，自引：0，他引：1
一、文秘专业（计算机速记专门化方向）人才需求情况
计算机速录师最早出现在司法系统,专门从事庭审记录和为律师取证服务。随着社会对速录价值的认可,许多工作领域开始选用速录师配合工作。他们的从业范围十分广泛,像政府机关、公检法部门、企事业单位、网站、电视台等媒体都需要速录秘书从事资料整理工作和现场会议服务工作。像完成会议记录、庭审记录、谈判记录、询问记录、口授记录、采访记录、录音录像记录等。 相似文献
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学业考核是高职语文教学的重要组成部分，是评价教学目标实现与否的重要途径，也是督促学生加强学习的重要方法之一．文中分析了高职语文教学考核中存在的问题，论述了建立考核体系的必要性及具体做法． 相似文献
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In order to deal with torque pulsation problem caused by traditional control method for brushless DC （BLDC） motor and to achieve high precision and good stability, a novel control strategy is proposed. Compared with the traditional control scheme, by using phase voltage as a control objective and making waveform of phase current approximately quasi-sinusoidal, torque ripple of BLDC motor is reduced from the original 14% to 3.4%, while toque is increased by 3.8%. Furthermore, by detecting zero-crossings of back electromotive force （BEMF） with non-conducting phases, sensorless control is realized. The new control strategy is simple. It can minimize torque ripple, increase torque, and realize sensorless control for BLDC motor. Simulation and experiments show good performance of BLDC motor by using the new control method. 相似文献
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首先对高科技创业企业风险的内涵与特征进行分析,而后基于生命周期理论对其风险因素进行识别,确定出技术风险等8类风险因素,并运用专家评价方法对风险因素进行初步筛选,得到核心风险因素集;在此基础上,运用决策特征根对核心风险因素进行进一步筛选,得到由资金因素、管理因素、技术因素和市场因素构成的高科技创业企业风险关键评价指标集;运用层次分析法对评价指标权重进行测度,并构建高科技创业企业风险度量模型;最后,以典型高科技创业企业为例进行实证研究,并基于实证研究结果提出了"加强技术创新、强化经营管控和规范营销网络"等高科技创业企业风险防范的策略建议。 相似文献
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This paper presents a novel current feedforward control strategy for a three-phase pulse-width modulation (PWM) DC voltage-type converter based on phase and amplitude control (PAC).With right-angle triangle relation of phasors and principle of conservation of energy, a phasor adjustment method and the relevant low-frequency mathematical model of the system are analyzed in detail, both in rectification and regeneration modes for the converter, are discussed.For improving the traditional PAC dynamic performance, variable load current is detected indirectly by the change of the DC voltage, which is fed to the control system as an additional control variable to generate modulation index and phase angle.Also, the algorithm is derived and the system principle is introduced.Experimental results from a 3 kw laboratory device are included to demonstrate the effectiveness of the proposed control strategy. 相似文献
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孙承波 《辽宁教育行政学院学报》2006,23(4):165-167
著名教育家叶圣陶先生曾指出：“在语言文字的训练听、说、读、写四个字中间，‘说’字最基本，说的功夫差不多，听、读、写三项就容易办了。”叶老以精辟的语言阐明了口语训练的重要性和学习语文的基本规律。美国口语专家罗斯的一份研究报告也表明，在语言交流活动中，听说方面所占比重很大，听是获得信息、摄取知识和发展智力的重要途径；说是输出信息，用口语表达形式表述自己所见所闻所想。由此看来，听说是语言交流活动的重要组成部分。只有提高听说能力，才能全面提高学生语文素质。 相似文献
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学生平时交际、思维、想象等能力都要以说话表现出来。现在有的学一旦当众说话便面红耳赤、手足无措,吞吞吐吐,前言不搭后主。在人才市场中一些毕业生因为表达迟钝而被用人单位淘汰的现象绥见不鲜。因此 相似文献
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A novel software implementation for current polarity detection and current compensation is presented. For a three-phase zero-voltage soft-switching （ZVS） PWM converter based on phase and amplitude control （PAC）, when saw-tooth carriers with alternate positive and negative slopes are adopted, the positive or negative slopes are chosen according to the phase current polarity. Since polarity reversal causes current distortion, current at the instant of reversal should be compensated for. Based on the characteristic of unity power factor converter in rectification and regeneration modes, a software implementation for current polarity detection is proposed. Distortion of current zero-crossing caused by using saw-tooth carriers with alternate positive and negative slopes is analyzed, and the relevant compensation method is proposed. Experimental study with a 1.5 kW device shows that phase current has a small harmonic content and power factor is high both in rectification and regeneration modes. 相似文献