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1.
A desktop magnetic resonance imaging system.   总被引:2,自引:0,他引:2  
Modern magnetic resonance imaging (MRI) systems consist of several complex, high cost subsystems. The cost and complexity of these systems often makes them impractical for use as routine laboratory instruments, limiting their use to hospitals and dedicated laboratories. However, advances in the consumer electronics industry have led to the widespread availability of inexpensive radio-frequency integrated circuits with exceptional abilities. We have developed a small, low-cost MR system derived from these new components. When combined with inexpensive desktop magnets, this type of MR scanner has the promise of becoming standard laboratory equipment for both research and education. This paper describes the development of a prototype desktop MR scanner utilizing a 0.21 T permanent magnet with an imaging region of approximately 2 cm diameter. The system uses commercially available components where possible and is programmed in LabVIEW software. Results from 3D data sets of resolution phantoms and fixed, newborn mice demonstrate the capability of this system to obtain useful images from a system constructed for approximately $13,500.  相似文献
2.
中国推进售电侧市场化的制度设计与建议   总被引:2,自引:0,他引:2       下载免费PDF全文
回顾了中国电力体制改革的历史进程,剖析了当前形势下中国推进售电侧改革的困难所在,研究了售电侧改革在电力体制改革中的定位,揭示了售电侧改革对形成市场化交易机制、促进分布式能源发展和开展需求响应的重要意义。在此基础上,提出了中国稳步推进售电侧改革的制度设计与建议,其中包括采用管制化与市场化相结合的方式培育售电主体、采用互联网手段加强用户侧的培育、分类建立差异化的售电主体准入机制、采用市场机制激励售电商提供多样化的增值服务、建立公益性售电商应对托管状态下的用电需求、建立公平透明的售电监管和信息披露制度等。  相似文献
3.
Two of the most expected objectives of transmission line protection are: 1) differentiating precisely the internal faults from external and 2) indicating exactly the fault type using one end data only. This paper proposes an improved solution based on wavelet transform and self-organized neural network. The measured voltage and current signals are preprocessed first and then decomposed using wavelet multiresolution analysis to obtain the high frequency details and low frequency approximations. The patterns formed based on high frequency signal components are arranged as inputs of neural network #1, whose task is to indicate whether the fault is internal or external. The patterns formed using low frequency approximations are arranged as inputs of neural network #2, whose task is to indicate the exact fault type. The new method uses both low and high frequency information of the fault signal to achieve an advanced line protection scheme. The proposed approach is verified using frequency-dependent transmission line model and the test results prove its enhanced performance. A discussion of the application issues for the proposed approach is provided at the end where the generality of the proposed approach and guidance for future study are pointed out  相似文献
4.
Five-phase permanent-magnet motor drives   总被引:1,自引:0,他引:1  
A five-phase brushless permanent-magnet (PM) motor is introduced. The proposed motor has concentrated windings such that the produced back electromotive force is almost trapezoidal. The motor is supplied with the combined sinusoidal plus third harmonic of currents. This motor, while generating the same average torque as an equivalent PM brushless dc motor (BLDC), overcomes its disadvantages. The motor equations are obtained in the d/sub 1/q/sub 1/d/sub 3/q/sub 3/0 rotating reference frame. Therefore, the so-called vector control is easily applicable to this kind of motors and the motor has the same controllability as a PM synchronous motor (PMSM). For presenting the superior performance of the proposed five-phase motor, its three and five-phase PMSM and BLDC counterparts are also analyzed. Finite element method is used for studying the flux density and calculating the developed static torque. Also, the developed torque is obtained using the mathematical model in the d-q reference frame. The average torque and the torque ripple for all cases are calculated and compared. Experimental results are in good agreement with the simulation results.  相似文献
5.
We present a six-axis magnetic-levitation (maglev) stage capable of precision positioning down to several nanometers. This stage has a simple and compact mechanical structure advantageous to meet the performance requirements in the next-generation nanomanufacturing. It uses the minimum number of linear actuators required to generate all six axis motions. In this paper, we describe the electromechanical design, modeling, and control, and the electronic instrumentation to control this maglev system. The stage has a light moving-part mass of 0.2126 kg. It is capable of generating translation of 300 /spl mu/m in the x, y, and z axes, and rotation of 3 mrad about the three orthogonal axes. The stage demonstrates position resolution better than 5 nm rms and position noise less than 2 nm rms. Experimental results presented in this paper show that the stage can carry, orient, and precisely position a payload as heavy as 0.4 kg. The pull-out force was found to be 8.08 N in the vertical direction. Furthermore, under a load variation of 0.14 N, the nanopositioner recovers its regulated position within 0.6 s. All these experimental results match quite closely with the calculated values because of the accurate plant model and robust controller design. This device can be used as a positioning stage for numerous applications, including photolithography for semiconductor manufacturing, microscopic scanning, fabrication and assembly of nanostructures, and microscale rapid prototyping.  相似文献
6.
Automated monitoring and analysis of circuit breaker operation   总被引:1,自引:0,他引:1  
This paper describes a solution for automated analysis of circuit breaker operation. The analysis is based on a record of waveforms taken from the circuit breaker control circuit by using a portable recorder and manually forcing an operation of the breaker. This solution was driven by a need to perform the analysis in a more timely and consistent manner than what is available with existing technology. The solution is implemented using advanced wavelet transforms for waveform feature extraction and an expert system for decision making. A web-based database solution for storing and retrieving both the field-recorded and processed data is also implemented. The software is developed in two versions: for field (substation) as well as off-line (office) applications.  相似文献
7.
Optical tomography for biomedical applications   总被引:1,自引:0,他引:1  
Nonionizing optical tomography is a new and active research field although projection-light imaging was investigated as early as 1929. The optical properties of normal and diseased tissues are usually different despite the large variation of values in optical properties of the normal tissues alone. Therefore, it is possible to detect some breast cancers based on measurements of optical properties. In summary, optical techniques have the following advantages: (1) the use of nonionizing radiation, (2) the capability of measuring functional (physiological) parameters, (3) the potential high sensitivity to pathologic state of biological tissues, and (4) low cost. However, biomedical optics is a challenging research field requiring the participation of many diverse, talented scientists and engineers  相似文献
8.
In-line Sagnac interferometer current sensor   总被引:1,自引:0,他引:1  
The authors demonstrate for the first time a near shot noise limited in-line Sagnac interferometer current sensor. It is shown to have a number of advantages over the optical current sensors based on polarimetric Faraday and Faraday/Sagnac loop interferometer topologies, including lower sensitivity to environmental disturbances, less demanding optical components, and easy installation. Its application to power system measurements is also detailed  相似文献
9.
10.
With the appearance of deregulation, distribution transformer predictive maintenance is becoming more important for utilities to prevent forced outages with the consequential costs. To detect and diagnose a transformer internal fault requires a transformer model to simulate these faults. This paper presents finite element analysis of internal winding faults in a distribution transformer. The transformer with a turn-to-earth fault or a turn-to-turn fault is modeled using coupled electromagnetic and structural finite elements. The terminal behaviors of the transformer are studied by an indirect coupling of the finite element method and circuit simulation. The procedure was realized using a commercially available software. The normal case and various faulty cases were simulated and the terminal behaviors of the transformer were studied and compared with field experimental results. The comparison results validate the finite element model to simulate internal faults in a distribution transformer.  相似文献
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