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Hu R.,Nanjing University of Aeronautics and Astronautics | Deng Z.,Nanjing University of Aeronautics and Astronautics | Cai J.,Nanjing University of Information Science and Technology | Cai J.,Jiangsu Engineering Research Center on Meteorological Energy Using and Control | And 3 more authors.
Hangkong Xuebao/Acta Aeronautica et Astronautica Sinica | Year: 2015

Sensorless control at low and medium speeds is still a challenging problem for switched reluctance motors. In order to estimate the rotor position continuously, a sensorless method based on PWM control is proposed. High-frequency triangle carrier waves intersect with low-frequency reference signals, which correspond to the rotor position. And the resultant PWM signals are used to drive the switching transistors of asymmetric half bridge converter. Spectral analysis shows that it is equivalent to inject sinusoidal signals with the same frequency as carrier waves. Then, after filtering and coordinate transformation, continuous rotor position information can be obtained. For the bad quality of sinusoidal inductance curve, phase shift and subtraction are applied to remove second harmonics, thus simplifying the process of estimation. Without injecting extra high-frequency signals, the method possesses the same accuracy and is easy for implementation. Finally, simulations and experiments are conducted on a 12/8 prototype to verify the validity of this method. ©, 2015, AAAS Press of Chinese Society of Aeronautics and Astronautics. All right reserved. Source


Yang C.,Nanjing University of Information Science and Technology | Yang C.,Jiangsu Engineering Research Center on Meteorological Energy Using and Control | Shen W.,Nanjing Institute of Environmental Sciences | Li H.,Nanjing Institute of Environmental Sciences
Nongye Gongcheng Xuebao/Transactions of the Chinese Society of Agricultural Engineering | Year: 2015

With less cultivated land, and sensitive to climate change, grain yield of Tibet is low and unstable, which has become an important factor to affect the social and economic development. In this study, the relationship between Tibet's grain yield and cultivated land during 1985-2010 were analyzed to find the main factors affecting grain yield in the background of climate warming, and the countermeasures and suggestions were put forward to guarantee of grain production in Tibet. Meteorological data were from 38 weather stations in Tibet, grain yield was from 2011 Tibet Automatic Yellow Book, and cultivated land area was from the remote sensing images from multi spectral scanner in 1980, thematic mapper in 1990, enhanced thematic mapper in 2000, and HJ-satellite in 2010. The four series of remote sensing data were corrected before use. In order to reveal the changes and influencing factors of grain yield, the crop climatic potential productivity was calculated by using Thornth waite model, and the crop trend production and climate production were separated by exponential curve based on the grain production and the meteorological data and four series of remote sensing data. The trend of climate change was analyzed using climatic trend rate and accumulative anomaly method. With the aid of visual interpretation and land change rate, changes of cultivated land were also discussed. The relationship between grain yield and climate change and the change of cultivated land was explored systematically using grey correlation analysis method. The results showed that: 1) From 1985 to 2010 in Tibet, the average total grain production was 77.83×107 kg, while average climatic potential productivity and grain production per unit area was 7419.58 kg/(hm2·a) and 1948.93 kg/hm2 respectively; Tibet's total grain production, climatic potential productivity and grain production per unit area demonstrated a fluctuating growing trend during the period of 1985-2010; Grain production potential was achieved in an average rate of 25.86%, presenting a gradually increasing trend; Grain production was approaching climatic potential productivity and there was much space for improvement in it; 2) The ratio of climate production to grain production per unit area fluctuated between -10.74% and 8.03% and its variation range showed a decreasing trend while the influence on grain production by climate continued to decline; 3) The average accumulated temperature ≥0℃ in Tibet was 2224.53 ℃, mean temperature in growing season and precipitation changes were 11.41 ℃ and 387.48 mm, respectively, which showed an increasing trend, while average sunshine hours of growing season was 1095.45 h, which tended to decrease; Besides, the turning change of those climate factors appeared in 1994; and 4) Correlation degree between grain production and cultivated land, accumulated temperature ≥0℃ and precipitation changed from 0.633 to 1.321; Correlation degree between grain production and mean temperature in growing season and precipitation was 0.595 and 0.641. Cultivated land, accumulated temperature ≥0℃ and precipitation in growing season were the most important factors affecting food production. The research can provide valuble information for identification of grain yield and its influencing factors, and to further improve the grain yield and to ensure the sustainable development of social economy in Tibet. ©, 2015, Chinese Society of Agricultural Engineering. All right reserved. Source


Cao Y.,Nanjing University of Information Science and Technology | Cao Y.,Jiangsu Engineering Research Center on Meteorological Energy Using and Control | Yu L.,Nanjing University of Information Science and Technology | Yu L.,Jiangsu Engineering Research Center on Meteorological Energy Using and Control | And 2 more authors.
2015 IEEE International Magnetics Conference, INTERMAG 2015 | Year: 2015

Axial-flux permanent-magnet (AFPM) machine with its compact structure, flat shape and high torque-to-weight ratio is increasingly being popularized and applied in various applications, such as wheel motor drives and portable generator sets. The coreless stator design of AFPM machines reduces the weight and eliminates core losses, which can operate at higher efficiency than conventional machines. The coreless AFPM machine has two outer rotor discs and one inner coreless stator disc. The rotor discs play a role both in magnetic circuit and mechanical support. For the coreless AFPM machine, the axial and circumferential electromagnetic force in the rotors will be generated by air-gap magnetic field. The circumferential electromagnetic force leads to electromagnetic torque, which is necessary for the motor rotation. The axial electromagnetic force may cause the rotor's deflection and affect the machine's reliability. Several analysis methods of the axial electromagnetic force and structure deformation for the AFPM machine have been reported. In [1], a simplified two-dimensional (2-D) finite element method (FEM) and three-dimensional (3-D) FEM are used to analyze and design the rotor discs of a high-speed AFPM generators. In [2], the combination method of numerical calculation and FEM is used to study the stiffness of the rotors of an AFPM machine. But there still remain some problems. 3-D FEM is time consuming and 2-D FEM is not accurate enough. © 2015 IEEE. Source


Zhang Q.,Nanjing University of Information Science and Technology | Zhang Q.,Jiangsu Engineering Research Center on Meteorological Energy Using and Control | Chen Y.,Nanjing University of Information Science and Technology | Chen Y.,Jiangsu Engineering Research Center on Meteorological Energy Using and Control | And 2 more authors.
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2015

In this paper we have analyzed the effect of frequency dependent soil (FDS) on the lightning-induced voltages caused by lightning subsequent return stroke for strike to tall objects ranging from 100m to 300m. It is found that the effect of FDS on the induced voltages peak can be approximately ignored when the low frequency conductivity (LFC) is equal to or larger than 0.01S/m, and with the decrease of LFC, the effect of FDS on the lightning induced voltages is more obvious. Compared with the constant LFC, the induced voltage peak becomes less for FDS. For example, for a constant LFC of 0.001S/m, the ratio of the induced voltages peak value for FDS to that for LFC is 83.2% at the line center and 66.8% at the line end for strike to 300-m-tall object, respectively. By using the decomposition method, we divide the lightning induced voltages into two components named by the incident induced waves (Vi) related with the vertical field and scattered induced waves (Vs) related with horizontal field, and it is found that FDS results into a less initial peak of tangential horizontal field along the overhead line and further results into a less induced voltage. Also, compared FDS with LFC, the FDS reduces the disparity of lightning induced voltages caused by different tall objects. For example, for the constant LFC, the induced voltage peak for strike to 300-m-tall object is 1.69 times larger than that for strike to 50-m-tall object. However, for the case of FDS, the corresponding ratio is about 1.2. © 2015 Elsevier Ltd. Source

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