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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Maximum power transfer theorem ： ウィキペディア英語版 Maximum power transfer theorem In electrical engineering, the maximum power transfer theorem states that, to obtain ''maximum'' external power from a source with a finite internal resistance, the resistance of the load must equal the resistance of the source as viewed from its output terminals. Moritz von Jacobi published the maximum power (transfer) theorem around 1840; it is also referred to as "Jacobi's law". The theorem results in maximum ''power'' transfer, and not maximum ''efficiency''. If the resistance of the load is made larger than the resistance of the source, then efficiency is higher, since a higher percentage of the source power is transferred to the load, but the ''magnitude'' of the load power is lower since the total circuit resistance goes up. If the load resistance is smaller than the source resistance, then most of the power ends up being dissipated in the source, and although the total power dissipated is higher, due to a lower total resistance, it turns out that the amount dissipated in the load is reduced. The theorem states how to choose (so as to maximize power transfer) the load resistance, once the source resistance is given. It is a common misconception to apply the theorem in the opposite scenario. It does ''not'' say how to choose the source resistance for a given load resistance. In fact, the source resistance that maximizes power transfer is always zero, regardless of the value of the load resistance. The theorem can be extended to alternating current circuits that include reactance, and states that maximum power transfer occurs when the load impedance is equal to the complex conjugate of the source impedance. == Maximizing power transfer versus power efficiency == The theorem was originally misunderstood (notably by Joule) to imply that a system consisting of an electric motor driven by a battery could not be more than 50% efficient since, when the impedances were matched, the power lost as heat in the battery would always be equal to the power delivered to the motor. In 1880 this assumption was shown to be false by either Edison or his colleague Francis Robbins Upton, who realized that maximum efficiency was not the same as maximum power transfer. To achieve maximum efficiency, the resistance of the source (whether a battery or a dynamo) could be made close to zero. Using this new understanding, they obtained an efficiency of about 90%, and proved that the electric motor was a practical alternative to the heat engine. File:Source and load circuit.png File:Maximum Power Transfer Graph.svg The condition of maximum power transfer does not result in maximum efficiency. If we define the efficiency as the ratio of power dissipated by the load to power developed by the source, then it is straightforward to calculate from the above circuit diagram that :$\backslash eta\; =\; \backslash frac\; +\; R\_\backslash mathrm\}\; =\; \backslash frac\}.$ Consider three particular cases: * If $R\_\backslash mathrm\; =\; R\_\backslash mathrm$, then $\backslash eta\; =\; 0.5,$ * If $R\_\backslash mathrm\; =\; \backslash infty$ or $R\_\backslash mathrm\; =\; 0,$ then $\backslash eta\; =\; 1,$ * If $R\_\backslash mathrm\; =\; 0$, then $\backslash eta\; =\; 0.$ The efficiency is only 50% when maximum power transfer is achieved, but approaches 100% as the load resistance approaches infinity, though the total power level tends towards zero. Efficiency also approaches 100% if the source resistance approaches zero, and 0% if the load resistance approaches zero. In the latter case, all the power is consumed inside the source (unless the source also has no resistance), so the power dissipated in a short circuit is zero. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Maximum power transfer theorem」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.