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Doubly fed electric machines are electric motors or electric generators that have windings on both stationary and rotating parts, where both windings transfer significant active power between shaft and electrical system. Usually the stator winding is directly connected to the three-phase grid and the three-phase rotor winding is fed from the grid through a rotating or static frequency converter. Doubly fed machines are typically used in applications that require varying speed of the machine's shaft in a limited range around the synchronous speed, for example ± 30%, because the power rating of the frequency converter is reduced similarly. Today doubly fed drives are the most common variable speed wind turbine concept. ==History== The doubly fed machine has its origins in wound rotor induction motors with multiphase winding sets on the rotor and stator, respectively, with the rotor winding set connected to resistors via slip rings for starting. However, the slip power was lost in the resistors. Thus means to increase the efficiency in variable speed operation by recovering the slip power were developed. In Krämer (or Kraemer) drives the rotor was connected to an AC and DC machine set that fed a DC machine connected to the shaft of the slip ring machine.〔Leonhard, W.: Control of Electrical Drives. 2nd Ed. Springer 1996, 420 pages. ISBN 3-540-59380-2.〕 Thus the slip power was returned as mechanical power and the drive could be controlled by the excitation currents of the DC machines. The drawback of the Krämer drive is that the machines need to be overdimensioned in order to cope with the extra circulating power. This drawback was corrected in the Scherbius drive where the slip power is fed back to the AC grid by motor generator sets.〔(Shively, E. K.; Whitlow, Geo. S.: Automatic Control for Variable Ratio Frequency Converters. ) Transactions of the American Institute of Electrical Engineers, Volume: 51 Issue: 1 Date: March 1932 , Page(s): 121 - 127.〕〔(Liwschitz, M. M., Kilgore, L. A.: A Study of the Modified Kramer or Asynchronous-Synchronous Cascade Variable-Speed Drive. ) Transactions of the American Institute of Electrical Engineers, Volume: 61, Issue: 5, May 1942, Page(s): 255 - 260.〕 The rotating machinery used for the rotor supply was heavy and expensive. Improvement in this respect was the static Scherbius drive where the rotor was connected to a rectifier-inverter set constructed first by mercury arc-based devices and later on with semiconductor diodes and thyristors. In the schemes using a rectifier the power flow was possible only out of the rotor because of the uncontrolled rectifier. Moreover, only sub-synchronous operation as a motor was possible. Another concept using static frequency converter had a ''cycloconverter'' connected between the rotor and the AC grid. The cycloconverter can feed power in both directions and thus the machine can be run both sub- and oversynchronous speeds. Large cycloconverter controlled doubly fed machines have been used to run single phase generators feeding 16 2/3 Hz railway grid in Europe 〔(Pfeiffer, A.; Scheidl, W.; Eitzmann, M.; Larsen, E.: Modern rotary converters for railway applications. ) Proceedings of the 1997 IEEE/ASME Joint Railroad Conference, March 18–20, 2007, Boston, MA, Pages: 29-33.〕 and run the turbines in pumped storage plants.〔(A. Bocquel, J. Janning: 4 *300 MW variable speed drive for pump-storage plant application. ) EPE Conference 2003, Toulouse.〕 Today the frequency changer used in applications up to few tens of megawatts consists of two back to back connected ''IGBT'' inverters. Several brushless concepts have also been developed in order to get rid of the slip rings that require maintenance. 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「Doubly fed electric machine」の詳細全文を読む スポンサード リンク
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