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 Wound-Rotor [Synchronous] Doubly-Fed (WRSDF):


The attractive attributes of the wound-rotor [synchronous] doubly-fed (WRSDF) electric machine system (EMS), such as highest power density, highest efficiency, and lowest cost, have been known by experts since at least the early 1960's; however, true WRSDF operation was foreseen to be only possible with the future invention of brushless real-time control (BRTC) as only provided by the patented Synchro-Sym EMS (SS-EMS):


“Recently, it has been shown that a grid-connected wound-rotor [doubly-fed induction] motor with current injection on the rotor side can be operated in super-synchronous mode to produce up to two times the rated nominal power” [1] [see page 1089, column 2, 1st  paragraph]


 “Since the full torque can be obtained up to twice rated speed, the power output (of the doubly-fed electric machine), can be up to twice the rating of the machine.” [2] [See page 1329, 2nd column, paragraph 3]


Another well known attribute of the SS-EMS is the low power, low cost electronic control:


“ ..each inverter rated for 1 p.u. [1/2 power rating] is capable of producing rated torque at rated frequency…Thus, the motor output power becomes twice the rated power.” [1] [See page 1093, column 2, 1st paragraph]


"The rating of the power converters used in the rotor circuit (of the doubly-fed motor) is substantially lower than the machine rating and is decided by the range of operating speed (around synchronous speed)." [3]


Doubly-Fed Machine – “…This is mainly due to the fact that the power electronic converter only has to handle a fraction (20-30%) of the total power. Therefore, the losses in the power electronic converter can be reduced, compared to a system (singly-fed) where the converter has to handle the total power. In addition, the cost of the converter becomes lower.” [4] [See page 227, column 1, 1st paragraph]


The wound-rotor [Induction or Asynchronous] doubly-fed electric machine system (WRIDF) or any derivative of the so-called brushless or unlike pole-pair doubly-fed electric machine systems (BDFM) do not show the attractive attributes of the wound-rotor [Synchronous] doubly-fed electric machine or SS-EMS:


“One notable development in this direction is the two-stator (self-cascaded) induction machine…the stator winding into two displaced identical windings …is actually a drawback…The reason is the mutual coupling between the two stator windings will cause circulating harmonic currents which will lead to extra stator losses.” [5] [See page 315, column 1, 1st and 2nd paragraph]


[1] “Sensorless Field-Oriented Control for Double-Inverter-Fed Wound-Rotor Induction Motor Drive,” Gautam Poddar and V. T. Ranganathan, IEEE Transactions On Industrial Electronics, VOL. 51, NO. 5, October 2004, pp. 1089-1096.

[2] “Direct Torque and Frequency Control of Double-Inverter-Fed Slip-Ring Induction Motor Drive,” Gautam Poddar and V. T. Ranganathan, VOL. 51, NO. 6, December 2004, pp 1329-1337.

[3] Rajib Datta and V.T. Ranganathan, "A Simple Position-Sensorless Algorithm for Rotor-Side Field-Oriented Control of  Wound-Rotor Induction Machines," IEEE Transactions On Industrial Electronics, Vol. 48, No. 4, August, 2001, pp. 786-793.

[4] “Evaluation of Current Control Methods for Wind Turbines Using Doubly-Fed Induction Machines,” Andreas Petersson, Lennart Harnefors, and Torbjorn Thiringer, IEEE Transactions On Power Electronics, Vol. 20, No. 1, January 2005, pp. 227-235.

[5] “A New Energy Recovery Double Winding Cage-Rotor Induction Machine,” Lili Bu, Chun Li, Jeff Krukowski, Wilsun Xu, and Xian Liu, IEEE Transactions On Energy Conversion, VOL. 18, NO. 2, June 2003, pp. 315-320.

[Just The Facts For Comparisons]

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Note: The Radial Flux prototyping of the Advanced Brushless Wound-Rotor Synchronous Doubly-Fed Electric Motor Or Generator System is shown as our Icon, which predates the axial-flux SS-EMS Technology.


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