The following bibliography references 60 years of selected academic studies (with clarifying comments enclosed in {}), which confirms the benefits and challenges surrounding the brushless symmetric multiphase doubly-fed “synchronous” electric motor or generator system with active multiphase winding sets symmetrically placed on the rotor and stator assemblies to inherently maintain the same electric motor package footprint of materials, cost, and loss of all others but only with the practical enabling invention of a brushless real time emulation controller (BRTEC)) to eliminate multiphase brush-slip-ring assemblies and the known instability issues. As a result, the symmetric multiphase doubly-fed synchronous electric motor or generator system (as only now provided by SYNCHRO-SYM) provides at least provides double power density and octuple peak torque, while improving efficiency and lowering cost, over all other electric motors per unit of power rating:

“The double-armature machine {similarly known as the symmetric synchronous doubly-fed electric motor as only provided by SYNCHRO-SYM} has many merits…continuous power rating is double…in addition…a maximum pull-out torque of…eight times nominal frame size torque rating.” [See page 95, 1st column, paragraph 3] [1]

“…the power electronic converter {of SYNCHRO-SYM} only has to handle a fraction of the total power…losses in the power electronic converter can be reduced…the cost…becomes lower.” [See page 227, 1st column, paragraph 1] [2]

“…the doubly-fed synchronous electric machine {of SYNCHRO-SYM}…which allows full advantage…from the possibility of delivering energy to both the rotor and stator…create unstable operation…the desirable steady-state features of this machine are not available…” [See page 653, 1st column, paragraph 1] [3]

“The operation of an ideal control circuit would be independent of the amplitude and frequency of the input signal.” [See page 656 3rd column paragraph 1] [3]

“The controller requires too many measurements and off-line computations… {unlike BRTEC of SYNCHRO-SYM, the field-oriented controller (FOC) is not brushless, uniformly bidirectional, instantaneous, sensor-less, automatic, which quickly leads to instability} [See page 1651, 1st column, paragraph 3] [4]

“The problem of inherent instability…uncontrollable torque angle is an old one … the problem of accelerating the machine and synchronizing it to the power system has continued.” [See page 526, Column 2, Paragraph 2] [5]

“Since realization of such a control which requires…zero time is almost impossible…” [See page 803 paragraph 6] [6]

“…peak-power capability of this machine when acting as a motor is greater than that of any comparable form of machine.” [See Abstract] [7]

“The power converters rating…is substantially lower than the machine rating…” [See page 787 column 1, paragraph 1] [8]

“The higher cost of the machine (due to the slip rings of the Doubly-fed machine) is compensated by a reduction in the sizing of the power converters {of the Doubly-Fed Machine}.” [See page 414, column 2, paragraph 3] [9]

With the formidable challenges of inventing a practical BRTEC followed by the timely discovery of a practical rare-earth permanent magnets or RE-PM (circa 1980) that eliminated the cost, size, and loss of electric motor magnetizing MMF, research on the pinnacle of electric motor systems, which is the symmetric multiphase doubly-fed synchronous electric motor or generator system as only now provided by SYNCHRO-SYM, was effectively abandoned in favor of RE-PM electric motor systems. Because of blantant disregard to the environmental impact and human suffering of mining and manufacturing RE-PMs, the CCP has become the major producer of RE-PMs with serious geopolitical consequences, such as insidiously becoming the market controller of RE-PMs and the major manufacturer and researcher for all electric motor systems, which are propelling the revolution in electric transportation. For example, the cost, size, and loss of magnetizing MMF are being retrofitted into RE-PM electric motor systems to gain the coveted benefit of field weakening but ironically, field weakening (as only provided by magnetizing MMF) was already more reliably, efficiently, and cost effectively available with inductions electric motor systems, which could be comparably optimized.


[1]   W. F. Long and N. L. Schmitz, Cycloconverter Control of the Doubly-fed Induction Motor,” IEEE Transactions on Industry and General Applications, Vol. IGA-7, No. 1, January/February 1971, Page 95-99.

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

[3]   D. W. Novotny and N. L. Schmitz, “Parametric Pump-Down of Synchronous Machine Oscillations,” AIEE Great Lakes District Meeting, Fort Wayne, Ind., April 25-27, 1962. Page 652-657.

[4]   Y.L. Abdel-Magid, A. H. M. A. Rahim, M. A. Al-Yadoumi, “A Quasi-optimal Stabilizing Control of Power Systems With Dual-Excited Machines,” 1991 IEEE Industry Applications Society Annual Meeting, 28 Sept-4 Oct 1991, ISBN: 0-7803-0453-5.

[5]    Norbert L. Schmitz and Willis F. Long, “The Cycloconverter driven Doubly-fed Induction Motor,” IEEE Transactions on Power Apparatus And Systems, Vol. PAS-90, No. 2, March/April 1971, pp. 526-531

[6] A.H.M.A Rahim, “Stabilizing Controls for Doubly Fed Synchronous-Induction Machines,” IEEE Transactions on Energy Conversion, Vol. 3, No. 4, December, 1988, pp. 799-803.

[7]    Bird, B.M. Burbidge, R.F., Analysis of doubly fed slip-ring machines, Electrical Engineers, Proceedings of Institution of, Vlume: 113 Issue: 6, June, 1966, pp.1016-1020.

[8] 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.

[9] “Variable-Speed Wind Power Generation Using Doubly Fed Wound Rotor Induction Machine – A Comparison With Alternative Schemes,” Rjib Datta and V. T. Ranganathan, IEEE Transactions On Energy Conversion, VOL. 17, NO. 3, September 2002, pp. 414-421. http://ieeexplore.ieee.org/xpl/periodicals.jsp.