SIXTY YEARS OF APPLIED SYNCHRO-SYM STUDY

For more than sixty years, universities, laboratories, and industry researchers investigated electric machines employing the electromagnetic symmetry of both an active stator and an active rotor because they promised substantially higher utilization of the electromagnetic air gap than conventional passive-rotor machines. Although the electromagnetic principles were well understood, practical implementation remained limited by the inability to economically control an active rotor without brushes outside of controlled laboratory conditions. Advances in power electronics, digital control, and BRTEC™ now provide a practical commercialization path. With clarifying facts enclosed in [] and comments enclosed in {}, the following bibliography is a short list of these investigations:

* [DOUBLE CONTINUOUS POWER DENSITY and EIGHT TIMES PEAK TORQUE DENSITY] “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, 1^st column, paragraph 3] [1]

* [LOWER COST] “…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, 1^st column, paragraph 1] [2]

* [“SYNCHRONOUS DOUBLY-FED”] “…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, 1^st column, paragraph 1] [3]

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

* [REAL TIME CONTROL] “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, 1^st column, paragraph 3] [4]

* [INSTABILITY WITHOUT REAL TIME CONTROL] “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]

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

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

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

* [LOWER SIZE] “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]

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These publications do not validate SYNCHRO-SYM itself. Rather, they confirm the exceptional performance attributes and practical implementation challenges surrounding the brushless symmetric multiphase doubly-fed “synchronous” electric motor or generator system (S-EMS) with the unique, optimal electromagnetic symmetry of independently-excited multiphase (or active) winding sets symmetrically placed on the rotor and stator assemblies, respectively, which inherently maintains the same electric motor package footprint of materials, cost, and loss as conventional electric machine systems with the asymmetry of a passive rotor (A-EMS) but inherently doubles the working electromagnetic power. But the S-EMS is only possible with the practical enabling invention of a brushless real time emulation controller (BRTEC) to eliminate the multiphase brush-slip-ring assembly and its known instability issues of “synchronous” operation from sub-synchronous to super-synchronous speeds, including at (or about) synchronous speed, when subjected to random line or shaft perturbations.

By hypothesizing a stabilizing brushless real-time emulation control means, the fundamental classical study of AC electric machines begins with the electromagnetic symmetry of active winding sets placed on the rotor and stator assemblies, respectively. With an “active rotor,” the symmetric multiphase doubly-fed “synchronous” electric motor or generator system (S-EMS) becomes the foundational study for all other electric machine systems of today by deoptimizing its electromagnetic symmetry with the non-optimal electromagnetic asymmetry of a “passive rotor” employing permanent magnets, reluctance saliencies, DC field windings, or slip-induction windings. The passive rotor of the asymmetric EMS (A-EMS) cannot contribute working electromagnetic power or torque to the electromechanical energy conversion process, together with the universally essential active stator, but still consumes reasonably half of the A-EMS loss, cost, and size. In contrast, the active-rotor of the S-EMS contributes additional working electromagnetic power and torque to the electromechanical energy conversion process, together with the universally essential active stator. Inherently, the S-EMS delivers twice the constant-torque speed range with a given torque, voltage, and frequency of excitation as the passive rotor A-EMS (i.e., 7200 RPM at 60Hz with one pole-pair for the S-EMS versus 3600 RPM for the A-EMS), which is tantamount to double the power density and octuple the peak torque at half the cost and half the loss per unit of power rating with the same packaging and design rating of the A-EMS.

With the formidable challenges of inventing a practical BRTEC with the limited technology of the time, followed by the timely discovery of a practical rare-earth permanent magnets or RE-PM (circa 1980s) that would effectively eliminate the cost, size, loss, and provisioning of electric motor magnetizing MMF, research on the pinnacle of electric motor systems, which is the S-EMS, was abandoned and left to obscurity in favor of the RE-PM A-EMS without forecasting today’s environmental impact, human exploitation, and geopolitical consequences of mining and manufacturing RE-PMs.

Widely regarded as today’s highest-performing and most efficient electric-machine architecture, the rare-earth permanent-magnet (RE-PM) passive-rotor A-EMS has become the dominant propulsion technology because it generally outperforms induction, reluctance, and field-wound A-EMS alternatives. Ironically, however, many manufacturers are now reintroducing synthesized field-excitation to recover the highly desirable field-weakening capability that was largely sacrificed when permanent magnets displaced induction, reluctance, and field-excited A-EMS designs. In doing so, they also reintroduce much of the cost, size, complexity, and excitation losses that permanent magnets originally sought to eliminate. At the same time, growing geopolitical uncertainty and supply-chain risks associated with rare-earth permanent-magnet materials are increasing demand for propulsion systems that eliminate—or substantially reduce—their dependence on rare-earth permanent magnets.

Conventional slip-ring and so-called brushless doubly-fed machines remain passive-rotor induction A-EMS architectures. In contrast, the S-EMS, uniquely embodied by SYNCHRO-SYM™, employs an active stator and an active rotor operating under coordinated brushless, sensorless real-time emulation control. Both the rotor and stator independently contribute working electromagnetic power while inherently providing field-weakening capability without relying on slip-induced rotor currents. Unlike every passive-rotor A-EMS alternative, SYNCHRO-SYM™ eliminates rare-earth permanent magnets by employing an active-rotor S-EMS architecture that inherently provides field-weakening capability while delivering the performance advantages of the Symmetric Electric Machine System.

The historical record demonstrates that active-rotor electric machines have been investigated for decades because of their theoretical advantages. Advances in digital control, power electronics, and BEM’s BRTEC™ architecture now provide a practical path toward commercial implementation. SYNCHRO-SYM represents BEM’s extensive ongoing research and invention effort to translate this long-standing body of research into a manufacturable electric-machine platform for transportation, aerospace, industrial, and defense applications.

SYNCHRO-SYM™ is not presented as a rejection of six decades of electric-machine research, but as its commercial continuation—combining established electromagnetic principles with modern control technology and BEM invention to realize a practical active-rotor electric machine system.

The SYNCHRO-SYM™ electromagnetically symmetric active rotor architecture is fundamental rather than incremental. BEM’s invention is BRTEC™, the practical enabling control technology that makes this historically recognized architecture commercially realizable.

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References

[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.