Letter to the Editor - Why Bother With Rare-Earth Permanent Magnet Electric Motor Or Generator Systems?
High energy product Neodymium rare-earth permanent magnets (RE-PM) were discovered in the early 1980s and later, Dysprosium doping was discovered to provide reasonable RE-PM operating life by mitigating demagnetization with improved coercivity and high-temperature operation; but also, sacrificed the precious high energy product. Despite ongoing research and development (R+D) for at least a viable Dysprosium alternative, strategically tailoring doping with extremely rare and expensive Dysprosium provides the best permanent magnets for a practical RE-PM electric motor and generator (or electric machine) system (EMS), which is now considered to be the highest performing (and most expensive) EMS available.
Since the discovery of RE-PMs, which were economically viable at the time and provided the allure of eliminating the electrical provisioning and inefficiency of magnetizing magneto-motive-force (MMF), virtually all EMS R+D was blindly directed to the RE-PM EMS (RE-PM-EMS) without considering the obvious environmental issues and political consequences of the RE-PM supply chain. As supporting evidence, today’s EMS R+D is again blindly directed to awkwardly introducing magnetizing MMF into the RE-PM-EMS for its coveted attributes of improving EMS efficiency and reliability at various speed by varying air-gap flux density, which ironically disregards the original intent of eliminating magnetizing MMF most conveniently provided by the slip-induction (or “asynchronous”) EMS being replaced.
With the ever-increasing application of the RE-PM-EMS, it became evident that the RE-PM global supply chain could not satisfactorily meet demand. Now with strategic ownership of the global Neodymium and Dysprosium reserves, China controls the ever-rising price of RE-PMs and also, to the chagrin of the original R+D investors, China is the de facto manufacturer of the RE-PM-EMS for the foreseeable future. In response, EMS R+D, such as the 2011 ARPA-E REACT program, is devoted to finding RE-PM material reduction or alternatives but after nearly a decade, the results are mixed. For example, one aspect of the program developed high-frequency electronic control for a higher speed (with comparable power) and therefore smaller (with less RE-PM material) RE-PM-EMS but seemingly overlooked the “compounded” inefficiency, cost, and size of the necessary transmission addition on the overall system. More troubling, the RE-PM-EMS has become the EMS of choice for at least electric vehicles and advanced wind turbines with “recycling” RE-PM-EMSs (with finite magnetization life expectancy) as a helping supply chain solution.
The Rare-Earth Permanent Magnet Solution:
While “Innovating for our clean, efficient, and sustainable energy future,” Best Electric Machine (BEM) pursued R+D into the symmetrical multiphase wound-rotor “synchronous” doubly-fed EMS (S-MWRSDF-EMS), which is without exotic or delicate components, such as RE-PMs. Since at least the 1960s, electric machine experts have theoretically considered the S-MWRSDF-EMS to be the best EMS possible, because of the optimized electromagnetic symmetry of electrically accessible balanced multiphase winding sets on each side of the air-gap between the rotor and stator bodies, respectively, but only with the invention of a real-time (or instantaneous) emulation (or sensor-less and automatic) control means to eliminate the slightest operational control dependency on the deoptimizing electromagnetic asymmetry of slip-induction. As a result, stable and contiguous “synchronous” doubly-fed control of air-gap flux (for field weakening), speed, torque, and torque angle (for leading-to-lagging power factor) from zero (or sub-synchronous) to super-synchronous speed is guaranteed, particularly about synchronous speed where slip-induction becomes unstable or ceases to exist. The hypothetical real-time emulation control means was far beyond any technology of the time or even today’s state-of-art field-oriented control (FOC). As a result, virtually all multiphase wound-rotor doubly-fed EMSs of today rely on “slip-induction” with the classic potential of unstable rotor oscillation from the slightest rotor perturbation (unless mechanically damped as in modern wind turbine “generator” system).
By brushlessly, sensorlessly, automatically, and instantaneously providing bi-directional, multiphase excitation power (with speed synchronized frequency and phase) “directly” to at least the rotor active multiphase winding set at any speed, including at synchronous speed, and thereby, eliminating the multiphase slip-ring-brush assembly and control dependency on slip-induction, the patented SYNCHRO-SYM (as only provided by the Multiphase Brushless Real-Time Emulation Controller or BRTEC) is the only brushless and true S-MWRSDF-EMS. As a conceptual demonstration, envision the common axial-flux RE-PM-EMS footprint (or pancake motor) comprising adjacent rotor and stator disks separated by an air-gap and electronically controlled by FOC. It is reasonable to assume that 1) the physical size and the amount (and cost) of materials, such as frame, electrical steel, and copper or RE-PMs, are similar between the “active” stator disk (that actively generates or motors electromechanical power via an active multiphase winding set) and the “passive” rotor disk (that dissipates electromechanical power via RE-PMs or slip-ring assemblies, reluctance saliencies, field windings, or slip-induction multiphase windings), and 2) the power rating and associated electrical and core losses are determined by the “active” stator disk. By simply replacing the “passive” rotor disk with another “active” stator disk and replacing FOC with BRTEC, the resulting SYNCHRO-SYM with the continuous power rating of two similar “active” winding sets on the stator and rotor disks, respectively, immediately provides at least the following conceptually obvious transformational advantages:
• Double the power rating within the same physical footprint and packaging techniques of the original RE-PM-EMS (or any other retrofitted EMS), such as legacy or future advanced design, material, winding, construction, or manufacturing techniques, which is tantamount to:
o Half the size, weight, or cost (per kilowatt of total power) or twice the power density at half the cost, without including the size, weight, and cost savings from eliminating the expensive, exotic, or wasteful “passive” components, such as RE-PMs;
o Up to half the electrical and core loss (per kilowatt of total power) of the RE-PM-EMS or the copper rotor slip-induction EMS, by including the electrical loss associated with the electromagnetic core, the small orthogonal vector magnitude of magnetizing MMF, and the much lower compounded system loss associated with the rotor or stator active winding set of SYNCHRO-SYM controlled by BRTEC, each of which is half the total power rating of the single stator active winding set of the RE-PM-EMS controlled by FOC;
o Double the performance enhancement (per kilowatt of total power) enabled by the best packaging techniques, which today’s most advanced EMSs are constrained to strategically use on the same century old asymmetric circuit and control architecture for their “incremental” performance enhancement or invention, such as square copper wire, etc.
• At least quadruple the peak torque density of the original RE-PM-EMS, because in accordance with the classic operating physics of the symmetrical (or dual ported) transformer circuit topology (as provided by SYNCHRO-SYM), air-gap (and core) flux density remains constant with increasing torque current beyond magnetizing MMF, which is unlike all passive rotor (or asymmetric) EMSs that quickly reach core flux saturation with increasing torque current.
By retrofitting the RE-PM-EMS (or any EMS) with SYNCHRO-SYM, the limited supply of precious and expensive RE-PMs with finite operating life are saved for other more strategic applications! Immediately providing twice the power density of the best RE-PM-EMS, electric airplanes will fly higher, faster, and longer while reliability will improve! Immediately providing quadruple the peak torque of the best RE-PM-EMS, electric vehicles will be without the compounded size, weight, cost, and loss of a transmission system to provide longer battery range!
Mr. Frederick Klatt: Chief Technology Officer (CTO) of Best Electric Machine (BEM):
· IEEE Life Member 02432870.
· Master of Science Electrical Engineering and Computer Science, University of California, Berkeley.
· Forty years of research and development and production experience as principle and founder of Engineering Devices Inc., a technology consulting firm.
· Thirty-five years of electric machine research and development and self-study.
· Presenter invitations to IEEE sponsored symposiums, e.g., SLED2012, ITEC2013, ESTS2013, IEMDC2013, & ITEC2014, with Published IEEE conference articles.
· Keeper of SYNCHRO-SYM Technologies knowledge base, IP (Patents, Trade Secrets, etc.), which includes the SYNCHRO-SYM, the only wound rotor doubly-fed synchronous electric machine, MOTORPRINTER, the only 3D Printer of amorphous metal axial-flux electric machines, BEM-CAD, SYNCHRO-SYM computer-aided design tool, etc.