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Rare-Earth Permanent Magnets (RE-PM):


High performance rare-earth permanent magnets (RE-PM) with the high coercivity only associated with expensive dysprosium-doped neodymium and samarium cobalt rare earth materials: a) establish similar air-gap flux density to any other electric machine (EM) or electric machine system (EMS) but within a smaller footprint than an electromagnet[1] (e.g., up to a 20% smaller footprint depending on air-gap depth); and b) eliminate magnetizing magneto-motive-force (MMF)[2], which reduces electrical loss and improves electric machine efficiency, but as already discussed, the essential stator armature predominantly determines the physical size of any EM.


CAVEAT: RE-PMs are "passive" devices that only occupy precious EM real-estate, such as the rotor real estate, without "actively" participating in the energy conversion process (e.g., after all, PM have no ports for electrical-mechanical power flow or fuel). As a passive device, a PM by itself cannot produce or consume energy over time (power). Also, the remanence (or flux density) of RE-PMs with acute sensitivity to temperature cannot achieve the same level of flux density potential as a relatively temperature insensitive electromagnet[3] (e.g., after all, it is an electromagnet that magnetizes the PM beyond remanence). Although RE-PMs eliminate magnetizing MMF, which was always associated with the lower efficiency of the PM-EMS nemesis, which is the Induction EMS (I-EMS), a newer family of RE-PM EMS(s), sometimes called a hybrid-field EMS, are ironically re-introducing magnetizing MMF (with virtually the same associated inefficiency and increased size as the I-EMS replaced) to provide field weakening capability to the PM-EMS for at least increasing the speed-range (already provided by the I-EMS without expensive RE-PM). The safety and difficult manufacturing associated with persistent magnetic fields of RE-PMs, the environmentally unfriendly mining methods associated with extracting the raw RE-PM materials, and the cartel control of the materials are serious issues with no foreseeable relief.[4]. Finally, a fully electromagnetic alternative, such as Synchro-Sym, saves precious rare earth (RE) materials for more strategic technologies without alternatives.


NOTE: Regardless of vehicle type, such as electric or conventional vehicles, the electric drivetrain, which includes electric motor and generator systems, in contrast to a mechanical drivetrain is the simplest, most reliable, and most efficient form of drivetrain, particularly if regeneration capability and micro-integrated electronic control are considered. When the transition to the electric drivetrain for all vehicles occurs, the total known global RE-PM mining capacity is not expected to meet the demand and as a result, the expected RE-PM cost will continue to increase for the foreseeable future. Demonstrating the relentless intrigue surrounding the RE-PM, some motor manufacturers are now studying ferrite PM as a replacement for RE-PM but the low B-H product of the ferrite PM make the ferrite PM electric machine system non-competitive with an "optimally" designed Induction electric machine system.


[1] An electromagnet is a winding with applied Magneto-Motive-Force (MMF), which is the product of current and the number of winding-turns.

[2] Magneto-Motive-Force (MMF) is the product of current and the number of winding-turns. The two orthogonal types of MMF for electric machines are Torque MMF, which is active or real MMF, and Magnetizing MMF, which is imaginary MMF. Although Induction EMS provide 5-30% of the Torque MMF as Magnetizing MMF, vector arithmetic shows the total MMF magnitude is  (MMFT2+MMFM2)1/2 = ((1)2 + (0.3)2)1/2 = 1.04 or only 4% more MMF magnitude with a 30% magnetizing MMF or only 9% more current and electrical loss than a PM EMS without Magnetizing MMF.  

[3] In most cases, air-gap flux density is bounded by the saturation limits of the electrical steel material of the core and not by the remanence of PM(s) or the flux density potential of electromangnets.

[4] Because of the progressively high cost of RE-PM, PM-EMS manufacturers are considering Ferrite PMs as a RE-PM alternative but because of the low energy product, Ferrite PM EMS are larger, less efficient, and more costly than even the Induction EMS.

[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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