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Performance Comparison Between Electric Motors or Generators:




Bottom line



There are only three means to improve cost-performance of any electric machine system:

  1. Optimize with the same packaging techniques available to all that improve footprint (e.g., axial-flux or disk shape), tighten winding sets, utilize the best electrical core materials, improve frame and bearing assembly for tighter air-gap, improve cooling, etc.

  2. Incorporate brushless wound-rotor doubly-fed synchronous electric machine technology as only provided by the patented Synchro-Sym with Brushless Real Time Control (BRTC) for twice the constant-torque speed (or power) with a given pole count and frequency of excitation.

  3. Incorporate advanced manufacturing techniques, such as 3D Printing as only provided by MotorPrinter, that enable electric machine manufacture with advanced core materials.


Only Synchro-Sym of Best Electric Machine successfully comprises all three means!   



"Constant-Torque Power Rating" for an Equitable Performance Baseline:



To guarantee an equitable comparison between electric machine system contestants without concern of marketing manipulation:


Analyze the loss, cost, and size of all "electric machine system” contestants with the same given constant-torque that is designed to the same speed, the same frequency of excitation, the same port voltage, and the same duty cycle (e.g., temperature rise, etc), which is the constant-torque design of the stator armature.[3]


Otherwise, contestants are allowed to quote peak torque, peak horsepower, peak speed, etc. by fiat.


Note: Enabled by its simple wound-rotor synchronous doubly-fed physics of operation as only provided by the patented brushless real time control (BRTC) as hypothesized by electric machine experts since the 1960's, Synchro-Sym EMS technology shows twice the constant-torque speed range for a given frequency of excitation and physical size.  As a result, Synchro-Sym EMS technology shows a significant improvement in cost-performance (e.g., nearly double) over any other type of electric machine system, including the rare-earth permanent magnet EMS, by reasonably assuming that the rotor and stator occupy the same physical volume as is always the case for the axial-flux (or disk shape) footprint.


Without a common standardized metric for equitable comparison between electric machine (EM) system contestants, specialty electric machine system (EMS) manufacturers, such as manufactures serving the electric vehicle industry, provide impressive electric machine system (EMS) power densities, efficiencies, and costs seemingly by fiat, such as specifying impressive peak torque or peak power without detailing the impractical short duty cycle of operation before destruction or without holistically including the compounded inefficiency, size, or cost of the necessary active cooling methods, the transmissions, or the electronic control for practical application. For instance, a 95% efficient EM may sound impressive but when mated with an equally impressive 95% efficient electronic controller for practical operation, the actual compounded EMS efficiency is significantly reduced to 90% (95% x 95%).


For a reasonable cost-performance comparison between electric machine systems, the following should be always be considered:

  • All magnetic electric motors and generators (i.e., electric machines) must obey the same laws of physics while leveraging the same packaging techniques. But some electric machines (EM), such as the reluctance, permanent magnet, and doubly-fed EM, etc., are impractical without an electronic control system. With the exception of the protected intellectual property of Synchro-Sym Technologies, all electric machine systems (EMS) follow the same electronic control methods, which are always a derivative of field oriented control.

  • All electric machine system (EMS), including superconductor or rare-earth permanent magnet (RE-PM) EMS(s), have a conventional stator armature (or multiphase AC winding set that is capable of flowing bi-directional magneto-motive-force (MMF) to produce (generating) or consume (motoring) energy over time (power) through an independent electrical excitation port), which can potentially provide higher flux density than any RE-PM, and as a result, the traditional stator armature ultimately determines the air-gap area (size) and efficiency of the EM with the high coercivity of RE-PMs being less relevant.

  • Virtually all EM(s) have the same highly permeable laminated electrical steel core with the same saturation limit and with the same shallow air-gap that in accordance to Ampere Circuital Law significantly reduces armature magneto motive force (MMF). Therefore, it is unfair (and unrealistic) for PM-EMS manufacturers or even the US Department of Energy [1] to compare the performance of a standalone RE-PM to a standalone electromagnet that is outside of its design environment, such as without a shallow air-gap or without an electrical steel core that is tantamount to an impractically large air-gap depth and MMF.   In reality, the high cost of RE-PM(s) have compelled motor manufacturers to revisit the induction electric machine design, which is now showing nearly similar efficiency and power density as the RE-PM EMS, as physics and packaging techniques always suggested.

  • The necessary slot volume where the winding conductors reside and the necessary frame for precision bearing and structural support of the air-gap contribute a significant portion of any EM real-estate size and weight. Based on the air-gap flux density, the static attractive magnetic forces between adjacent air-gap core faces (and not the rotational forces) require a robust frame and bearing assembly while contributing nothing (passive) to the electromechanical conversion and power production. Any suggested means to specifically utilize the attractive magnetic forces in the the electromechanical conversion (e.g., power production) must satisfy the laws of conservation with equal electrical power rating, which would mitigate the single effect.  Slot shapes are important for providing better winding packing, better utilization of the air-gap area, such as one wire width opening for winding placement, lower noise, and lower torque ripple. Available to virtually all electric machine types, packaging techniques are basic means to improve electric machine performance and power density; for instance, composites are being investigated to reduce structural frame size and weight.

  • Some electric machine manufacturers flaunt the use of concentrated windings, increasing the number of poles, or increasing the number of phases as value added but in fact, the mechanical form of concentrated windings is tantamount to increasing the number of phases or poles of an electric machine and changing the number of phases or poles has been a packaging method since the dawn of electric machines over a century ago. In addition, concentrated winding arrangements may increase the number of air-gaps (or air-gap depth).  

  • It is highly unfair to realistically compare the power density, cost, and efficiency between EMS(s) with different "operational speeds for a given excitation frequency," particularly without at least including into the system performance the compound affects of the power density, cost, and efficiency of the necessary transmission and electronic controller to match the high speed to the load for practical application, such as comparing an 1800 RPM Induction EMS to a 10,300 RPM RE-PM EMS or even comparing a 10,300 RE-PM EMS to a 40,000 RPM RE-PM EMS motor.[2] Higher speeds of operation without considering the transmission to translate the application speed, if needed, is a clear means to improve the power density, cost, and efficiency of any EMS.  For instance, the Synchro-Sym EMS (SS-EMS) is the only EMS with inherently twice the constant-torque speed range for a given frequency of excitation [or twice the synchronous speed for a given frequency of excitation] and as a result, inherently shows nearly twice the power density of any other EMS with the same packaging techniques. In fact, today's method of reducing the amount (and cost) of RE-PM material in an EMS is by increasing "speed of operation and frequency of excitation" to make the RE-PM EMS smaller in order to use less high cost RE-PM material; but the compounded effects of the transmission for practical "system" application are rarely included in the overall "system" analysis, such as the obvious additional size of the transmission with respect to the high speed EMS.

  • The electric motor for electric propelled airplanes is a clear example of an electric machine system packaging dilemma. Without dramatic improvement in material science, efficient electric motors always incorporate copper (for winding conductors) and electrical steel, which are incompatibly heavy materials to the airplane platform. Just reducing the amount of these materials will reduce the performance of the EMS. Active multiphase alternating current windings, which are the main component in any electric machine, are not compatible with superconductors. DC field windings or permanent magnets are passive devices that only consume real-estate (and some harmonic loss) without directly contributing to active power production. Only the wound-rotor [synchronous] doubly-fed electric machine system (as only provided by the patented SS-EMS technology) with independently excited multiphase AC windings on the rotor and stator respectively, gives twice the constant torque speed range for a given frequency of excitation, which immediately doubles the power rating for a given electric machine weight and size by reasonably assuming the rotor and stator occupy the same physical volume as is the case for the axial-flux (or disk shape) footprint.  SS-EMS Technology should be the propulsion choice for electrically propel airplanes. 

  • All electric machines can be categorized as singly-fed with one armature, which comprises a multiphase AC winding set with an independent electrical excitation port, or doubly-fed with two armatures with at least one of the armatures generally as the stator. Electric machines relying solely on passive permanent magnets are singly-fed. Electric machines relying solely on a passive DC electromagnet (with a slip-ring assembly) are singly-fed (and not doubly fed as some incorrectly assume because the DC electromagnet is without an multiphase AC excitation port). Reluctance or Induction electric machines may be singly-fed or doubly-fed but will always have a passive rotor assembly to promote back electromotive force (EMF) as a result of operational reliance on induction due to the asynchronous speed between the rotor and stator (i.e., slip or speed-based induction). Only the multiphase AC wound rotor electric machine (with of course a multiphase AC wound stator) could be an asynchronous (i.e., induction) doubly-fed electric machine (with operational reliance on slip-induction) or a synchronous doubly-fed electric machine (without operational reliance on slip-induction but may experience slip-induction).


Note: Once an electric machine is designed to the specified port voltage, constant-torque, and excitation frequency for the same constant torque speed range and temperature rise and is cost-effectively optimized with the best packaging techniques available to all, such as construction techniques, winding arrangements, and materials, "electrical" efficiency can only be improved by increasing the amount of copper in the windings, which increases winding conductor size and slot size, or by increasing the amount electrical steel in the wound core, both of which increase size and weight of the entire EM.  Said differently, there is always a tradeoff between high efficiency and small  size once the EM design is optimized with the best available packaging techniques.


[1] Susan Rogers, "Advanced Power Electronics and Electric Motors Program," Annual Progress Report, US Dept. Of Energy, Energy Efficiency and Renewable Energy, DOE/EE 1040, 2013 Vehicle Technologies Office, pp 162.

[2] Susan Rogers, "Advanced Power Electronics and Electric Motors Program," Annual Progress Report, US Dept. Of Energy, Energy Efficiency and Renewable Energy, DOE/EE 1040, 2013 Vehicle Technologies Office, pp 168.

[3]  Preferably, the duty cycle is measured without active cooling to avoid qualifying the contestant's active cooling technology instead of the performance of the EMS.

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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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