ELECTRIC MACHINE DESIGN DISTINCTIONS & CONSTRAINTS

ELECTRIC MACHINE DESIGN DISTINCTIONS & CONSTRAINTS

The following notes provide a knowledge base of best practices and lessons learned for electric machine design and distinction; and accordingly, provide the basis for an equitable comparison between electric machine systems with SYNCHRO-SYM:

NOTE: All electric machines comprise a rotor (or moving) body and a stator (or stationary) body, each with a moving magnetic field that is developed by the coercivity of  a moving body of permanent magnets (PM), the reluctance of a moving body of saliencies, or the magneto-motive-force (MMF) of a moving body of electromagnets (or windings with current). The moving magnetic fields must be synchronized for average torque production in accordance with the synchronous speed relation (±WS±WR±WM=0, where WR is the electrical angular frequency of the rotor winding excitation, WS is the electrical angular frequency of the stator winding excitation, and WM is the mechanical angular frequency of the rotor).  For a permanent magnet or field wound synchronous electric machine (i.e., singly fed), (|WR| = 0) and (|WM| = |WS|). For a multiphase wound-rotor synchronous “doubly-fed” electric machine as only provided by SYNCHO-SYM,  (|WR| = |WS|) and (|WM| = |2xWS|), or twice the speed for a given torque, air-gap flux density, frequency and voltage of excitation (or twice the power within the same footprint)).

NOTE: Winding magneto-motive-force (MMF), which is the product of current and winding turns, or its scalar component, Magnetic Flux, tells how easy it is to magnetize an electromagnet. MMF may be “passive,” such as the magnetizing MMF of DC field winding, a salient pole, or a squirrel cage winding, or “active,” such as the torque MMF of a directly excited multiphase winding set, and may be located on the rotor or stator. In contrast, permanent magnet coercivity tells how easy it is to demagnetize a permanent magnet, for instance by MMF.

NOTE: The sole purpose of the “passive” rotor assembly of the century old componentized electric machine circuit and control architecture of all electric machine systems with rare earth permanent magnets (RE-PM), slip-induction dependent windings, reluctance saliencies, or DC field winding is to close the magnetic path through the air-gap in order to establish the air-gap flux density. In contrast, the purpose of an “active” rotor assembly (as only provided by the patented electric machine circuit and control architecture of SYNCHRO-SYM) is to “actively” contribute to the electromechanical energy conversion and power production process (in addition with the “active” stator assembly) as well as establish the air-gap flux density.

NOTE: Sometimes called an “armature,” only a multiphase alternating current (AC) winding set that is “directly” excited (at its multiphase terminals) with multiphase AC develops a moving (or rotating) magnetic field relative to its frame and (accordingly, only a directly excited multiphase AC winding set) “actively” contributes to electromechanical power conversion and production (i.e., electrical to mechanical power).  Therefore, all electric machines operate on AC, regardless of their naming convention, and must comprise at least one “directly” excited multiphase AC (or active) winding set or armature (singly-fed), which is generally located on the stator for electrical connection convenience, or at most two active winding sets (doubly-fed), after which electric machine circuit topology replicates, with the power rating of the optimally designed electric machine solely determined by the sum of the power ratings of all active winding sets.

NOTE: The Multiphase Wound-Rotor “Synchronous doubly fed” Electric Machine System (as only provided by SYNCHRO-SYM) uniquely shows the performance and power of two active multiphase winding sets on the rotor and stator bodies, respectively, and within the same package footprint as other electric machines, which equates to twice the power density.

NOTE: The symmetrical mathematical relationships describing the synchronized moving magnetic fields of the multiphase AC wound-rotor synchronous doubly-fed electric machine in accordance to the synchronous speed relationship (as only provided by SYNCHRO-SYM‘s brushless real time emulation control or BRTEC) become the classic textbook study for all asymmetrical electric machines by “de-optimizing” the symmetrical relationships (and associated optimized performance, noise, harmonic content, etc.) with “asymmetry,” such as with the asymmetry of DC field windings, permanent magnets, slip-induction dependent windings, reluctance saliencies, etc.  

NOTE: In contrast to the “armature” as just discussed, reluctance saliencies, Direct Current (DC) field windings, slip-induction windings, or permanent magnets occupy precious rotor real estate with associated cost, size and loss but are without a direct multiphase electrical power port to “actively” contribute to “real” electromechanical power conversion and production. Worse, slip-induction dependent windings (e.g., rotor squirrel cage winding set) are completely dependent on the mutual slip-induction (or speed-based frequency induction) with the armature (e.g., stator armature) and as a result, any active or passive power induced onto the rotor multiphase winding set (e.g., squirrel cage winding) of a slip-induction electric machine is solely provided by the extra rated stator active winding set.  Therefore, any rotor slip-induction dependent winding set is simply a winding extension of the stator active winding set (as is any single coil in a winding of coils), which increases the reactance (e.g., Inductive),  as well as associated cost, and size, to the stator winding set without contributing to “additional” active power production.  Ideally, the best electric machine would eliminate the extraneous and “passive” components, if possible.

NOTE: Packaging, winding, and thermo management techniques equally consume significant size, loss, and cost of any electric machine. For instance, the packaging must be structurally robust to dynamically support the enormous dynamic and magnetic forces exerted on the bearing and frame assembly; and the thermo management must be capable of dissipating the electrical loss as a result of current and harmonic content in the winding sets and core, such as the active multiphase winding set(s), found in all electric machines, including permanent magnet (PM) electric machines.

IMPORTANT DESIGN CONSIDERATION NOTE: All electric machines with “designed optimized” materials, packaging, winding, and thermo management techniques will show similar effective air-gap area, similar “rated” torque, and similar active multiphase winding set physical size (per unit of rated continuous design power at synchronous speed), because the determining design factor (which is the effective air-gap flux density) is predominantly constrained by the same flux saturation limit of the magnetic core material used by all (particularly at the teeth of the winding or permanent magnet (PM) slots where concentration of flux is highest) with little regard to the high coercivity (or compact size) of rare-earth permanent magnets (RE-PM) (as popularly suggested) or the even higher peak magneto-motive-force (MMF) potential of a winding set (after all it takes winding MMF to actually magnetize or demagnetize a PM).  As supporting evidence, today’s specialty induction electric machine systems (with optimized copper squirrel cage winding rotors) are achieving similar size and efficiency for a given rated torque, air-gap flux density, and speed as optimally designed RE-PM electric machine systems (but without the extravagant cost, safety, manufacturing, and lack of field weakening of RE-PMs).  Overall, the physical size, the continuous torque rating, and the continuous power rating of the active multiphase winding set will be similar across electric machine types, including rare earth permanent magnet electric machines, by optimally designing to the same air-gap flux density (close to the flux saturation limits of the core material) and to the same constant torque speed range with the same continuous torque rating (i.e., same rated synchronous speed and continuous power).  Furthermore, the physical size of the active multiphase winding set (and effective air-gap area) ultimately determines the physical size of the rotor and stator bodies; and together, the rotor and stator bodies (with their structurally robust frames) determine similar overall physical size amongst electric machines, regardless of the high coercivity (or compact size) of the rare-earth permanent magnets (RE-PM) as commonly suggested or the even higher peak magneto-motive-force (MMF) potential of a winding. Disregarding magnetic flux radiation, superconductor electric machines as the exception are less constrained by the operational flux saturation limit of the core material.

NOTE: Without considering the formidable issues of cryogenics, superconductor electric machine systems achieve many times more winding  magneto-motive-force (MMF) (and resulting air gap flux density) than conventional windings or PM coercivity, regardless of core saturation limits. Note: SYNCHRO-SYM Technologies can bring superconductor electric machines closer to practical reality). 

NOTE: “Induction (or asynchronous)” electric machines (doubly fed or singly fed) rely on slip-induction (for establishing the air-gap flux) for operation, which is current induction into the rotor winding set do to the asynchronous speed (or slip) between the rotor and active stator winding sets, and as a result, singly fed or doubly fed induction (or asynchronous) electric machines rely on slip-induction for operation and cannot controllably operate at (or about) synchronous speed, where slip-induction ceases to exist.  In contrast, singly-fed “synchronous” electric machines (or synchronous doubly-fed as only provided by SYNCHRO-SYM) do not “rely” on stochastic slip-induction (for establishing the air-gap flux and must provide another means, such as permanent magnets or DC field windings) for operation and as a result, “synchronous” electric machines will controllably operate at synchronous speed, regardless of the loss of slip-induction.  

NOTE: Since flux density is proportional to the vector sum of all flux producing components of the system, such as PM coercivity or winding MMF, asymmetric electric machines (or single ported transformer topology), such as singly fed synchronous (e.g., PM and field wound)  and singly or doubly fed induction electric machines, must lower the rated steady state core flux density design to compensate for the combined peak flux with increasing active (or torque) MMF (or flux density) to stay comfortably within the saturation limits of the core material (e.g., field weakening).  But in accordance with dual ported electromagnetic transformer physics, only the air-gap flux density of symmetric electric machines (or true dual ported multiphase wound rotor “synchronous” doubly fed transformer circuit topology as only provided by SYNCHRO-SYM) remains constant with increasing active (or torque) MMF beyond magnetizing MMF; and as a result, symmetric electric machine systems (as only provided by SYNCHRO-SYM) can be designed closer to the flux density saturation limit of the core material for another degree of power density. 

NOTE: Asymmetric electric machines with permanent magnets show “peak” torque (and “peak” power) potential 2x higher than its “rated design torque.” Asymmetric electric machines with a primary and secondary winding transformer circuit topology, such as single and doubly fed “induction or asynchronous” electric machines, show “peak” torque (and “peak” power) potential 3x-4x “rated design torque” (because of its single ported but dual winding transformer topology) than asymmetric electric machines with no rotor or secondary multiphase winding set, such as electric machines with PMs, field windings, or saliencies (reluctance). The highest peak torque electric machine, such as the universal electric machines or electromechanically commutated DC electric machine, show peak torque potential 5x rated design torque. But only SYNCHRO-SYM with a symmetric (or truly dual ported) transformer circuit topology shows factors of higher peak torque potential than rated torque (e.g., 8x rated design torque). See electric machine torque 101 whitepaper for details.

NOTE: The high peak torque density of SYNCHRO-SYM is essential for direct drive (gearbox-less or magnetic gear) drive-train systems for electric vehicles, which are simpler, more reliable, less costly, lower maintenance, and most likely smaller than an electric motor and transmission combination. For instance, Rimac is selling a complete motor/transmission package for electric vehicles because of limited peak torque density of their RE-PM electric machine. BEM is proposing its own “direct drive or magnetic gear” Universal Electric Vehicle Powertrain Chassis Module (BEM-UPM) with the superior performance of SYNCHRO-SYM. For utility vehicles (without performance suspension), SYNCHRO-SYM is the best electric machine alternative for in-wheel motor applications, such as presently provided by Protean and Elaphe.  

NOTE: Synchronous electric machines allow a selectable magnetic field position (i.e., torque angle) that is fixed between the synchronized rotor and stator rotating magnetic fields but in contrast, the torque angle of induction electric machines is stochastically the result of changing rotor time constant and rotor slip-induction due to at least shaft perturbations. Consequently, for both induction and traditional synchronous singly-fed electric machines, delays in measurement and excitation synthesis by state of art offline processing, such as derivatives of field oriented control (FOC), always contribute to instability issues, particularly at speeds or frequencies with large time constants (or shallow slopes), such as at low speeds.   With the contrasting control provisions of BRTEC, SYNCHRO-SYM is a synchronous doubly fed electric machine with rotating magnetic flux automatically phased locked (fixed) to a selectable phase position (without regard to speed) without processing delays (sensor-less and real time) or large time constant signal measurement issues, which eliminates reliance on stochastic slip-induction for rotor magnetic field production and as a result, makes SYNCHRO-SYM a synchronous electric machine system (with an inherent absolute resolver). For both induction and synchronous electric machines, an axle speed and position resolving sensor must be provided for estimating the flux position. 

NOTE: The passive, delicate, short lived, expensive, environmentally unfriendly, and cartel controlled permanent magnets, such as rare earth permanent magnets (RE-PMs), show degrading performance over normal operational life, such as demagnetization, which accelerates during peak torque stress, such as experienced in an electric vehicle application. More daunting, the globally minable supply of RE-PM materials may not meet the expected global demand (even with difficult recycling), particularly if RE-PM electric machines become the electric machine of choice, as anticipated. In consideration, comparable alternatives are being aggressively researched but only SYNCHRO-SYM, which is without permanent magnets,  already provides a better alternative.

NOTE: To reduce the amount of expensive RE-PM material, RE-PM electric machine systems with electronic control support are designed to operate at much higher speeds (i.e., electric machine size is inversely proportional to speed). However, to match the high speed electric machine system to the low speed application, the compounded size, cost, complexity, and maintenance (such as lubrication) of a transmission (i.e., gearbox) must be introduced. For instance, if the transmission and the electric machine each show 95% efficiency, the overall compounded efficiency of the electric machine and transmission system is 90.25% efficiency (i.e., 95% x 95%), which is far from 95% and a considerable 5 percentage point reduction in system efficiency from the component efficiency. In contrast, a direct drive would show the efficiency of the electric machine component or in this case 95%.  Optimizing design focus should be on the entire compounded system of systems performance, such as the electric machine, the electric machine controller, and the transmission. Improving the performance of an individual component in detriment to the other components of the system, such adding a necessary transmission for  the rare-earth permanent magnet electric machine, may have hidden consequences.

NOTE:  The “coreless” or yokeless terms seem to suggest there is no core material but in fact, the magnetic path is routed through the PM core material, which exhibits the density of steel but the permeability of air, and the back-iron with the density and permeability of electrical steel. Also, there must be mechanical structure (i.e., framework) for the active winding set.

NOTE: Compared to the common radial flux form of electric machines (i.e., rotor cylinder inside the annulus of stator cylinder), axial flux electric machines (i.e., rotor disk adjacent to stator disk) are known to use up to 10-20% less copper and steel while providing higher efficiency and torque (see axial-flux whitepaper with MAGNAX marketing remarks).  As a symmetrical (i.e., fully electromagnetic dual ported) transformer electric machine system (without permanent magnet asymmetry), SYNCHRO-SYM most conveniently accommodates the axial flux form of electric machine.

NOTE: Compared to the high speed radial-flux form, which RE-PM electric machines consider as their packaging advantage, the axial-flux electric machine (as SYNCHRO-SYM proposes), inherently contain rotor windings and air-gap tolerance in high speed operation (because rotor centripetal contention is tangential to the air-gap), provides reduced (or shimmed) air-gap depth for lower magnetizing MMF or RE-PM volume, and provide comparable rotor cooling as the stator disk.

NOTE: Amorphous Metal Ribbon shows: 1) extremely high permeability (e.g., for less rare earth permanent magnet material or less magnetizing MMF), 2) extremely high resistivity (e.g., for up to 80% lower core loss than the best grain-oriented electrical steel), and 3) high flux density over a wideband of power supporting frequencies (e.g., for high power transformers from 10s of Hz to 100s of kHz). In consideration, amorphous metal ribbon is an ideal core material for high performance, high power, axial-flux electric machines, such as low and high frequency transformers for the smart grid or electric motors and generators for electric vehicles or to meet progressively mandated efficiency standards for electric machines, such as IE4, IE5, etc.  But until the electric machine 3D Printing provided by MOTORPRINTER, the attractive electromagnetic properties of high performance electromagnetic materials, such as amorphous metal ribbon, also make these materials difficult to manufacture into an electric machine core without losing their delicate and coveted properties or without experiencing extreme manufacturing tool wear. For instance, although attempted many times, amorphous metal ribbon (discovered in the 1950s) has yet to show practical use in an electric motor or generator.

NOTE: If amorphous metal ribbon (for instance) is wrapped in an axial-flux core form without damaging the perfectly aligned slots and flat air-gap surface (as only provided by MOTORPRINTER), the changing magnetic field of an electric machine ideally travels directly through the solid high permeability material ribbon until reaching the air gap junction between the rotor and stator assemblies. In contrast, the magnetic flux of an axial-flux core made of soft magnetic core (SMC) material always pass through  a multitude of insulation material (or air-gap) surrounding each minute particle of the SMC, which significantly lowers permeability and structural integrity of the entire core.

NOTE: Regardless of conflicting terminology, all electric machines, including so-called DC and permanent magnet (PM) electric machines, only function with alternating current and as a result, comprise AC windings, such as the stator “active” winding set, with end turns that do not actively participate in the power conversion process but instead, contribute to parasitical electrical loss and flux leakage (i.e., reactive impedance) in accordance to the total length of the end-turns. Note: a winding is a closed loop with only two sides of the four sided loop actively contributing to power productions (by Lorentz force) and with the other two sides closing the loop and always contributing to reactive power. Compared to distributed windings, smooth air-gap, etc., some winding methods, such as concentrated windings, segmented cores, etc., may reduce the length of the end turns but sacrifice effective air-gap area and core volume while increasing harmonic content. Segmented (or concentrated) cores may improve automated winding placement but increase air-gap length (and reluctance) in the magnetic path. Traditional electric machine design is always juggling (or optimizing) between pros and cons of practical winding, packaging, and manufacturing techniques, which is available to all.  Likely tried during the century of electric machine development, it is easy to confuse a focused improvement as invention, such as a new winding arrangement or permanent magnet placement, without grasping its compounding negative effects on other aspects of practical electric machine operation. In contrast, SYNCHRO-SYM, which can virtually use all of the optimizing techniques and so-called inventions, such as winding arrangements, packaging techniques, or manufacturing techniques, provides brushless real time control (BRTEC) that experts have extensively studied and theorized would provide the best possible electric machine system, which is the multiphase wound-rotor synchronous doubly-fed electric machine system or SYNCHRO-SYM.  

NOTE: Since squeezing more copper equates to more power density, the copper wire slot fill factor is a necessary optimization goal. All specialty motor manufacturers are achieving upwards of 80-90% fill factor, which is considered best case, with orthocyclic windings, concentrated windings, distributed windings, square copper wires, etc. Compared to the customary non-optimized 40-50% fill factor, motor manufacturers are seemingly promoting optimized motors (and resulting power density) as “invention.”

NOTE: Distorted by years of extensive research and development, rare earth permanent magnet (RE-PM) electric machine systems are now considered the highest performing. But combinational RE-PM materials, such as Samarian cobalt, neodymium, dysprosium, etc., are difficult to unearth (mine), impose considerable environmental impact, or are globally and politically supply limited to country cartels. Strategic research, such as by ARPA-E, has been directed: 1) to reducing the needed quantity of RE-PM material in a RE-PM electric machine, 2) to comparably improve the PM free induction electric motor performance by using 3D printing (for instance), or 3) to bring superconductor windings to practical reality, such as the idyllic alternating current (instead of available direct current) superconductor winding. Already without RE-PMs or superconductors, the patented SYNCHRO-SYM doubles the overall performance expected from a RE-PM electric machine form factor, such as MAGNAX, and would be the natural evolution if induction electric motors or AC superconductors became the norm.

NOTE: Electronic control is the practical means to optimize the performance of the electric machine to the application, such as variable speed and torque. For singly-fed synchronous, such as permanent magnet electric machines, doubly fed, or reluctance electric machine, electronic control is necessary for practical operation. When appropriately including electronic control, the electric machine is a system with the electronic controller compounding the overall size, weight, loss, and cost of the electric machine system of the traditional componentized electric machine circuit and control architecture (as shown in illustration). As a result, the compounded size, cost, and efficiency should be prominently included in the performance specification. For instance, if the electronic controller and the electric machine each show 95% efficiency, the overall compounded efficiency of the electric machine system is 90.25% efficiency (i.e., 95% x 95%), which is far from the 95% component efficiency and a considerable 5 percentage point reduction in system efficiency from the individual component efficiency.

NOTE: Neglecting the safety issues, the active winding voltage rating of any electronically controlled electric machine should be as high as possible to reduce the loss effects of the semiconductor junction voltage drops. For instance, with a 400v supply rating and a reasonable fixed 0.6v semiconductor junction drop, 0.15% (0.6/400) of the power will be dissipate by the semiconductors junctiion drop but in contrast, a significant 1.2% (0.6/48) of the power will be dissipated by the semiconductors with a 48 volt supply rating. BRTEC can conveniently allow high voltage electric machine (i.e., SYNCHRO-SYM) windings with low voltage power supplies.

NOTE: As a result of electromechanical symmetry in an axial-flux form (as only provided by the integrated control architecture of BRTEC without the size, cost, and inefficiency of large reactive components, such as large capacitors and chokes, of a DC Link Stage), the BRTEC of only SYNCHRO-SYM can be located in the otherwise wasted annulus space of the low frequency axial-flux electric machine to provide another level of power density and  the simplicity of duplicate rotor and stator assemblies.

NOTE: Field Weakening is the coveted means of controlling air-gap flux density for improving efficiency and reliability at differing speeds and for limiting core saturation by controlling “magnetizing” MMF and resulting air-gap flux density. Ironically, the original intention of permanent magnet electric machine was to eliminate magnetizing MMF and associated electrical loss characteristics of the slip-induction electric machine system; but it is becoming very common to incorporate magnetizing MMF, which negates the original benefit of incorporating RE-PMs.

NOTE: Unlike active power that does real work, reactive power is neutral overall but reactive current (or magnetizing current), like active current, shows electrical loss (I2R) in the circuit, where I, is the current flowing in the circuit and R is the resistance of the circuit. [The resistance of a copper winding is lower than the resistance of an aluminum winding, which promotes the use of a copper rotor for slip-induction electric machines.] For an electric machine with a vector winding magnetizing current, IM, orthogonal to the vector torque current, IT, electrical loss is proportional to the vector magnitude (IM2 + IT2)-1/2. Note: PM have a persistent magnetic field and have no electrical port for magnetizing current, IM = 0. For an electric machine, IM is reasonably set to 10% (to 30%) of IT and in accordance to the vector magnitude, electrical loss would increase by only 1% (to 9%) and air-gap flux density would increase by 0.4% (to 4.4%), which is not excessive for induction machines. By adjusting IM between 10% and 30% of IT (i.e., field weakening) in accordance to speed or torque, there would be a dramatic change in air-gap flux density and loss.

NOTE: Since physics dictates electric machine torque and physical size are inversely proportional to speed, designing to a higher speed is a practical means of reducing RE-PM material but the compounded cost, size, and loss of a transmission (or gearbox) to match the high speed to the application should be included. If the efficiency of the high speed electric machine system is 95% and the efficiency of the transmission is 95%, the compounded efficiency of the system is 90.25%, a direct drive, low speed electric machine system with 95% efficiency would be better.

NOTE: RE-PM versus an electromagnet (or coercivity versus magnetizing MMF):

  • RE-PM can support up to five times the air-gap depth? 
    • With the same packaging techniques and core flux density, five times the air-gap depth is tantamount to five time the amount of RE-PM material; but the necessary goal of today is to reduce the amount of extravagantly costly RE-PM material (not increase it). To reduce RE-PM material actually requires a smaller air-gap depth, such as for the magnetizing MMF of an electromagnet or the higher speed of operation (and the compounded cost, size, inefficiency, and complexity of a gearbox addition).
  • RE-PM electric machine systems (EMS) do not have the electrical provisioning or the inefficiency of magnetizing MMF? 
    • Actually, it is becoming common to awkwardly introduce the electrical provisioning and inefficiency of Magnetizing MMF (which is orthogonal to Torque MMF) in the RE-PM EMS to acquire improved its efficiency and reliability over a larger operating range of speeds, as already conveniently provided by the fully electromagnet EMS.
  • The power density of a RE-PM EMS is substantially better than any other EMS? 
    • Reasonably considering the size of an electric machine system (EMS) is predominantly determined by the active power multiphase electromagnet winding set of the stator, which all EMS must incorporate, the power density of a RE-PM EMS is similar to any EMS with similar packaging techniques, although the actual size of the RE-PM assembly may be smaller than a comparable field-wound electromagnet assembly.
  • RE-PM EMS can operate at higher speeds? 
    • With a larger air-gap capability, radial-flux form factor electric machine systems can operate at higher speed without the rotor cylinder (or windings) colliding with the annulus of the stator cylinder. But in an axial-flux form factor, which is known to be more efficient with higher torque density than the radial-flux form, the centripetal forces are tangential to the stator or rotor structure without the possibility of colliding under centripetal force. Also, the axial-flux form has better control with air-gap depth (such as shimming) for reduced magnetizing MMF or RE-PM volume. The axial-flux EMS may need a more robust frame and bearing assembly.
  • RE-PM EMS can operate at higher speeds without RE-PM dislodging?
    • Under centripetal forces, the rotor RE-PM assembly is satisfactorily supported in some radial flux forms but a rotor electromagnet winding set assembly may dislodge; however, an electromagnet winding set assembly is well supported by the core in an axial flux form.