COMPARISON TRADE SPACE

COMPARISON TRADE SPACE BETWEEN ALL ELECTRIC MOTOR AND GENERATOR SYSTEMS:

  • All electric machines can be characterized as synchronous or asynchronous electric machines. 1) By definition, an asynchronous (or induction) electric machine indisputably relies on slip-induction, which is the mutual induction arising from the asynchronous movement or slip between the rotor and stator multiphase winding sets, to remotely produce speed synchronized torque magneto-motive-force (MMF) on the rotor winding set and therefore, an asynchronous electric machine has indirect (i.e., estimated) control of the torque MMF (or torque flux) on the rotor winding set due to at least remoteness and impedance inconsistencies, such as stochastic rotor winding set resistance temperature dependency; but has direct (i.e., independent and deterministic) control of the magnetizing MMF (or magnetizing flux) on the stator winding set. 2) By strictly following traditional definition, which ignored the electromagnetic/electromechanical symmetry between the rotor and stator because of practical real time control technology limitations, a traditional synchronous electric machine indisputably does not rely on slip-induction to produce torque MMF with the rotor speed synchronized to the revolving magnetic flux in the air-gap and therefore, a synchronous electric machine has independent and deterministic control of the torque MMF by direct application to the stator multiphase or active winding set and also, effectively has independent and deterministic control of the magnetizing MMF by explicitly knowing the magnitude of magnetizing MMF or predesigned PM coercivity x depth, which are conveniently fixed to the known position and speed of the rotor axis (albeit with potentially one degree of adjustable rotor flux magnitude by field weakening). Following this analysis, asynchronous and synchronous electric machines are more accurately characterized with three criteria: an asynchronous (or slip-induction) electric machine: a) does not have independent and deterministic control of both the rotor and stator torque and magnetizing MMFs at any speed and as a result, b) relies entirely on slip-induction for operation and c) cannot continuously and stably operate at synchronous speed where slip-induction ceases to exist; but in contrast to the traditional definition, a synchronous electric machine: a) has independent and deterministic control of both the rotor and stator torque and magnetizing MMFs at any speed and as a result, b) does not rely on slip-induction for operation and c) can continuously and stably operate at synchronous speed. Note: This synchronous electric machine definition satisfies the operation of the theoretical brushless symmetric multiphase wound-rotor “synchronous” doubly-fed electric machine system, which is taught in the “classic” theoretical study for all electric machine systems to provide twice the maximum load speed (MLS) of any other electric machine system with a given torque, package, and voltage and frequency of excitation; but only by postulating the invention of a brushless real-time emulation controller for independent and deterministic control of the rotor multiphase winding (active winding) set in order to simplify the study with steady-state synchronous stabilization from sub-synchronous to super-synchronous speeds, including zero and synchronous speeds.
  • Only a directly (e.g., bidirectional) excited multiphase winding set (or active winding set) produces a rotating magnetic field that contributes active (or working) power to the electromechanical energy conversion process in accordance with its power rating while consuming loss, cost, or size of its rotor or stator mounting assemblies. Generally, the universally essential active winding set is located on the stator to avoid the complexity of moving rotor electrical provisioning with stable excitation control. In contrast, a) slip-induction dependent windings, which are inductively powered through the extra sized stator active winding set, b) DC field windings, which are not multiphase AC winding sets, c) reluctance saliencies, which obviously have no electric port for active power connection, or d) permanent magnets, which obviously have no multiphase electrical port for active power connection, cannot contribute additional active power to the electromechanical energy conversion process but consume loss, cost, or size (together with their mounting assemblies).
  • All electric machines (i.e., electric motors, generators, and transformers) are optimally designed with similar air-gap flux density because the air-gap flux density is determined by the permeability and saturation limit of the same electrical steel core material available to all and not by the limited residual flux density potential of rare-earth permanent magnets (RE-PM) or the boundless flux density potential of an electromagnet. Because high air-gap flux density determines at least torque production (i.e., Lorentz Relation), establishing the largest possible airgap flux density within the flux saturation limit of the same electrical steel core is the first steady state design criteria for any electric machine. Although RE-PMs are shallower than an electromagnet when producing an air-gap flux density under one Tesla at a reasonable operating temperature and air-gap depth, the necessary stator active winding set still determines the associated torque and effective air-gap area (e.g., overall size) of any electric machine.
  • Commonly misunderstood, all optimized electric machines of comparable power rating, including so-called yokeless or ironless electric machines, have similar amounts of core iron to at least provide the universally essential back-iron for optimally closing the magnetic path through the airgap without saturating the back-iron with increasing air-gap flux density and to reduce the amount of magnetizing MMF or expensive RE-PM material. However, yokeless or ironless electric machines admirably reduce stator loss but by sacrificing thermal management, structural integrity, and increasing amounts of magnetizing MMF or expensive RE-PM material.
  • Today, all electric machine circuit and control architectures (EM-CCA) incorporate electronic excitation control for optimum performance or for practical operation and therefore, an equitable comparison between competing electric machines should always be a “full system” comparison by at least including the compounding loss, cost, and size of the electronic controller at the designed Maximum Load RPM (or constant torque speed range).
  • In accordance to the classic textbook study, there are only two electric machine circuit and control architectures (EM-CCA) for comparative convenience: 1) the optimal symmetric EM-CCA with the symmetry of an “active” rotor assembly, which comprises another directly excited multiphase winding set (or active winding set) on the rotor in addition to universally essential active winding set on the “active” stator assembly as only possible with the patented and practical brushless real time emulation controller (BRTECTM) for synchronous stabilization; otherwise, falls into the asynchronous category of so-called doubly-fed electric machines, and 2) the non-optimal asymmetric EM-CCA with the asymmetry of a passive rotor assembly, which comprises slip-induction dependent windings (singly-fed or so-called doubly-fed asynchronous), reluctance saliencies (reluctance), permanent magnets (synchronous), or DC field windings (synchronous), all under estimating field oriented excitation control (FOC).
  • Commonly confused with the asynchronous (or slip-induction) doubly-fed electric machine system or asymmetric EM-CCA, a practical “synchronous” symmetric multiphase doubly-fed electric machine system (or symmetric EM-CCA) has never materialized, because of the formidable challenges of realizing the essential BRTEC for “continuously synchronous stability” from sub-synchronous to super synchronous speeds, including zero and synchronous speed, although early symmetric EM-CCA research began with the advent of practical high speed electronic and magnetic control (circa 1960’s).
  • With over a century of legacy practice, electric machine design knowledge and manufacture are straight-forward, regardless of the EM-CCA. But coupled with the past elusiveness of a practical BRTEC for implementing the most “optimum” EM-CCA, which is the symmetric EM-CCA, and with the advent of a high energy product rare-earth (RE) PM (circa 1980’s) that provided a practical means of eliminating the provisioning, cost, size, and loss of Magnetizing Magneto-Motive-Force (MFF), electric machine research was conveniently redirected to the development and empirical application of readily available electric machine material, such as RE-PM materials, winding, packaging, high speed electronic control, and manufacturing techniques for performance enhancement of the century old asymmetric EM-CCA, such as the permanent magnet (PM) EM-CCA. But ironically, the provisioning, loss, cost, and size of Magnetizing MMF is being redesigned into the RE-PM EM-CCA to regain the coveted attribute of field weakening capability, instead of just leveraging the slip-induction EM-CCA, which more effectively provides field weakening, and as a result, this irony indisputably shows the RE-PM producer’s control dominance over all electric machine system innovation and manufacturing.
  • With the optimal symmetry of equally rated active winding sets on the rotor and stator, respectively, the symmetric EM-CCA shows up to half the loss, half the cost, and half the size as the asymmetric EM-CCA per unit of power rating (with a passive rotor assembly of slip-induction windings, RE-PM, reluctance saliencies, or DC field windings) by reasonably assuming the rotor of any EM-CCA consumes the same size, loss, and cost as the stator, particularly if the EM-CCA is an axial flux slip-induction asymmetric EM-CCA.
  • The symmetric EM-CCA doubles the performance gain expected from the same readily available electric machine material, winding, packaging, and electronic component techniques that were empirically applied to the asymmetric EM-CCA for performance distinction because of the magnifying working power contribution of two active winding sets on the rotor and stator, respectively.
  • The symmetric EM-CCA inherently: a) has the coveted field weakening capability for extended speed range, b) is without cogging drag from permanent magnet persistent magnetism of RE-PMs, or c) is without the extravagant cost, environmental harm, unsustainable global supply chain, and geopolitical consequences of RE-PMs.
  • The symmetric EM-CCA brings superconductor electric machine systems closer to practical reality by contactlessly (i.e., brushlessly or wirelessly) relocating the superconductor field windings to the stator assembly and by eliminating the harmonic heating of FOC power conditioning; but when AC superconductors become a practical reality, the fully electromagnetic SYNCHRO-SYM, which is without delicate and limited lifetime permanent magnets, will become the electric machine system of choice.
  • Particularly in a single air-gap configuration, the axial-flux electric machine (e.g., side by side rotor and stator disks separated by a bearing assembly) is proven to provide higher electrical performance than the radial-flux electric machine (e.g., rotor cylinder inside the annulus of a stator cylinder separated by a bearing assembly), to provide incremental increases in power by conveniently stacking lengthwise, and to provide equal cooling surfaces between rotor and stator but may require a more robust bearing assembly.
  • Aften overlooked, the essential electronic controller has compounding effects on the overall loss, cost and size of any electric machine “system.” For instance, if the efficiency of the motor is 96% and if the efficiency of the essential electronic controller is also an impressive 96%, the actual compounded efficiency of the system is only 92% (i.e., compounding 96% x 96%).