COMPARISON TRADE SPACE
COMPARISON TRADE SPACE BETWEEN ALL ELECTRIC MOTOR AND GENERATOR SYSTEMS
To provide an equitable comparison for cost, size, and loss (per unit of power rating) between electric machine systems, a common baseline must be established that is bounded in accordance with the following trade space, which is detailed in Electric Machine Design Distinctions & Constraints, and would result in the same stable MLS (or constant-torque speed range) for the same continuous torque, excitation frequency and voltage, and air-gap flux density design and with the same packaging of material, winding, electronic component, thermal management, and manufacturing techniques; otherwise, comparative results can be conveniently skewed in favor of a single contestant. For instance, there are dramatic publicized performance differences between optimized electric machine systems from different manufacturers, although all use the same “me-too” electric machine circuit and control technology with the asymmetry of the universally essential “active stator assembly” for torque production but a “passive rotor assembly” with rare-earth permanent magnets (RE-PM), reluctance saliencies, slip-induction dependent windings, or DC field windings, which in accordance with electric machine physics, would result in the same MLS for the same port voltage, excitation frequency, continuous torque, air-gap flux density, if designed with the same packaging, winding, thermal management, electronic component, and material techniques:
- All electric machine systems can be characterized as “synchronous” or “asynchronous.” 1) By definition, an asynchronous (or induction) electric machine indisputably relies entirely on slip-induction to remotely induce speed synchronized torque magneto-motive-force (MMF) on the rotor multiphase winding set without physical electrical contact and as a result, the rotor of an induction electric machine cannot rotate at synchronous speed, where slip-induction ceases to exist. Since slip-induction results from the mutual induction arising from the asynchronous movement or slip between the rotor and stator multiphase winding sets, 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 with temperature dependency, but the induction electric machine does have direct (i.e., independent and deterministic) control of the magnetizing MMF (or magnetizing flux) on the stator winding set. 2) In contrast, a synchronous electric machine indisputably does not rely on slip-induction to produce torque MMF and as a result of control technology limitations of the time, the rotor assembly speed of the traditional synchronous electric machine must be synchronized to the revolving magnetic flux in the air-gap to avoid slip-induction but in overview, the synchronous electric machine has independent and deterministic control of the torque MMF by directly applying bi-directional, speed synchronized excitation to the stator multiphase (or active) winding set and also, effectively has independent and deterministic control of the rotor flux, which is a result of explicitly knowing the magnetizing MMF magnitude (albeit with potentially one degree of adjustable rotor flux magnitude by field weakening) or the predesigned permanent magnet (PM) coercivity and depth product (i.e., PM flux), both of which are conveniently fixed to the known position and speed of the rotor axis.
Following this analysis without invoking mechanical limitations on the mechanical aspects of the electric machine, such as the speed of the rotor, asynchronous and synchronous electric machines are more accurately characterized with three criteria: 1) an asynchronous (or slip-induction) electric machine: i) does not have independent and deterministic control of both the rotor and stator torque and magnetizing MMFs at any speed, ii) relies entirely on slip-induction for operation, and iii) cannot continuously and stably operate at synchronous speed where slip-induction ceases to exist; but in contrast to the traditional definition, 2) a synchronous electric machine: i) has independent and deterministic control of both the rotor and stator torque and magnetizing MMFs at any speed, ii) does not rely on slip-induction for operation, and iii) can continuously and stably operate at synchronous speed. By satisfying the three criteria, the symmetric multiphase wound-rotor (synchronous) doubly-fed electric machine, which is the classic introductory theoretical study for both synchronous and asynchronous electric machine systems, is now properly defined as a “synchronous” electric machine system; but only by postulating the invention of a brushless, automatic, and sensorless real time emulation control means during its study for continuously stable operation, which cannot rely on slip induction that ceases to exist at synchronous speed.
NOTE: Only the synchronous electric machine definition provided will satisfy 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 continuous torque, package, air-gap flux density, and voltage and frequency of excitation, but only by postulating the invention of a practical brushless real-time emulation controller for independent and deterministic control of the rotor multiphase winding (active winding) set torque and magnetizing current in order to simplify the study with steady-state synchronous stabilization from sub-synchronous to super-synchronous speeds, including zero and synchronous speeds.
NOTE: The asynchronous (or induction) electric machine establishes the air-gap flux density with the magnetizing MMF of the stator multiphase winding set and then, establishes the torque MMF on the rotor multiphase winding set, which pushes and pulls the rotor, by the mutual slip-inductive coupling with a similar MMF on the stator winding set, which pushes and pulls on the stator frame. As a result, the stator size, cost and loss must support (e.g., structurally and electromagnetically) the electrical power, the core loss, and the electrical loss of the magnetizing and torque MMF combination, which is the vector magnitude of orthogonal vectors in accordance with Lorentz Force Law, and the rotor size, cost, and loss must support the friction loss, which include stray, windage, bearing, etc., the core loss, the electrical power, and the electrical loss of torque MMF. With similar analysis: 1) the stator size, cost, and loss of the DC field wound synchronous electric machine, which only has magnetizing MMF on the rotor, must support the core loss, the electrical power, and the electrical loss of torque MMF and the rotor size, cost, and loss must support the friction loss, the core loss, the electrical power, and the electrical loss of magnetizing MMF, 2) the stator size, cost, and loss of the RE-PM synchronous electric machine, which has no electrical power on the rotor, must support the core loss, the electrical power, and the electrical loss of stator torque MMF and the rotor size, cost, and loss must support only the friction loss and a small core loss, which is due to harmonics with precise synchronous control, 3) the stator size, cost, and loss of the reluctance electric machine, which has no electrical power on the rotor, must support the core loss, the electrical power, and the electrical loss of the torque and magnetizing MMF combination and the rotor size, cost, and loss must support the friction loss and the small core loss, which is due to harmonics with precise synchronization control.
- Only a directly excited (e.g., bidirectional), multiphase winding set (or active winding set) produces a rotating magnetic field relative to its frame that contributes active (or working) power to the electromechanical energy conversion process in accordance with its power rating, while consuming loss, which includes core, electrical, and friction losses, cost, and 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 providing moving rotor electrical provisioning with stable excitation control. In contrast, a) slip-induction dependent windings, which are inductively powered through the extra capacity stator active winding set, b) DC field winding, c) reluctance saliencies, or d) permanent magnets have no directly excited, multiphase electrical port for active power connection and as a result, cannot contribute additional active power to the electromechanical energy conversion process and therefore, waste precious electric machine real estate with loss, cost, or size.
- All electric machines must have at least one active winding set (i.e., singly-fed) for electromagnetic energy power conversion and at most two active winding sets (i.e., doubly-fed) before circuit topology duplicates.
- All electric machines (i.e., electric motors, generators, and transformers) are optimally designed with similar air-gap flux density because air-gap flux density is determined by the permeability and saturation limit of the same available electrical steel core material 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 at least torque production is the cross-product of air-gap flux density and torque MMF (i.e., Lorentz Force Law), establishing the highest possible airgap flux density within the same flux saturation limit of the same electrical steel core of all electric machines is the first steady-state design criteria for any electric machine.
- With similar steady-state air-gap flux density, all optimally designed electric machines have similar effective air-gap area and similar stator active winding set size for a given continuous torque, which determines the size and volume of the electric machine,: a) when designed to the same maximum load speed (MLS) for a given air-gap flux density, continuous torque, and voltage and frequency of excitation, b) regardless of the shallowness of RE-PMs versus an electromagnet but only when producing an air-gap flux density under one Tesla at a reasonable operating temperature and air-gap depth, and c) with the universal exception of super conducting electric machine systems with super high air-gap flux density.
NOTE: The torque limit of any electric machine depends on the magnitude and orthogonality control limits of the winding torque current vector, while disregarding the thermal management limits of electrical loss and heat dissipation, because in accordance to Lorentz Force Law, torque depends on the magnitude of air-gap flux density vector, which is orthogonal to the magnitude of the torque current vector, and since the magnitude of the air-gap flux density is similar amongst all electric machines.
NOTE: The magnetic flux density of the airgap and core is the combined result of all torque MMF generated flux, all RE-PM generated flux, which is the product of RE-PM coercivity and its physical depth, and all magnetizing MMF generated flux, which must be designed within the core saturation limit.
- All optimized electric machines of comparable power rating, including so-called yokeless or ironless electric machines, have similar amounts of core iron: a) to at least provide the universally essential back-iron for optimally closing the magnetic path through the airgap without saturating the magnetic core with peak torque current, b) to reduce the amount of magnetizing MMF or expensive RE-PM material by reducing effective air-gap depth or core reluctance. Yokeless or ironless electric machines sacrifice thermal management, structural integrity, and air-gap depth by eliminating the stator back-iron (and resulting core loss), while increasing magnetizing MMF or expensive and environmentally unfriendly RE-PM material, or c) to add structural integrity without exotic composite materials, such as carbon fiber, or unfriendly manufacturing methods.
- Today, all electric machine circuit and control architectures (EM-CCA) or system architectures incorporate electronic excitation control for optimum performance or for practical operation. 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).
- The essential electronic controller introduces compounding effects on the overall loss, cost and size of any electric machine “system,” and therefore, is often overlooked in electric machine specifications. For instance, if the efficiency of the motor component is 96% and if the efficiency of the essential electronic controller component is also an impressive 96%, the actual compounded efficiency of the system is only 92% (i.e., compounding 96% x 96%).
- With over a hundred years of practical electric motor application and risk mitigation, all magnetic electric machine circuit and control architectures are straight-forward and must simultaneously obey the three empirically derived laws of physics, Ampere Circuital Law, Faraday’s Law, and Lorentz force Law, which Maxwell Equations generalized and integrated with time and space.
- 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 synchronous EM-CCA, as only provided by SYNCHRO-SYM, 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 the universally essential active winding set of the stator assembly as only possible with the patented and practical brushless real time emulation controller (BRTECTM) for automatically exact, synchronous stabilization to avoid falling into the asynchronous category of so-called doubly-fed electric machine systems, 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).
NOTE: Excluding the symmetric synchronous electric machine system with an active rotor assembly of a directly excited multiphase winding set (i.e., active winding set) because of the formidable challenges of inventing a practical brushless real time emulation controller, electric machine system categories were limited to only the multiplicity of asymmetric electric machine systems with a derivative of an estimating field oriented controller and a passive rotor assembly of either practical RE-PMs, slip-induction dependent winding sets, such as the multiphase wound-rotor induction doubly-fed electric machine system, reluctance saliencies, or DC field windings.
NOTE: Commonly confused with the asynchronous (or slip-induction) doubly-fed electric machine system, which is an 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).
NOTE: Coupled with the formidable challenges of inventing a practical BRTEC for implementing the most “optimum” EM-CCA, which is the symmetric synchronous EM-CCA, and with the advent of a high energy product rare-earth permanent magnets (RE-PM) (circa 1980’s) of neodymium with dysprosium doping that seemingly provided a practical means of effectively eliminating the provisioning, cost, size, and loss of Magnetizing MMF, electric machine research was conveniently redirected to just the development and empirical application of readily available material, such as RE-PMs, winding, packaging, high speed electronic control, and manufacturing techniques for performance enhancement of the century old, me-too, asymmetric EM-CCA, such as the RE-PM EM-CCA in particular. 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 for instance, optimizing the slip-induction EM-CCA that more effectively provides field weakening, which indisputably shows the recent control dominance over all electric machine system innovation and manufacturing by the producer of RE-PMs.
NOTE: Because of the expensive, geopolitical, environmental, human suffering, safety, and reliability (e.g., demagnetization and life expectancy) consequences of producing RE-PMs, such as dysprosium doped neodymium or samarium cobalt permanent magnets, there are efforts to use the abundant but low energy product ferrite permanent magnets in high performance permanent magnet electric motor systems but these efforts have not proven to achieve comparable air-gap flux density and as a result, comparable reliability, efficiency, and performance as an optimized induction or DC field wound synchronous electric machine system.
NOTE: The symmetric synchronous 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.
- All electric machine price-performance distinction is based on one of only two distinct electric machine circuit and control architectures (EM-CCA) and the empirical application of the same available present or futuristic material, winding, packaging, construction, electronic component, thermal management, and manufacturing performance enhancing techniques.
- The symmetric synchronous EM-CCA shows up to half the loss, half the cost, and half the size with the optimal symmetry of equally rated active winding sets on the rotor and stator, respectively, compared to the asymmetric EM-CCA with a passive rotor assembly of slip-induction windings, RE-PM, reluctance saliencies, or DC field windings (per unit of power rating) and a single active winding set on the stator by reasonably assuming the rotor of any EM-CCA consumes the same size, cost, and loss, which includes electrical, core, or friction losses, as the stator, particularly if the EM-CCA is an axial flux slip-induction asymmetric EM-CCA with similar adjacent rotor and stator disks.
- The symmetric synchronous EM-CCA doubles the performance gain expected from the same readily available electric machine material, winding, packaging, and electronic component techniques, which 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 synchronous EM-CCA has twice the constant-torque speed range for a given continuous torque and frequency and voltage of excitation (i.e., 7200 RPM @ 60 Hz and 2 poles versus 3600 RPM for all others), which is tantamount to twice the power rating.
- The symmetric synchronous EM-CCA provides at least eight times (octuple) more peak torque potential than the asymmetric EM-CCA, including asymmetric RE-PM EM-CCA, because unlike the asymmetric EM-CCA with increasing torque MMF quickly leading to core saturation, the torque MMF on each side of the air-gap is symmetrically neutralized in accordance with the conservation laws of dual ported transformer physics before leading to core flux saturating as the result of parasitic circuit impedances, which leaves the symmetric synchronous EM-CCA providing another level of power density and efficiency over at least the asymmetric RE-PM EM-CCA with two additional advantages: 1) inherent field weakening and 2) operational air-gap flux density can be designed closer to the flux saturation limit of the core.
- 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.
- The axial-flux electric machine (e.g., side by side rotor and stator disks separately articulated with a bearing assembly), which may require a more robust 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 articulated with a bearing assembly), particularly in a single air-gap configuration, to provide an simpler outside-to-inside winding approach, to provide incremental increases in power by conveniently stacking lengthwise, to provide a shallower air-gap, and to provide equal cooling surfaces between rotor and stator.