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This paper presents and evaluates a dual rotor axial flux permanent magnet motor for electric aircraft applications. Several features, including grain oriented electrical steel (GOES), magnet segmentation, and wires with rectangular cross-sections, are used to improve torque density and efficiency. Rather than simply optimizing the motor by itself, this paper evaluates the tradeoffs between motor performance and its interfaces with the drive, thermal management system (TMS), and mechanical structure. This information can be used along with similar analyses of the drive, TMS, and structure to select a design that achieves the system-level optimal performance. The paper uses finite element simulations to characterize tradeoffs between active mass, efficiency, fundamental frequency, power factor, axial forces on the rotors, and cooling surface area. Several designs exceed 95% efficiency at takeoff with less than 8 kg of active mass. While high pole counts, a large outer radius, and short stator teeth tend to optimize the magnetic performance at takeoff, this can reduce cruise efficiency, reduce the surface area through which the TMS can extract heat, increase the fundamental frequency the drive must supply, and increase the structural mass required to support the rotors. Additionally, designs with 20 °C cooler magnets were simulated to evaluate the impact of a more effective TMS, but the improvements in magnetic performance were relatively small.

Magnetic gears perform the same function as mechanical gears using magnetic fields instead of interlocking teeth. A review of the design processes used in the literature demonstrates that a critical design parameter, pole pair count, is often given inadequate consideration. In addition to reviewing existing prototypes, this paper uses a parametric simulation study to analyze the impacts of pole pair counts on gear performance and illustrate how the optimal pole counts vary with gear ratio and various design parameters. This paper also introduces new ripple factors, which better correlate with torque ripple than the cogging factor used in previous papers, and illustrates why designs with non-integer gear ratios tend to have much smaller torque ripples than designs with integer gear ratios. While selecting the pole counts to minimize symmetry can reduce torque ripple, designs without any symmetry are shown to experience unbalanced magnetic forces on each rotor. Thus, it is recommended to select pole counts that result in an even number of modulators but not an integer gear ratio. This paper also reveals that, for a fixed gear ratio, a nontrivial optimum pole count minimizes the electromagnetic losses.

Magnetic gears, like mechanical gears, transform power between different speeds and torques; however, magnetic gears’ contactless nature provides inherent potential benefits over mechanical gears. A genetic algorithm was used to optimize magnetic gears at different temperatures across a range of gear ratios. Using different magnet material grades on the different rotors and for the tangentially and radially magnetized magnets can slightly increase the specific torque relative to designs with a single magnet material. The high pole count rotor requires a magnet material with higher coercivity than that of the low pole count rotor magnet material, especially for designs with a large gear ratio. While increasing the temperature produces an exponential decay in the achievable specific torque, with a compounding reduction of about 0.4% for each degree Celsius, the temperature does not significantly affect the optimal geometric parameters and primarily affects the optimal materials. The gear ratio significantly affects the optimal geometric parameters and can impact the optimal magnet materials. Additionally, the genetic algorithm was employed to characterize the impact of stack length using 3D finite element analysis. Designs with shorter stack lengths favored thinner magnets and higher pole counts and may be able to use magnet materials with lower coercivities.

This study investigates a novel hybrid cooling method for more weight efficient thermal management of aerospace electric propulsion motors using thermal energy storage (TES) elements. The proposed system utilizes the latent heating of TES in the form of SAPO-34 zeolite slabs hydrated with water to maintain stable operating temperatures during takeoff. The TES operates in parallel with a fluid cooling system comprised of minichannel heatsinks attached to the stator windings and exterior air heat exchanger. Thermoelectric performance benefits of TES inclusion are evaluated using network analysis under assumed flight path load. Complex non-linear thermofluid and electromagnetic behaviors in the network are replaced with lookup table interpolants generated using results of numerical simulations swept across a series of input parameters. Subsequent solution of two-hundred systems with varying TES volume indicated a maximum TMS mass savings of 14.8% compared to the lightest thermal management system without TES inclusion.

This paper describes the detailed analysis for the design of a linear generator developed for a Surface Riding Wave Energy Converter (SR-WEC), which was designed to improve energy capture over a wider range of sea states. The study starts with an analysis of the power take-off (PTO) control strategy to harness the maximum output power from given sea states. Passive, reactive, and discrete PTO control are explored. For the random wave excitation and limited sliding distance of the generator, the discrete strategy provides the highest average power output. The paper discusses the sizing requirement for the linear generator. Based on the force and power rating of the system and the application requirements, a slotless permanent magnet tubular generator is designed for the wave energy converter., To be published in Energy Conversion Congress and Expo 2021

Magnetic gears employ magnetic fields, rather than teeth, to perform the gearing action. At large gear ratios, cycloidal magnetic gears (CyMGs) can achieve higher torque densities than the more common coaxial magnetic gears. This paper compares CyMG topologies with consequent pole (CP) rotors against CyMGs with more conventional surface permanent magnet (SPM) rotors. CyMGs with CP rotors require less PM pieces than CyMGs with SPM rotors, which may simplify manufacturing. Each topology is optimized using a genetic algorithm with 2D finite element analysis (FEA). The simulation results demonstrate that CyMGs with CP rotors achieve lower VTD values than CyMGs with SPM rotors. However, if using a CP inner rotor eliminates the need for a PM retention sleeve and enables the use of a smaller effective air gap, CyMGs with CP rotors can achieve higher PM STs at high gear ratios than CyMGs with SPM rotors. CyMGs with CP rotors generally experience similar perpendicular magnetic forces and slightly higher eccentric magnetic forces, relative to CyMGs with SPM rotors. The optimal 2D designs are further investigated using 3D FEA, and, within this design set, the CyMGs with CP rotors experience more significant end effects than the CyMGs with SPM rotors.

Magnetic gears perform the same fundamental power conversion as mechanical gears. However, magnetic gears have significant potential benefits due to their noncontact operation. This paper compares three different ways to achieve high gear ratios using coaxial magnetic gears by evaluating trends for radial flux coaxial magnetic gears with surface mounted permanent magnets. First, a single-stage design can be used, but the torque density and efficiency both decline as the gear ratio increases. Additionally, the gear ratio achievable with a single-stage coaxial magnetic gear is limited by practical considerations, such as the maximum number of modulators and pole pairs that can be used within the given space. Second, a multistage design can be formed by connecting single-stage designs in series. Multistage designs can achieve much higher net gear ratios with much less of a torque density penalty, especially as the number of stages increases, but this entails greater complexity. Third, the compound differential coaxial magnetic gear (CDCMG) is proposed. The CDCMG is formed by interconnecting two single-stage coaxial magnetic gears and can achieve gear ratios much higher than the product of the gear ratios of the individual stages. However, the circulating power in the CDCMG leads to poor efficiencies.

Wide bandgap (WBG) device-based power electronics converters are more efficient and lightweight than silicon-based converters. WBG devices are an enabling technology for many motor drive applications and new classes of compact and efficient motors. This paper reviews the potential applications and advances enabled by WBG devices in ac motor drives. Industrial motor drive products using WBG devices are reviewed, and the benefits are highlighted. This paper also discusses the technical challenges, converter design considerations, and design tradeoffs in realizing the full potential of WBG devices in motor drives. There is a tradeoff between high switching frequency and other issues such as high dv/dt and electromagnetic interference. The problems of high common mode currents and bearing and insulation damage, which are caused by high dv/dt , and the reliability of WBG devices are discussed.

Future space missions require new robotic technologies, such as precision gearboxes capable of achieving speed-reduction ratios in excess of 1000:1, specific torques exceeding 50 N*m/kg, operation in environmental temperatures as low as 40 K, and operation in low-atmosphere or hard vacuum, with high reliability and lifetime. The tribological challenges associated with lengthy missions in harsh space environments may be ameliorated with contactless magnetic gearing solutions. This paper employs an extensive parametric 2-D finite element analysis (FEA) study to optimize the high-torque stage of a two-stage surface permanent magnet radial flux cycloidal magnetic gear train, accounting for the impacts of practical fabrication constraints, which degrade the achievable performance and shift the optimal gear ratio. This paper presents a novel discussion of the internal stress distribution and the reaction forces acting on the structure of the cycloidal magnetic gear. This paper also proposes a rotor with three sections to simultaneously balance the center of mass, radial magnetic forces, and off-axis torques. A proof-of-concept prototype was developed, and the experimental slip torque and specific torque are presented. The specific torque of the prototype discussed in this paper is competitive with commercial mechanical cycloidal-type drives with a similar torque rating.

Magnetic gears have recently received significant interest as a potential replacement for mechanical gears. Some papers have proposed that magnetic gears may be more tolerant to misalignment than their mechanical counterparts. This paper describes a parametric finite element analysis (FEA) study of the effects of misalignment in an axial flux magnetic gear (AFMG). A base design is selected and important parameters, including gear ratio, pole count, and outer radius, are varied to illustrate the effect of misalignment on different designs using simulation results. The torque signature of the gear tends to be more sensitive to misalignment on the high pole count rotor than on the low pole count rotor, and increasing the pole counts makes the gear more sensitive to misalignment. However, other parameters, such as magnet thicknesses, have much smaller impacts on the gear’s sensitivity to misalignment. Experimental results confirm the simulated trends for the slip torque.

The noncontact operation of magnetic gears offers significant potential advantages over mechanical gears. Reluctance magnetic gears (RMGs) have been proposed as an alternative to surface permanent magnet gears (SPMGs) at high speeds. This paper explains the operating principles of RMGs and provides a thorough parametric comparison between RMGs and SPMGs using 2-D and 3-D finite element analysis (FEA). SPMGs are found to outperform RMGs in torque density, as expected. However, contrary to the results of nonoptimized comparisons in previous papers, optimal SPMGs also can achieve better magnet utilizations and efficiencies than optimal RMGs. Additionally, the RMGs with the highest torque densities suffer from significantly more torque ripple than the SPMGs with the highest torque densities. Also, the simulations demonstrate that the optimal SPMGs and optimal RMGs with the same stack lengths tend to suffer comparable reductions in torque from end effects.

This paper provides a thorough, parametric comparison of axial and radial flux coaxial magnetic gears with surface-mounted permanent magnets. While both topologies share similar operating principles and can achieve comparable shear stresses, they exhibit different scaling trends with respect to key design parameters. Both topologies’ volumetric torque densities (VTDs) increase with the outer radius, but the axial flux topology's VTD increases with the radius at a much faster rate than the radial flux topology's VTD. Another difference between the topologies involves their cross-sectional scaling parameters. The stack length axially scales radial flux gear cross sections, and, as it increases, the impact of end-effects decreases, which improves the performance. For axial flux gears, the radii ratio scales the cross section's radial width between different radii. This fundamentally changes optimal parameter values and design performance tradeoffs. Finally, axial flux rotors experience significant axial forces, but the radial forces on radial flux rotors can be balanced out. Based on these trends, radial flux gears are superior to axial flux gears for most applications; however, axial flux gears have a significant advantage when a large radius and a small axial length are permissible or when mechanical power needs to be transmitted across a flat barrier.

A magnetically geared machine (MGM) integrates a magnetic gear with a low-torque, high-speed electric machine to create a single compact high-torque, low-speed device with the size advantages of a mechanically geared system and the reliability of a direct drive machine. This work investigates the use of MGMs for wave energy conversion through the development of a large-scale magnetically decoupled inner stator radial flux magnetically geared generator rated for 10 kW at an input speed of 30 r/min. Critical design trends are illustrated using parametric two-dimensional and three-dimensional finite element simulation results. Information is also provided about the prototype's mechanical structure and key magneto-mechanical design considerations, including the impact of modulator bridges and the extent of axially escaping leakage flux. The prototype's experimental stall torque of 3870 Nm represents a 99.1% match with the simulated stall torque and corresponds to volumetric and gravimetric torque densities of 82.8 kNm/m3 and 14.5 Nm/kg, respectively. Additionally, the prototype achieves an experimental efficiency of approximately 90% for operation near rated torque.

This is Part II of a two part paper on the development of a parameterized linear magnetic equivalent circuit (MEC) for radial flux magnetic gears with surface permanent magnets. Part I describes the MEC implementation. This section, Part II, evaluates the MEC model's accuracy by comparing its results against those produced by a nonlinear finite-element analysis (FEA) model. Simulation results demonstrate that the linearity approximation does not prevent the MEC from accurately matching the torque and air-gap flux densities predicted by a nonlinear FEA model for three diverse magnetic gear base designs. The impacts of the MEC discretization parameters introduced in Part I are also investigated using the same base designs, and guidelines for those settings are developed. Additionally, single design parameter sweeps illustrate the MEC's ability to track these changes over most practical design ranges and reveal where the MEC's accuracy degrades due to the linearity approximation. Finally, the results of a 46,656 case parametric optimization study demonstrate the MEC's ability to match the nonlinear FEA model's torque predictions within a few percent over a wide range of designs while achieving average simulation speeds 44 to 271 times faster than those of the FEA model.

This study employs a genetic algorithm (GA) to optimize surface mounted permanent magnet coaxial radial flux magnetic gear designs using both 2-D finite element analysis (FEA) and 3-D FEA. Specifically, the GA optimizes different designs, which are all rated for a stall torque of 500 N·m and a gear ratio of approximately 5, to independently maximize volumetric torque density (VTD), torque per dollar (TPD), and gravimetric torque density (GTD). Maximum VTDs of 274 and 210 kN·m/m3 were obtained with 2-D and 3-D simulations, respectively. Including the space required to provide an axial buffer for leakage flux resulted in a maximum leakage adjusted VTD of 162 kN·m/m3. Maximum TPDs of 5.86 and 5.47 N·m/$ were obtained with 2-D and 3-D simulations, respectively. Maximum GTDs of 102.8 and 86.8 N·m/kg were obtained with 2-D and 3-D simulations, respectively. The results demonstrate that independently maximizing these three metrics leads to markedly different designs with widely varying performance characteristics. The most significant differences occur between the maximum VTD and maximum TPD designs, and the analysis includes a thorough discussion of the dominant design parameters driving this phenomenon. Finally, the impacts of end effects on the optimal design parameters are also illustrated to demonstrate that consideration of these 3-D effects leads to significantly different performance predictions and to different optimal design selections.

Magnetic gears promise the benefits of mechanical gears with added advantages from contactless power transfer. Although most of the literature focuses on minimizing the size of magnetic gears, their material costs must also be reduced to achieve economic feasibility. This paper compares the active material costs of NdFeB and ferrite radial flux coaxial magnetic gears with surface permanent magnets through a parametric two-dimensional (2-D) and 3-D finite element analysis study. Differences in optimal design trends such as pole counts and magnet thicknesses are illustrated for the two materials. The results demonstrate that, for most historical price rate scenarios, NdFeB gear designs are capable of achieving lower active material costs than ferrite gear designs, and they are always capable of achieving much higher torque densities. Based on the selected design constraints, relative to a nominal ferrite cost of $10/kg, NdFeB must cost more than $90/kg before ferrite is cost competitive. However, ferrite gears can achieve higher efficiencies than NdFeB gears, especially at high speeds, and generally emit less axial leakage flux. Additionally, contour plots are provided to show the impact of material price rate variation on the cost break points.

This paper evaluates the impacts of various means of securing the magnetically active materials in a radial flux coaxial magnetic gear with Halbach arrays and air cores. Rods through holes in the modulators and spacers between adjacent modulators can be used to attach the modulators to their end caps with a minimal reduction of the design’s slip torque. A bridge between adjacent modulators simplifies assembly but significantly reduces the slip torque. Retention sleeves can secure the magnets, but, due to an increase in the effective air gaps, these sleeves drastically reduce the slip torque, especially if the effective outer air gap is increased. Nonmagnetic walls between adjacent magnets facilitate positioning and holding the magnets but significantly reduce the slip torque. However, extending the magnets axially beyond the modulators provides additional space for structural support while increasing the slip torque. A prototype with a 4.67:1 gear ratio was fabricated and tested. Its active material torque density of 25.2 N∙m/kg is competitive with commercially available mechanical planetary gear examples. However, when the structural mass of the prototype is considered, its torque density is 7.1 N∙m/kg. It achieves 99% efficiency, which is higher than the efficiency of the example mechanical gears.

This paper provides a loss breakdown of the NovaMAX® 215 frame, 1800 rpm motor at 5 kW. Mechanical losses, copper loss, and various types of electromagnetic losses are identified through experiment and simulation. The motor is able to achieve a very high efficiency, up to 96% at some operating points. A comparison with another highly efficient permanent magnet motor illustrates the benefits of the dual rotor axial flux topology, the use of grain oriented electrical steel, and the conical air gap and rotor flux focusing topology together with ferrite magnets. Additionally, a motor with a variable air gap was tested at different air gaps, and its losses are shown. At low speed and high torque, the losses are minimized with relatively small air gaps. However, at higher speeds and lower torques, the optimal air gaps are larger.

This paper presents an analytical approach to the nonlinear dynamics of magnetic gears. Without compromising the system's nonlinearity, a magnetic gear's motion can be separated into rigid body motion and motion about a fixed center. Then, neglecting losses, the behavior of the torque angle can be evaluated using the conservation of energy principle. This approach shows excellent agreement with a Simulink model and a transient finite element analysis (FEA) model of a magnetic gear with sinusoidal torque angle curves. However, there are some discrepancies in the predicted velocities and oscillation frequencies compared to results produced by FEA models of designs with significant torque ripples, but the proposed approach still agrees with each of the FEA models about whether the gear will slip for over 98.5% of the cases. Also, cases with viscous friction are considered, but this has a limited effect on the system's dynamic behavior immediately after a change in the applied torques. With viscous friction, the proposed approach still agrees with each of the FEA models about whether the gear will slip for at least 93.5% of the cases. This analysis also demonstrates the significant impact of the effective inertia ratio on the system's dynamic performance.

Magnetic gears transform power between low-torque, high-speed rotation and high-torque, low-speed rotation using magnetic fields instead of interlocking teeth. This paper uses an extensive parametric finite element analysis study to quantitatively compare the optimal performances of two of the most promising magnetic gear topologies, radial flux coaxial magnetic gears and radial flux cycloidal magnetic gears, in terms of volumetric torque density and permanent magnet gravimetric torque density. At low gear ratios, optimal coaxial gears generally achieve higher torque densities than optimal cycloidal gears. However, at medium and high gear ratios, the cycloidal topology can generally outperform the coaxial topology in both of these metrics unless very thick magnets are used. Additionally, the cycloidal magnetic gear can realistically achieve much higher gear ratios than the coaxial magnetic gear, but the optimal gear ratio for a cycloidal design varies with other design parameters, such as outer radius. However, cycloidal designs suffer from significant fabrication challenges because one rotor's axis orbits the axis of the other rotor. Additionally, cycloidal magnetic gear rotors experience strong magnetic forces, which must be supported by the bearings, whereas the net magnetic forces on each rotor in a coaxial magnetic gear can be canceled out using symmetry.

A magnetically geared machine (MGM) integrates a magnetic gear with a low torque, high speed electric machine to create a single compact high torque, low speed device with the size advantages of a mechanically geared system and the reliability of a direct drive machine. This work investigates the use of MGMs for wave energy conversion through the development of a large scale magnetically decoupled inner stator radial flux magnetically geared generator rated for 10 kW at an input speed of 30 rpm. Critical design trends are illustrated using parametric 2D and 3D finite element simulation results. Information is also provided about the prototype's mechanical structure and key magneto-mechanical design considerations, including the impact of modulator bridges and the extent of axially escaping leakage flux. The prototype's experimental stall torque of 3870 N·m represents a 99.1% match with the simulated stall torque and corresponds to volumetric and gravimetric torque densities of 82.8 kN·m/m3 and 14.5 N·m/kg, respectively. Additionally, the prototype achieves an experimental efficiency of approximately 90% over much of its operating range.

This study employs a genetic algorithm (GA) to optimize coaxial radial flux magnetic gear designs using both 2D Finite Element Analysis (FEA) and 3D FEA. Specifically, the GA optimizes different designs, which are all rated for a stall torque of 500 N·m and a gear ratio of approximately 5, to independently maximize volumetric torque density (VTD), torque per dollar (TPD), and gravimetric torque density (GTD). Maximum VTDs of 274 kNm/m3 and 210 kNm/m3 were obtained with 2D and 3D simulations, respectively. Maximum TPDs of 5.86 N·m/$ and 5.47 N·m/$ were obtained with 2D and 3D simulations, respectively. Maximum GTDs of 102.7 N·m/kg and 86.8 N·m/kg were obtained with 2D and 3D simulations, respectively. The results demonstrate that independently maximizing these three metrics leads to markedly different designs with widely varying performance characteristics. The most significant differences occur between the maximum VTD and maximum TPD designs, and the analysis includes a thorough discussion of the dominant design parameters driving this phenomenon. Finally, the impacts of end-effects on the optimal design parameters are also illustrated to demonstrate that consideration of these 3D effects leads to significantly different performance predictions and different optimal design selections.

Magnetic gears promise the benefits of mechanical gears with added advantages from contactless power transfer. Although most literature focuses on minimizing the size of magnetic gears, their material costs must also be reduced to achieve economic feasibility. This work compares the active material costs of NdFeB and ferrite radial flux magnetic gears with surface permanent magnets through a parametric 2D and 3D finite element analysis (FEA) study. Differences in optimal design trends such as pole count and magnet thicknesses are illustrated for the two materials. The results demonstrate that, for most historical price combination scenarios, NdFeB gear designs are capable of achieving lower active material costs than ferrite gear designs, and they are always capable of achieving much higher torque densities. Based on the selected design constraints, relative to a nominal ferrite cost of $10/kg, NdFeB must cost more than $90/kg before ferrite is cost competitive. Additionally, contour plots are provided to show the impact of material price rate variation on the cost break points.

Axial flux magnetically geared machines offer the potential to achieve exceptional torque densities at high diameters. This paper proposes a topology for an axial flux magnetically geared machine, in which the electric machine is placed in the bore of the axial flux magnetic gear. This new topology offers the advantage of correctly matching the size of the electric machine to its required torque without increasing the volume of the system beyond that of the magnetic gear. A proof-of-concept prototype with a torque density of $\text{7.8 kN}\cdot\text{m/m}^{3}$ was designed, fabricated, and tested to evaluate the novel topology. A less conservative alternate design capable of achieving $\text{60.6 kN}\cdot\text{m/m}^{3}$ was also simulated to demonstrate the topology's potential for high torque densities, and even higher torque densities can be reached at larger scale designs.

Magnetic gears can scale the torque and speed of rotary systems, while avoiding problems associated with transferring torque through physical contact. This work uses 3D finite element analysis to parametrically characterize the performance benefits which can be obtained from applying Halbach magnet arrays to axial field magnetic gears. It is found that Halbach arrays decrease the flux density in the adjacent back iron, allowing the thickness of these yokes to be reduced. Also, using Halbach arrays increases the magnitude of the desired magnetic flux spatial frequencies in the air gaps, increasing torque, and decreases the magnitude of undesired spatial frequencies, decreasing torque ripple. Additionally, the use of Halbach arrays on both rotors increased the torque volumetric torque density for two different design points by 43.6% and 65.7% to 183.9 and 155.8 kN∗m/m3, respectively, and decreased the cogging torque on both rotors.