A fractional slot multiphase air-core compulsator with concentrated winding

Compulsator is a specially designed generator capa¬ble of delivering high current pulses to a low-impedance load, such as the electromagnetic railgun. In order to increase the tip speed of the rotor, advanced composite materials have been used in the recent compulsator prototype, which is mentioned as air core instead of the traditional iron core. For typical air-core compulsators, there are no slots and steel teeth to place the armature windings due to the nonmachinability of composite materials. Therefore, concentric windings in racetrack style are often adopted instead of traditional lap winding in most cases, since it is more convenient to be fixed by composite materials. However, overlap occurs at the end winding part for multiphase compulsators, which are not easy to be formed during the manufacture process. In this paper, a fractional slot multiphase air-core compulsator with concentrated windings is proposed and analyzed. The main advantage of fractional slot configuration is that it can offer a concentrated winding structure under certain conditions, which means each coil only spans one “tooth,” and will not cause any intersection between each phase at the end winding. Two referenced fractional slot air-core compulsators with two phases, six poles, and four “slots” or eight “slots” (q = 1/3 and q = 2/3, q is the “slot” per pole per phase) are analyzed and compared with the performance of a traditional integral slot machine. The results indicated that the output voltage and self-excitation performance of a fractional slot compulsator can reach the same level with an integral slot one, and the discharging performance can reach an acceptable level. Thus, the fractional slot multiphase concept can be further used to improve the manufacture process of the winding in the future

Self-excitation and energy recovery of air-core compulsators

As power supplies, compulsators are popular choices for high-end railgun power supplies. In order to increase power and energy density, air-core compulsators are proposed by using composite materials instead of traditional iron-core compulsators. Due to the absence of ferromagnetic material, the flux density in the air-core compulsator can reach to 4–6 T instantaneously, which is much higher than the saturation field strength in traditional iron-core machines. Therefore, self-excitation topology is essential for the air-core compulsator to obtain up to 100-kA field current. This paper carried out research on the key parameters of self-excitation efficiency first, and then focus on the large magnetic energy remained in the inductive field winding after one shot, an implementation scheme and control strategy of energy recovery of air-core compulsator was proposed and analyzed. By controlling the field rectifier working at active inverter state after one discharge process, the magnetic energy stored in the field winding can be converted to rotor kinetic energy again. The simulation results indicate that the energy recovery efficiency can reach to 70% for a reference air-core compulsator. The continuous discharge number of times increased from 3 to 4 during one kinetic energy charging, which means that the delivered energy density increases 33.3%

Challenges and Opportunities for Wound Field Synchronous Generators in Future More Electric Aircraft

Electrical machines and drives keep moving away from traditional technologies such as brushed machines and wound field machines towards lighter, ‘easier to maintain’ machines. A very interesting aspect is that certain transport applications, especially the aerospace industry, still favour the classical wound field machine for its main generating system such as the Boeing 787. This paper focuses on investigating this particular trend by presenting a detailed overview of historical power generation systems on aircraft. This paper compares the current state of the art of wound field machines with other generator families. The results of this analysis are then projected into the needs of the electrical power generation and distribution system on aircraft. While power density is a major objective for any aerospace application, however the extra benefits associated with wound field systems are still essential in modern aircraft. The paper then focuses on the main challenges for improving power density of wound field machines. Recommendations, opportunities and improvements related to wound field machines are discussed. In conclusion, if robust designs for higher speed wound field generators were consolidated, it would be very probable that these classical machines might still be implemented on future MEA platforms

Active thermal control for modular power converters in multi-phase permanent magnet synchronous motor drive system

Modular winding structure has been employed in the Permanent Magnet Synchronous Motors (PMSMs) to increase the reliability and reduce the torque ripple. Nevertheless, the reliability of the motor system depends on the lifetime of the power semiconductor devices. Since the thermal cycles, which can generate the mechanical stress between the different material layers in power devices, are the key factors to influence the lifetime of power devices, in this paper, an Active Thermal Control (ATC) for modular power converters in PMSM drive is proposed to extend the system lifetime. The power routing method is employed to balance the power in a quadruple modular winding PMSM system. The Rainflow Counting Algorithm is used to calculate the thermal cycles with a load mission profile, and estimate the lifetime of the power converters. The proposed method is validated by both simulation and experiments

Two-dimensional interlocked pentagonal bilayer ice: how do water molecules form a hydrogen bonding network?

The plethora of ice structures observed both in bulk and under nanoscale confinement reflects the extraordinary ability of water molecules to form diverse forms of hydrogen bonding networks. An ideal hydrogen bonding network of water should satisfy three requirements: (1) four hydrogen bonds connected with every water molecule, (2) nearly linear hydrogen bonds, and (3) tetrahedral configuration for the four hydrogen bonds around an O atom. However, under nanoscale confinement, some of the three requirements have to be unmet, and the selection of the specific requirement(s) leads to different types of hydrogen bonding structures. According to molecular dynamics (MD) simulations for water confined between two smooth hydrophobic walls, we obtain a phase diagram of three two-dimensional (2D) crystalline structures and a bilayer liquid. A new 2D bilayer ice is found and named the interlocked pentagonal bilayer ice (IPBI), because its side view comprises interlocked pentagonal channels. The basic motif in the top view of IPBI is a large hexagon composed of four small pentagons, resembling the top view of a previously reported ‘‘coffin’’ bilayer ice [Johnston, et al., J. Chem. Phys., 2010, 133, 154516]. First-principles optimizations suggest that both bilayer ices are stable. However, there are fundamental differences between the two bilayer structures due to the difference in the selection among the three requirements. The IPBI sacrifices the linearity of hydrogen bonds to retain locally tetrahedral configurations of the hydrogen bonds, whereas the coffin structure does the opposite. The tradeoff between the conditions of an ideal hydrogen bonding network can serve as a generic guidance to understand the rich phase behaviors of nanoconfined water

Two-dimensional monolayer salt nanostructures can spontaneously aggregate rather than dissolve in dilute aqueous solutions

It is well known that NaCl salt crystals can easily dissolve in dilute aqueous solutions at room temperature. Herein, we reported the first computational evidence of a novel salt nucleation behavior at room temperature, i.e., the spontaneous formation of two-dimensional (2D) alkali chloride crystalline/non-crystalline nanostructures in dilute aqueous solution under nanoscale confinement. Microsecond-scale classical molecular dynamics (MD) simulations showed that NaCl or LiCl, initially fully dissolved in confined water, can spontaneously nucleate into 2D monolayer nanostructures with either ordered or disordered morphologies. Notably, the NaCl nanostructures exhibited a 2D crystalline square-unit pattern, whereas the LiCl nanostructures adopted non-crystalline 2D hexagonal ring and/or zigzag chain patterns. These structural patterns appeared to be quite generic, regardless of the water and ion models used in the MD simulations. The generic patterns formed by 2D monolayer NaCl and LiCl nanostructures were also confirmed by ab initio MD simulations. The formation of 2D salt structures in dilute aqueous solution at room temperature is counterintuitive. Free energy calculations indicated that the unexpected spontaneous salt nucleation behavior can be attributed to the nanoscale confinement and strongly compressed hydration shells of ions. Supplementary files, including 6 movies, attached below

Two-dimensional interlocked pentagonal bilayer ice: how do water molecules form a hydrogen bonding network?

The plethora of ice structures observed both in bulk and under nanoscale confinement reflects the extraordinary ability of water molecules to form diverse forms of hydrogen bonding networks. An ideal hydrogen bonding network of water should satisfy three requirements: (1) four hydrogen bonds connected with every water molecule, (2) nearly linear hydrogen bonds, and (3) tetrahedral configuration for the four hydrogen bonds around an O atom. However, under nanoscale confinement, some of the three requirements have to be unmet, and the selection of the specific requirement(s) leads to different types of hydrogen bonding structures. According to molecular dynamics (MD) simulations for water confined between two smooth hydrophobic walls, we obtain a phase diagram of three two-dimensional (2D) crystalline structures and a bilayer liquid. A new 2D bilayer ice is found and named the interlocked pentagonal bilayer ice (IPBI), because its side view comprises interlocked pentagonal channels. The basic motif in the top view of IPBI is a large hexagon composed of four small pentagons, resembling the top view of a previously reported ‘‘coffin’’ bilayer ice [Johnston, et al., J. Chem. Phys., 2010, 133, 154516]. First-principles optimizations suggest that both bilayer ices are stable. However, there are fundamental differences between the two bilayer structures due to the difference in the selection among the three requirements. The IPBI sacrifices the linearity of hydrogen bonds to retain locally tetrahedral configurations of the hydrogen bonds, whereas the coffin structure does the opposite. The tradeoff between the conditions of an ideal hydrogen bonding network can serve as a generic guidance to understand the rich phase behaviors of nanoconfined water

Electrical Machines for the More Electric Aircraft: Partial Discharges Investigation

Modern electrical machines employed in transportation applications are required to provide high performance in terms of power (and torque) density. At the same time, being these applications safety-critical, a significant level of reliability and/or fault tolerance is expected. Among all the factors which can compromise motors reliability, partial discharges (PDs) inception is one of the most crucial, in particular for low voltage, random wound machines. This article presents an extensive experimental investigation on PDs in electrical machines for aerospace applications. Measurements are carried out using both sinusoidal and fast-rising pulses in a representative aerospace environment, emulating the typical ambient conditions encountered throughout a commercial aircraft mission, and beyond (i.e., down to 30 mbar). As a main result of the investigation, it is proved that electrical machines employed for actuating primary flight control surfaces feature a higher risk of PDs inception. Therefore, their insulation system demands an extremely careful design

Modeling and stability enhancement of a permanent magnet synchronous generator based DC system for More Electric Aircraft

The concept of the More Electric Aircraft (MEA) is aimed at electrifying the mechanical, hydraulic, and pneumatic subsystems on aircraft. With increasing usage of power electronics, the architecture of on-board electrical power distribution systems (EPDS) becomes more complicated. Therefore, it is necessary to analyze the stability of the system. This paper firstly presents and validates an impedance model of a permanent magnet synchronous generator (PMSG) as a source and dual active bridge (DAB) converter as a load. These models are used for the stability analysis of a simple DC power system. In addition, two new control strategies are proposed to enhance the stability of the system. The stabilization effects of the new control strategies are verified with experimental results