Gap Flux Research Articles

Anomalous open-circuit voltage from a high-Tc superconducting dynamo

We report on the behavior of a high-Tc superconducting (HTS) homopolar dynamo which outputs a DC open-circuit voltage when the stator is in the superconducting state, but behaves as a conventional AC alternator when the stator is in the normal state. We observe that this time-averaged DC voltage arises from a change in the shape of the AC voltage waveform that is obtained from a normal conducting stator. The measured DC voltage is proportional to frequency, and decreases with increasing flux gap between the rotor magnet and the HTS stator wire. We observe that the DC output voltage decreases to zero at large flux gaps, although small differences between the normal-conducting and superconducting waveforms are still observed, which we attribute to screening currents in the HTS stator wire. Importantly, the normalised pulse shape is found to be a function of the rotor position angle only. Based on these observations, we suggest that the origin of this unexpected DC effect can be explained by a model first proposed by Giaever, which considers the impact of time-varying circulating eddy currents within the HTS stator wire. Such circulating currents form a superconducting shunt path which “short-circuits” the high field region directly beneath the rotor magnet, at those points in the cycle when the rotor magnet partially overlaps the superconducting stator wire. This reduces the output voltage from the device during these periods of the rotor cycle, leading to partial rectification of the output voltage waveform and hence the emergence of a time-averaged DC voltage.

Investigation of modular odd-pole PM linear synchronous motors with flux gaps

Purpose – Nowadays, to simplify manufacture process and improve fault-tolerant capability, more and more modular electrical machines are being applied in industrial areas. The purpose of this paper is to investigate a novel modular single-sided flat permanent magnet linear synchronous motor (PMLSM), which adopts segmented armature with the required flux gaps between segments to enhance the performance. Design/methodology/approach – Using 2D finite element analysis, the performances, such as open-flux linkage, back-EMF, average thrust force, thrust ripple, etc., are compared in different values of flux gaps, as well as different slot/pole number combinations (mainly odd numbers of poles). Finally, to show the difference of linear motor from rotary one, the detailed comparison is made between modular PMLSM and rotary PMSM. Findings – Due to flux gaps, it is found the electromagnetic performances are worsened along with flux gap width increasing to modular PMLSMs having slot number higher than pole number, but some aspects of performances such as winding factor, open-circuit flux linkage, back-EMF and average thrust can be improved to those having slot number lower than pole number. Due to the end effect of linear format, the thrust ripple is not significantly improved. Originality/value – It is concluded the proper flux gaps can be chosen to improve the performance of PMLSM with certain slot/pole combinations. A new structure of 12-slot-13-pole (hereinafter referred to as 12s/13p) PMLSM with fractional slot and alternative-teeth wound winding is designed.

A Novel Rotating HTS Flux Pump Incorporating a Ferromagnetic Circuit

High-temperature superconductor (HTS) flux pumps enable large currents to be injected into a superconducting coil without requiring normal-conducting current leads. We present results from an experimental axial-type HTS rotating flux pump that employs a ferromagnetic circuit to focus incident flux upon a coated-conductor stator wire. We show that this device can inject currents of > 50 A into an HTS coil at 77 K and is capable of operating at flux gaps greater than 18 mm. Accommodating a cryostat wall within this flux gap will enable future flux pump designs, in which all moving parts are located outside the cryostat.

Cogging Torque Mitigation of Modular Permanent Magnet Machines

This paper proposes a novel cogging torque mitigation method for modular permanent magnet (PM) machines with flux gaps in alternate stator teeth. The slot openings (SOs) of the modular PM machines are divided into two groups in a special way. By shifting the SOs of two groups in opposite directions with the same angle, the machine cogging torque can be significantly reduced. An analytical formula of the desired shift angle is derived, and can be applicable to other modular machines with different slot/pole number combinations. Meanwhile, the influence of the proposed method on phase back electromotive force (EMF) is investigated, and the three-phaseback EMF waveforms remain balanced for all slot/pole number combinations after slot shifting. Experiments based on the existing prototypes are carried out to validate the finite-element modeling.

Mathematical Modeling of Thermal Resistances of Mold Flux and Air Gap in Continuous Casting Mold Based on an Inverse Problem

Mathematical Modeling of Thermal Resistances of Mold Flux and Air Gap in Continuous Casting Mold Based on an Inverse Problem

Modular Permanent-Magnet Machines With Alternate Teeth Having Tooth Tips

This paper presents single-layer modular permanent-magnet machines with either wound or unwound teeth with tooth tips. The structures with wound teeth having tooth tips are suitable for modular machines with slot number higher than pole number to compensate for the drop in winding factor due to the flux gaps in alternate stator teeth, accordingly to maintain or even to increase their average torques. However, the structures with unwound teeth having tooth tips are suitable for modular machines with slot number lower than pole number to increase the winding factor and hence to further improve the machine performance. The phase back-EMF, on-load torque, iron and copper losses, as well as efficiency have been calculated using FE analysis for different slot/pole number combinations, and for different flux gap and tooth tip widths. It is found that by properly choosing the flux gap and tooth tip widths, both the on-load torque performance and the efficiency can be optimized for the investigated machines with different slot/pole number combinations. Experiments have been carried out to validate the FE results.

Impact of flux gap upon dynamic resistance of a rotating HTS flux pump

HTS flux pumps enable superconducting currents to be directly injected into a magnet coil without the requirement for thermally inefficient current leads. Here, we present results from an experimental mechanically rotating HTS flux pump employing a coated-conductor stator and operated at 77 K. We show the effect of varying the size of the flux gap between the rotor magnets and coated conductor stator from 1 to 7.5 mm. This leads to a corresponding change in the peak applied perpendicular magnetic field at the stator from approximately 350 to 50 mT. We observe that our experimental device ceases to maintain a measurable output at flux gaps above 7.5 mm, which we attribute to the presence of screening currents in the stator wire. We show that our mechanically rotating flux pump is well described by a simple circuit model which enables the output performance to be described using two simple parameters, the open-circuit voltage Voc and the internal resistance, Rd. Both of these parameters are found to be directly proportional to magnet-crossing frequency and decrease with increasing flux gap. We show that the trend in Rd can be understood by considering the dynamic resistance experienced at the stator due to the oscillating amplitude of the applied rotor field. We adopt a literature model for the dynamic resistance within our coated-conductor stator and show that this gives good agreement with the experimentally measured internal resistance of our flux pump.

Analytical Modeling of Modular and Unequal Tooth Width Surface-Mounted Permanent Magnet Machines

This paper presents a simple analytical model for two types of three-phase surface-mounted permanent magnet machines: modular and unequal tooth (UNET) width machines with different slot/pole number combinations. It is based on the slotless open-circuit air-gap flux density and the slotted air-gap relative permeance calculations. This model allows calculating the opencircuit air-gap flux density, phase flux linkage, back electromotive force, and average torque of both the modular and UNET machines. Its accuracy has been validated by the 2-D finite-element (FE) method. A flux focusing/defocusing factor has also been introduced to analyze the influence of flux gaps in alternate stator teeth of modular machines on the machine electromagnetic performances. It has been found that by properly choosing the slot/pole number combination and flux gap width, the open-circuit air-gap flux density, the winding factor, and the flux focusing effect can all be improved. As a result, the performance of the modular machine can be significantly boosted. The analytical and FE models have been validated by experiments.

Thermodynamic chemical energy transfer mechanisms of non-equilibrium, quasi-equilibrium, and equilibrium chemical reactions

Chemical energy transfer mechanisms at finite temperature are explored by a chemical energy transfer theory which is capable of investigating various chemical mechanisms of non-equilibrium, quasi-equilibrium, and equilibrium. Gibbs energy fluxes are obtained as a function of chemical potential, time, and displacement. Diffusion, convection, internal convection, and internal equilibrium chemical energy fluxes are demonstrated. The theory reveals that there are chemical energy flux gaps and broken discrete symmetries at the activation chemical potential, time, and displacement. The statistical, thermodynamic theory is the unification of diffusion and internal convection chemical reactions which reduces to the non-equilibrium generalization beyond the quasi-equilibrium theories of migration and diffusion processes. The relationship between kinetic theories of chemical and electrochemical reactions is also explored. The theory is applied to explore non-equilibrium chemical reactions as an illustration. Three variable separation constants indicate particle number constants and play key roles in describing the distinct chemical reaction mechanisms. The kinetics of chemical energy transfer accounts for the four control mechanisms of chemical reactions such as activation, concentration, transition, and film chemical reactions.

Physical states and finite-size effects in Kitaev's honeycomb model: Bond disorder, spin excitations, and NMR line shape

Kitaev's compass model on the honeycomb lattice realizes a spin liquid whose emergent excitations are dispersive Majorana fermions and static ${\mathrm{Z}}_{2}$ gauge fluxes. We discuss the proper selection of physical states for finite-size simulations in the Majorana representation, based on a recent paper by F. L. Pedrocchi, S. Chesi, and D. Loss [Phys. Rev. B 84, 165414 (2011)]. Certain physical observables acquire large finite-size effects, in particular if the ground state is not fermion-free, which we prove to generally apply to the system in the gapless phase and with periodic boundary conditions. To illustrate our findings, we compute the static and dynamic spin susceptibilities for finite-size systems. Specifically, we consider random-bond disorder (which preserves the solubility of the model), calculate the distribution of local flux gaps, and extract the NMR line shape. We also predict a transition to a random-flux state with increasing disorder.

Experimental study of the prediction of the ventilation flow rate through solar chimney with large gap-to-height ratios

In this study, experiment was carried out with a solar chimney model with large gap-to-height ratios between 0.2 and 0.6. The experimental results show that the existing prediction method available in the literature overpredicts the airflow rate for chimney geometry investigated in this work, especially for vertical solar chimney with large gap-to-height ratios. An improved prediction method, which takes into account the variation of pressure loss coefficient for different flow conditions at the chimney outlet, is presented and compared with the experimental results obtained in this work and available in the literature. It is shown that the improved prediction method are in better agreement with the experiment results than the existing prediction method available in the literature for both narrow and wide chimneys. On the other hand, it is also found that the temperature and velocity distribution of air and airflow rate in chimney are highly dependent on heat flux and chimney gap. Experimental results show that an optimum gap-to-height ratio that maximizes the airflow rate in chimney is around the gap-to-height ratio of 0.5.

Computer Aided Design of New Type PMSM Based on ISAD

Integrated starter alternator damper (ISAD) system requires motor: high-efficient, high-power density, low-ripple torque. Halbach motor is a new type of PMSM: unique permanent magnet structure makes motor have a sinusoidal magnetic field, can increase magnetic flux density of air gap and decrease magnetic flux density of rotor, which could enhance the power density and efficiency of motor as well as reduce its size. Software of Halbach motor design is given, also with the finite element analysis software with ANSYS language. Experiment proves the advantage of Halbach motor in ISAD system.

Experimental Investigation of Using Solar Chimney to Induce Natural Ventilation

The vertical panels of solar chimney have internal dimensions of 2000mm height、1000mm length. Under the condition of heat flux and chimney gap variety, we research chimney interior velocity field. Experimental Results show that airflow increased with chimney gap augmentation, the airflow and air velocity augment with the increase of solar radiant intensity, and air velocity decreases with the increase of solar chimney gap. Air velocity is higher near the heated surfaces than it in the middle chimney. Meanwhile velocity boundary layers form near the heated surface.

Influence of Flux Gaps on Electromagnetic Performance of Novel Modular PM Machines

In order to simplify manufacture processes and improve fault-tolerant capabilities, modular electrical machines, especially the ones with segmented stators, are increasingly employed. However, flux gaps between segments are often inevitable. In this paper, to take advantage of these flux gaps to enhance the machine performance, novel modular permanent magnet machines with different slot/pole combinations have been proposed. The influence of these flux gaps on the electromagnetic performance of modular PM machines, such as winding factor, open-circuit air-gap flux densities, back-EMFs, cogging torque, on-load torque, inductances, magnetic saturation and copper losses, are comprehensively investigated and general rules have been established. It is found that for modular machines having slot number higher than pole number, the flux gaps between stator segments degrade the electromagnetic performance due to the lower winding factor and the flux defocusing effect. However, for modular machines having slot number lower than pole number, the electromagnetic performances can be significantly improved using proper flux gap width due to the higher winding factor and the flux focusing effect. The finite element results are validated by experiments using two prototype modular machines.

Comparative Studies of Modular and Unequal Tooth PM Machines Either With or Without Tooth Tips

This paper presents four types of three-phase permanent magnet (PM) machines, that is, modular and unequal teeth (UNET) machines either with or without tooth tips. Two-dimensional finite element (FE) modeling has been carried out to analyze the influences of flux gaps (in alternate stator teeth) and UNET widths. These influences are mainly on winding factors, air-gap flux density due to PMs, flux linkage, back-electromotive force, and electromagnetic torques. For UNET machines without tooth tips, it has been found that the UNET widths affect both winding factor and air-gap flux density due to PMs. However, for UNET machines with tooth tips, the UNET widths have no effect on machines' electromagnetic performance. Nevertheless, for modular machines, the flux gaps influence three parameters: winding factor, air-gap flux density due to PMs, and flux focusing/defocusing. Meanwhile, for different slot/pole number combinations, the influences of flux gap and UNET widths are different. Several general rules concerning these influences have been established. These general rules can be extended to other single or dual three-phase modular or UNET machines with different slot/pole number combinations. Prototypes of modular machines have been built, and the predictions and conclusions have been validated.

A nonlinearity in permanent-magnet systems used in watt balances

Watt balances are used to measure the Planck constant and will be used in the future to realize mass at the kilogram level. They increasingly rely on permanent magnet systems to generate the magnetic flux. It has been known that the weighing current might affect the magnetization state of the permanent-magnet system used in these systems, causing a systematic bias that can lead to an error in the result if not accounted for. In this article, a simple model explaining the effect of the weighing current on the yoke of the magnet is developed. This model leads to a nonlinear dependence of the magnetic flux density in the gap that is proportional to the squared value of the coil current. The effect arises from changing the reluctance of the yoke by the additional field produced by the coil. Our analysis shows that the effect depends on the width of the air gap, the magnetic flux density in the air gap, and the BH curve of the yoke material. Suggestions to reduce the nonlinear effect are discussed.

Parameter Design by a Novel Method and Optimization of the Field Coil for a 500 kW Air-Gap HTS Generator

Due to the special structures of high temperature superconducting (HTS) generators, the traditional magnetic circuit method for electromagnetic designing is not yet appropriate. In this paper, the separated variables method is introduced to calculate the magnetic fields of an HTS generator, and by this novel method, parameters of a 500 kW HTS generator is designed. Furthermore, the variations of harmonics content and amplitude of flux density in the air gap with the different open angles of HTS field coil are evaluated. Results obtained by the analytical method are basically in agreement with that obtained by finite element method. It is shown that the analytical method is effective and convenient in the design of HTS generator.

Idealized numerical experiments on microscale eddies in the Venusian cloud layer

Three-dimensional microscale dynamics of convective adjustment and mixing in and around the Venusian lower cloud layer were investigated using an idealized Weather Research and Forecasting (WRF) model. As control parameters of the idealized experiment, the present work introduces an initial lapse rate in the convective layer and thermal flux associated with the infrared flux gap at cloud base. Eddy heat, material, and momentum fluxes increase in the convective layer with the increase of these two parameters. In the case of convective adjustment over a very short period, prior to formation of a large-scale convective cell, transient microscale eddies efficiently and rapidly eliminate the convective instability. In the case of convective mixing induced by cloud-based thermal flux, microscale eddies are induced around a thin unstable layer at the cloud base, and spread to the middle and upper parts of the neutral layer. For atmospheric static stability around 55 km, two types of fine structure are found: a wave-like profile induced by weak microscale eddies, and a profile locally enhanced by strong eddies.

Large DC Electromagnet for Semi-Industrial Thermomagnetic Processing of Nanocrystalline Ribbon

A large dc electromagnet with a 200 mm long air gap was designed, constructed, and tested. Analytical calculations provided the first approximation of required parameters. The design was refined with the aid of finite element modeling. Thermomagnetic processing required nominal flux density level to be 0.35 T. It was validated on a prototype that the average flux density in the air gap exceeded this value. The magnetizing coils were designed to be cooled by natural air convection. Despite a large size $({\sim}{\rm 1900}~{\rm kg})$ , the electromagnet was designed to consume less than 2.5 kW and could be energized from a single-phase mains socket.

Analyzing Thermal Shrinkage of Fire-Protective Clothing Exposed to Flash Fire

The thermal shrinkage of a fire-protective garment has significant influence on its thermal performance. However, there is a lack of systematic investigation on garment shrinkage under exposure to flash fire. Based on a posture-adjustable flame manikin, a new approach to characterize the thermal. The shrinkage during flash-fire exposure is proposed, and several factors were investigated shrinkage was measured in different test conditions by calculating the deformation of markers on the garment. Results showed that the shrinkage was affected by the exposure conditions, fabric properties, garment size, and body posture. The shrinkage showed different results in the four directions and was unevenly distributed over the manikin. The distribution of garment shrinkage was analyzed and correlated with the heat flux and air gap. The most severe shrinkage location was the arms and legs. The shrinkage at the rear was larger than at the front. There was significant difference between the chest and other parts such as the back, arms, and legs (P < 0.01). There was also significant difference between the back and the arms as well as legs (P < 0.01). This indicates that the correlation with shrinkage was weak for the air gap size but moderate for the heat flux. Design of fire-protective clothing should take the thermal shrinkage property into consideration, and this study provides a technical basis to engineer high-performance protective clothing.