Superconducting Field Winding Research Articles

A Superconducting Vernier Motor for Electric Ship Propulsion

A megawatts (MW)-class direct-drive vernier motor for electric ship propulsion is proposed in this paper. The proposed vernier motor adopts the homopolar structure and utilizes the high-temperature superconducting wire for electrical excitation. The stator houses the superconducting field winding and the copper armature winding. The rotor consists of an iron core without excitation sources. Thus, no cryo-moving components are required. A dual-body Dewar for cooling the pancake high-temperature superconducting field winding is proposed. The motor performance is analyzed and evaluated using three-dimensional finite-element analysis. Both the brushless ac and brushless dc operation modes for the proposed motor are assessed and compared.

Electromagnetic Design of 13.2 MW Fully Superconducting Machine

As low ac loss superconductors progress, the feasibility of fully superconducting (SC) machines is worth studying every few years. A 13.2 MW fully SC machine with MgB2 armature windings and low-temperature superconducting (LTS) field windings for direct drive wind turbines is proposed and electromagnetically designed in this paper. The performance of this machine is studied through finite elements analysis. Comparisons are made between the fully SC machine and a well-designed LTS machine in aspects of weight, initial cost of effective components, and so on. It is found that fully SC machines are of higher torque density and power density, while the improvement could hardly cover the high initial cost of the more complex mechanical support and cooling systems.

A Superconducting Linear Variable Reluctance Machine for Urban Transportation Systems

This paper presents a new superconducting linear variable reluctance machine for urban transportation applications. The proposed linear machine has a doubly salient structure. Its armature adopts segmented configuration and houses both the copper armature winding and the high-temperature superconducting field winding, while the salient-pole rail consists of iron core only with no excitation sources. Performances of the proposed machine are analyzed and evaluated by the finite element method. By using segmented armature design, the force ripple can be suppressed to 5.5% that is very desirable for urban transportation systems.

A Novel Double Stator Flux Modulation Machine With Low-Temperature Superconducting Windings

Flux modulation permanent magnet machines (FMPMMs) exhibit many superiorities over regular permanent magnet machines (PMMs), such as simple and robust configuration, high torque density, and efficiency; thus they are promising candidates for the motors in wind turbine powertrains, electric vehicles, and ship propulsion systems. However, since the PM flux linkage is almost constant, the flux-weakening capability of FMPMMs is usually inferior to that of the wound field machines (WFMs). Moreover, the power factors of FMPMMs are also lower. In order to incorporate the merits of FMPMMs and WFMs, this paper proposes a double-stator flux modulation machine with low-temperature superconducting field windings, which can exhibit higher torque density, stronger flux weakening capability, and larger power factor than the conventional FMPMMs. The configurations and working mechanism of the proposed machine are introduced, and its key performances are comparatively analyzed to a regular FMPMM based on a finite-element algorithm.

Potential of Partially Superconducting Generators for Large Direct-Drive Wind Turbines

This paper aims at assessing the potential of partially superconducting generators for 10 MW direct-drive wind turbines by investigating their performance for a very wide range of excitation currents. Performance indicators such as shear stress and efficiency and other generator characteristics are compared for 12 different generator topologies. To be sufficiently attractive, superconducting generators must have significant advantages over permanent magnet direct-drive generators, which typically have shear stresses of the order of 53 kPa and efficiencies of 96%. Therefore, we investigate what excitation is required to obtain a doubled shear stress and an efficiency of 98%. To achieve this, the different topologies require a range of excitation from 200 to 550 kAt (ampere-turns) with a low armature current density of 2 A/mm $^2$ . The more iron that is used in the core of these topologies, the easier they achieve this performance. By examining the maximum magnetic flux density at the location of the superconducting field winding, feasible superconductors can be chosen according to their engineering current density capabilities. It is found that high- and low-temperature superconductors can meet the performance criteria for many of the topologies. MgB $_2$ superconductors are feasible for the fully iron-cored topology with salient poles but need cooling down to 10 K.

Demonstration of a Practical Nb$_3$ Sn Coil for an Actively Shielded Generator

This paper describes a detailed design of a 6-T Nb 3 Sn superconducting racetrack coil designed for conduction cooling. We then describe a bench test pursued as a proof of concept for one winding of an actively shielded air-core electric machine with superconducting field windings. Electromagnetic design selection is drawn from previous optimization work. The coil former design is then discussed. Numerical simulations of thermal and structural features are pursued to determine temperature distribution and strain within the winding. A coil instrumentation and experimental setup of a quasi-conduction cooled system is described. Finally, test results are presented; a maximum critical current of 480 A was reached at a peak temperature of 7.9 K, surpassing the operational current goal of 435 A. Future work and planned improvements to the test setup are discussed.

Comparison of Peak Armature and Field Winding Currents for Different Topologies of 10-MW Superconducting Generators Under Short-Circuit Conditions

This paper studies the peak armature and peak field winding currents for three different topologies of 10 MW partial high temperature superconducting generators (HTSGs) under short-circuit conditions (SCC) by simulation. The investigated partial HTSGs employ copper armature windings and superconducting field windings with different armature and rotor topologies, i.e., iron-cored armature and rotor, air-cored armature and rotor, and iron-cored armature and air-cored rotor. For each HTSG topology, the investigation includes: 1) the field winding current control strategies; 2) the influence of operating field current; and 3) the ratings of circuit breakers for limiting the peak armature and peak field winding currents. The results can provide guidelines for determining the peak armature and peak field currents of HTSGs and also the possibility of limiting them by employing circuit breakers under SCC.

Electromagnetic Analysis of a HTS Linear-Rotary Permanent Magnet Actuator

In this paper, a novel linear-rotary permanent magnet actuator (LRPMA) is proposed with high-temperature superconducting (HTS) field winding imbedded in the stator. In order to show the merits of the HTS LRPMA, a same structure without HTS field windings and one with permanent magnet mounted on the surface of the mover with same magnetization direction are taken as the comparison. The electromagnetic characteristic and harmonic analysis of the three topologies are analyzed using finite element method, including flux density, back electromotive force, cogging torque and cogging force, torque, and thrust. The air-gap flux density can be easily controlled by adjusting dc current in the HTS field winding. Compared with the other two structures, the torque density of the proposed LRPMA can be easily increased. The computed results show that the performance of the HTS LRPMA, such as the torque density, can be greatly improved.

Design and Development of a Cryogen-Free Superconducting Prototype Generator With YBCO Field Windings

A prototype machine has been developed to study large-capacity wind turbine generators using high-temperature superconducting technology. The superconducting field windings, using 2G YBCO wires, are used in the rotor, whereas the conventional technology is employed in the stator. The racetrack superconducting magnets are cooled directly by two cryo-refrigerators and the operating temperature is 30 K. The cold heads of the refrigerators are installed on the rotor, which is driven by a drive motor, rotating synchronously with a vacuum chamber. A rotary gas seal is employed as the gas slip ring connecting the rotary cold head to a stationary compressor. This paper also presents the electromagnetic characteristics of the prototype machine and the feasibility of the cooling system. Experimental results of back electromotive force are shown at field current from 10 to 100 A and rotating speed from 10 to 30 r/min. The electromagnetic design, based on the finite-element method (FEM), is available in this paper, and the FEM data are coincident with the experimental results very well.

Ripple Field AC Losses in 10-MW Wind Turbine Generators With a MgB2Superconducting Field Winding

Superconducting (SC) synchronous generators are proposed as a promising candidate for 10-20-MW direct-drive wind turbines because they can have low weights and small sizes. A common way of designing an SC machine is to use SC wires with high current-carrying capability in the dc field winding and the ac armature winding is made with copper conductors. In such generators, the dc field winding is exposed to ac magnetic field ripples due to space harmonics from the armature. In generator design phases, the ac loss caused by these ripple fields needs to be evaluated to avoid local overheating and an excessive cooling budget. To determine the applicability of different design solutions in terms of ac losses, this paper estimates the ac loss level of 10-MW wind generator designs employing a MgB 2 SC field winding. The effects on ac losses are compared between nonmagnetic and ferromagnetic teeth with different numbers of slots per pole per phase. The necessity of an electromagnetic shield is then discussed based on the obtained loss levels. The results show that the total ac loss is so small that ferromagnetic teeth can be applied in the generator design without using an electromagnetic shield.

Design of a Superconducting Synchronous Generator With LTS Field Windings for 12 MW Offshore Direct-Drive Wind Turbines

Offshore wind generation is an important trend of global wind power generation. To reduce the cost of energy of offshore wind generation, high-power direct-drive wind turbines are being pursued around the world. With the development of superconductors and related technologies, superconducting (SC) generator offers a possible technological approach to produce large-scale direct-drive wind turbines with a modest cost. A design of a SC wind generator (SCWG) for offshore direct-drive wind turbines is presented in this paper. The proposed SCWG is an electrically-excited synchronous generator with stationary field windings made of low temperature superconductor wires. The electromagnetic design considerations on SCWG are studied. The description on design of copper rotor and SC stator are presented. Several key design technologies including rotor cooling and supporting structure design, SC field winding design, and cryogenic refrigeration system for SC magnets are analyzed. The weight and cost of the proposed SCWG are estimated and compared with that of a permanent magnet wind generator (PMWG) design and several existing similar SCWG designs. It is found that the proposed SCWG is 46% lighter than the PMWG with the same specifications. Moreover, a rough estimation of material cost of the SCWG is carried out, and comparison analysis of cost is also performed.

A modular and cost-effective superconducting generator design for offshore wind turbines

Superconducting generators have the potential to reduce the tower head mass for large (∼10 MW) offshore wind turbines. However, a high temperature superconductor generator should be as reliable as conventional generators for successful entry into the market. Most of the proposed designs use the superconducting synchronous generator concept, which has a higher cost than conventional generators and suffers from reliability issues. In this paper, a novel claw pole type of superconducting machine is presented. The design has a stationary superconducting field winding, which simplifies the design and increases the reliability. The machine can be operated in independent modules; thus even if one of the sections fails, the rest can operate until the next planned maintenance. Another advantage of the design is the very low superconducting wire requirement; a 10 MW, 10 rpm design is presented which uses 13 km of MgB2 wire at 30 K. The outer diameter of the machine is 6.63 m and it weighs 184 tonnes including the structural mass. The design is thought to be a good candidate for entering the renewable energy market, with its low cost and robust structure.

Study of Multiphase Superconducting Wind Generators With Fractional-Slot Concentrated Windings

Superconducting (SC) generator offers a promising candidate for offshore direct-drive wind generators with power of 10 MW or higher. Since copper losses commonly dominate in the SC direct-drive wind generators with SC field windings and copper armature windings, an important way to increase the generator efficiency is to improve the armature winding design. Compared to traditional overlapping windings, fractional-slot concentrated windings (FSCW) have shorter end connection and higher slot fill factor, which has a great contribution to copper loss reduction. However, high content of magnetic motive force (MMF) harmonics is a big challenge for applying FSCW in SC generators. The asynchronous MMF harmonics may cause high eddy-current losses in the rotor assembly, resulting in efficiency reduction and more refrigeration burden. A novel multiphase FSCW with fewer MMF harmonics for SC generators is presented in this paper, followed by an MMF harmonic analysis through winding function method. Comparison with three-phase and traditional dual three-phase FSCW is performed. Characteristics of the generators are compared, in terms of back electromotive-force, electromagnetic torque, and rotor eddy-current losses by using finite element analysis. The advantages of the proposed FSCW are verified. Further, it is observed that the alternating components of the flux density at SC coil area is attenuated significantly by the proposed topology, indicating that the AC losses of the SC coils may be substantially reduced. The feasibility of applying FSCW topology in the design of SC wind generator is enhanced through the study.

Coil Shape Optimization for Superconducting Wind Turbine Generator Using Response Surface Methodology and Particle Swarm Optimization

Changing the cross-sectional shape of superconducting field coils can reduce the total harmonic distortion (THD) of the air gap magnetic flux density (AGMFD) for superconducting wind turbine generators (SWTG). This paper proposes an approach to optimize the cross-sectional shape of the superconducting field windings for SWTG to minimize the THD of the AGMFD. This approach is based on the response surface methodology (RSM) and the particle swarm optimization (PSO). The RSM is used to construct the objective function and the PSO is used to calculate the optimal value of objective function swiftly. The optimized results would be compared with the initial results and thus prove the validity and accuracy of this approach.

Armature reaction effects on a high temperature superconducting field winding of an synchronous machine: experimental results

This paper presents experimental results from the Superwind laboratory setup. Particular focus in the paper has been placed on describing and quantifying the influence of armature reaction on performance of the HTS filed winding. Presented experimental results have confirmed the HTS field winding sensitivity to both armature reaction intensity and angular position with respect to the HTS coils. Furthermore, the characterization of the HTS field winding has been correlated to the electromagnetic torque of the machine where the maximal Ic reduction of 21% has been observed for the maximum torque.

Design of a High-speed Superconducting Bearingless Machine for Flywheel Energy Storage Systems

In this paper, an 8-pole/12-slot high-speed superconducting bearingless machine is proposed for flywheel energy storage systems. The proposed machine adopts a homopolar configuration: the rotor only consists of iron lamination with eight salient iron poles and the 12-slot stator accommodates all three groups of windings: the high temperature superconducting (HTS) field winding; the armature winding; and the suspension winding. With the HTS field winding, eight iron poles in the rotor are magnetized as 4-pole-pair electromagnets. Also, by adjusting the dc current in the HTS field winding, the air-gap flux can be controlled flexibly. By deploying the suspension winding which shares the same slot with the armature winding, a two-degree-of-freedom suspension force can be generated for bearingless operation.

Topology Optimization of a High-Temperature Superconducting Field Winding of a Synchronous Machine

This paper presents topology optimization (TO) of the high-temperature superconductor (HTS) field winding of an HTS synchronous machine. The TO problem is defined in order to find the minimum HTS material usage for a given HTS synchronous machine design. Optimization is performed using a modified genetic algorithm with local optimization search based on on/off sensitivity analysis. The results show an optimal HTS coil distribution, achieving compact designs with a maximum of approximately 22% of the available space for the field winding occupied with HTS tape. In addition, this paper describes potential HTS savings, which could be achieved using multiple power supplies for the excitation of the machine. Using the TO approach combined with two excitation currents, an additional HTS saving of 9.1% can be achieved.

The Difficulty of Modeling Ripple Field Losses in Superconductors Using the Eddy Current Model

The eddy current (EC) model is a widely used tool for simulating hysteresis losses in superconductors. It is a reliable way to estimate losses in 50 Hz transport current, applied field or AC-AC cases. However, in some applications, such as synchronous electric motors with superconducting field windings, a small multifrequency external magnetic field is present while a superconducting coil carries a DC current. In such cases, the use of the EC model becomes challenging. In this paper, we investigate the challenges confronted in such cases by using the EC model in their simulation. It is observed that the EC model using the power law resistivity leads to current density profile homogenization even when no changes happen in the driving quantities. Also, modeling uncoupled cases in which a DC bias current flows in the conductor is troublesome.

A Modular Superconducting Generator for Offshore Wind Turbines

In this study, a new claw-pole type transverse flux superconducting generator topology is presented. The machine has a stationary superconducting field winding, which eliminates electrical brushes and cryocouplers. The machine is specifically designed for low-speed high torque applications such as large offshore wind turbines. The proposed machine is robust and has a modular structure. A 30 kW, 100 rpm prototype is planned to be manufactured to prove the concept. MgB2, YBCO, and BSSCO wires are compared in terms of wire length, operating temperature, and critical temperature. The magnetic flux penetrating into superconducting wire has been simulated using 3D FEA software. Moreover, mechanical loads are estimated and the deflections in the structure are analysed.

Numerical Study of Optimization Design of High Temperature Superconducting Field Winding in 20 MW Synchronous Motor for Ship Propulsion

This paper presents the optimization study of the field winding structure of the 20 MW-class HTS motor for the electric ship propulsion based on the minimization of the flux flow loss. The examples of the optimized shape of the HTS field pole coils obtained by the numerical analysis are presented. By optimizing the cross section of the field pole coil, it is found that the magnetic field perpendicular to the tape surface in the optimized shape coil significantly decreases compared with that in the rectangular shape base coil. As the results of the reduction of the perpendicular magnetic field, it is shown that the flux flow loss remarkably decreases. It is also pointed that the necessary length of the HTS tape for the optimized shape coil increases.