Superconducting Armature Windings Research Articles

Ic measurement of twisted multifilamentary MgB2 wires with non-magnetic sheath over a wide range of temperatures and fields

All-superconducting rotating machines have the potential for meeting the high power density and high efficiency required for electrical aircraft applications. However, very high AC loss encountered in superconducting armature windings could hinder their development. Multifilamentary MgB2 wires are one of the promising candidates for the stator windings, due to their potentially low AC loss properties with small filament size and twist pitches. As the first step, the dependence of critical current and n-value on magnetic fields and temperatures Ic(B, T) and n(B,T), which are basic input parameters for AC loss simulation, needs to be measured. In this work, we present transport Ic measurements in three non-magnetic multifilamentary MgB2 wires (MgB2/Nb/CuNi/CuZn): one large wire with a 0.70 mm diameter and 25 mm twist pitch, and two small wires with a 0.48 mm diameter each and a 10 mm and 30 mm twist pitch respectively. A four-probe direct current method is used to measure Ic of the MgB2 wires with variations in temperature (15 – 35 K) and magnetic field (0 – 5.5 T). Full Ic data for the small wire with 10 mm twist pitch was obtained, and the n-values were mostly less than 20. While the Ic data for the large wire at low fields was more limited due to heating, the n-values were higher and could be up to around 100. The difference is attributed to the different filament sizes. Experiments also found that there is no significant hysteresis in the transport critical current measured by decreasing or increasing magnetic fields due to the non-magnetic sheaths. This non-hysteretic characteristic is critical for lowering AC loss because the additional losses from magnetic sheaths can be eliminated. From the magnetic-field dependence of critical current density, an empirical expression has been developed that provides suitable extrapolations to lower fields for the large wire.

Performance Analysis of HTS Armature Axial Flux Magnetic Field Modulation Direct Drive Wind Generator

Axial flux electrical machine has higher power density than the radial flux electrical machine, showing a promising application in the field of offshore wind power generation. However, the conventional copper armature winding will suffer from severe joule loss when improving the electrical loading, resulting in that the further improvement of the power density of axial flux electrical machine is limited. In order to overcome the limitation of the electrical loading improvement of the copper armature winding, this paper uses the high-temperature superconducting (HTS) armature winding instead of the conventional copper armature winding, proposes an HTS armature axial flux magnetic field modulation direct-drive wind generator (HTSAAFFM-DDG). In this paper, the topology and operational principle of the proposed HTSAAFFM-DDG is clearly demonstrated. Meanwhile, the electromagnetic performances are carefully analyzed, including airgap magnetic field, basic electromagnetic characteristics, and AC loss analysis of the HTS armature winding. The research results show that the torque density of the proposed HTS armature axial flux machine can be 24% higher than that of the copper armature axial flux machine with the same size and topology. Besides, the superconducting (SC) shielding layer used in the proposed HTSAAFFM-DDG exhibits an excellent inhibition effect on the PM excited field, and can reduce the AC loss of the armature winding obviously.

Thermal Characteristic Analysis of Fully Superconducting Motors Employing Dilute Gas Rotor Cooling Structure

We have proposed a rotor cooling structure using dilute gas for a fully superconducting motor for aircraft application. Numerical analysis was performed to investigate the feasibility of the proposed cooling method and structure, which can make a fully superconducting motor lighter and more compact than a partially superconducting motor. The helium gas pressure in the gap, the rotor rotational speed, and the rotor loss are key parameters of the cooling, and thermal analysis was performed to clarify the dependence of the field winding temperature on these parameters. In the analysis, the motor model has the maximum output of 5.5 MW and the maximum speed of 5000 rpm. The superconducting armature windings are cooled down to 20 K by conduction cooling with liquid hydrogen, and the superconducting field winding is cooled down to about 50 K indirectly using helium gas in the gap. Numerical analysis results indicate that the helium gas pressure should be increased when the rotor loss is larger, but if the pressure is increased too much, the temperature of the field winding becomes higher due to wind loss. When the rotor loss is several hundred watts, the helium gas pressure should be in the range of 104 to 105 Pa.

Development and testing of a cryogenic induction motor with a novel Dewar structure

A Dewar is the key structure of motors with high temperature superconducting (HTS) armature windings. Traditional Dewars use total immersion cooling, which can cause additional mechanical losses and challenges for rotary sealing. To avoid these problems, a novel Dewar structure was developed with partial immersion cooling. A 15 kW cryogenic induction motor with the designed Dewar was then developed. Its stator core and the coil were cooled with liquid nitrogen through partial immersion cooling. We studied the thermal insulation performance of the Dewar, and the running performance of the cryogenic induction motor. The temperature characteristics of the stator core were measured by PT-100 temperature sensors. Results show that the stable temperature of the stator is 76.5 K. Under no-load conditions, the slip rate of the motor is 0.02 with the no-load current being 30 A at 350 V of the stator voltage. The no-load loss power at low temperature is much larger than that at room temperature. These results suggest that the partial immersion cooling is effective, which validates the reliability of the designed Dewar structure, and that running performance of the motor designed is desirable.

Short-Circuit Characteristics of Superconducting Permanent Magnet Generators for 10 MW Wind Turbines

Superconducting permanent magnet generators (SCPMGs) are a potential candidate for 10 MW direct-drive wind turbine applications. This paper presents two 10 MW SCPMG designs using MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cables for the armature winding and investigates the short-circuit characteristics of the designed SCPMGs. The first part of the results shows that the SCPMGs can double the shear stress of a conventional low-speed permanent magnet (PM) generator (from 65 kPa to 130 kPa) whilst avoiding demagnetization of the PMs in rated-load operation. However, the power factor has to drop to a range of 0.7-0.8. The second part of the results shows that during a sudden three-phase short circuit, the superconducting armature winding is prone to quench and the PMs are likely to be demagnetized in both proposed designs.

Stator Optimization of Wind Power Generators With High-Temperature Superconducting Armature Windings and Permanent Magnet Rotor

Electrical machines with high-temperature superconducting (HTS) armature windings can benefit from high power density and efficiency. Furthermore, by using permanent magnets (PM) on the rotor, the cryogenic system can be significantly simplified compared with the fully superconducting (SC) machines or the machines with SC field windings. Compared with the conventional electrical machines, the performance of machines with HTS armature windings can be more sensitive to the design parameters due to the interaction between the operating current and the flux density. In this article, the influence of stator geometry on the machine with HTS armature windings and PM rotor is investigated. By considering the interaction between the operating current and the flux density, an iteration procedure for calculating the operating current of the HTS armature windings is proposed. A 10-MW surface-mounted PM wind power generator with HTS armature windings is studied by the finite element analysis. It is found that machines with different coil angle have different behaviors when changing the tooth width and the dimensions of tooth tips, which is different from conventional electrical machines. A new design and optimization method is then developed. Finally, the optimal design is identified with global optimization.

Electrical design and structure optimization of 10 MW fully superconducting wind turbine generators

In this paper, two types of conceptual structures of 10 MW fully superconducting offshore wind turbine generators (FSWTG) with an efficient electrical design method are presented. Four types of generator models, Gen. A - D, are compared according to different armature winding structures, and it's found that Gen. A is the preferable one. Optimization of the main generator parameters reveals that 36 poles with a pole pitch of 474 mm and electrical load of 6.0 kA/cm yields the best performance. The different design parameters of FSWTGs are compared to partially superconducting wind turbine generators (PWTG) with HTS field windings and copper armature windings and traditional wind turbine generators (WTG). Results show that FSWTGs are smaller in size, weigh 0.7 and 0.4 times less and are more efficient by 1.9–2.1% and 2.9–3.1% than PWTGs and WTGs, respectively. Lastly, the AC losses generated in the superconducting armature windings of the four types of FSWTGs are calculated and discussed.

Influence of Coil Location and Current Angle in Permanent Magnet Wind Power Generators With High-Temperature Superconducting Armature Windings

In this article, the operating current and torque of surface-mounted permanent magnet (SPM) wind power generators with high-temperature superconducting (HTS) armature windings are analyzed. The influence of coil location and current angle on the electromagnetic performance is investigated systematically. The results show that the location of the HTS coils significantly affects the operating current and torque. Furthermore, the maximum torque current angle of SPM wind power generators with HTS armature windings can be different from 180° due to the interaction between the armature and magnet fields. A design optimization method for wind power generators with HTS armature windings is developed in this article. The optimal design is then identified with consideration of the coupling influence of coil location and current angle.

Conceptual Design of Superconducting Induction Motors Using REBa2Cu3O y Superconducting Tapes for Electric Aircraft

High power density motors are required for electric propulsion aircrafts. The highest power density of the conventional motor is reported as 5.2 kW/kg up to now. Superconducting rotating machines have a potential to realize higher power density due to its high current density and a large magnetic field property. In this study, the induction motor with air-cored superconducting armature windings was conceptually designed. The output power range was 3 to 6 MW and the output power density aimed to be over 20 kW/kg. The armature voltage should be below kV-class and joule loss below 122 kW. The target efficiency was over 95%. The electromagnetic design and analysis were carried out by JMAG Designer. The number of magnetic poles and operating temperatures were set as two poles and liquid hydrogen temperature of 20 K, respectively. The parameter was power frequency in the range of 100-250 Hz. As a result, all the requirements were satisfied when the power frequency was 130 Hz. The feasibility of the MW-class superconducting induction motor was investigated, and in conclusion, the superconducting induction motor can be applied to the aircraft propulsion system.

К расчету высокодинамичных электродвигателей

The use of electric drive in industrial and specialized facilities, and transport, in particular aircraft, requires the formation of transient processes of the drive a number of requirements aimed at achieving the desired level of either maximum performance or minimum level of losses, or limitation of dynamic loads in elements of kinematic diagram connecting the actuator with the actuator, or combination of these parameters. However, often increased interest is shown to highly dynamic motors with maximum angular speedup and braking. As is known, the speed of the electric motor is characterized by its electromechanical constant, which depends on the moment of inertia of the rotor, determined by its dimensions. Based on the analysis of the equations of motion of the motor defined by the ratio between the moment of inertia of the actuator and the moment of inertia for highly dynamic drive motor which delivers maximum speedup. This analysis was carried out taking into account the resonant phenomena, which limits the axial length of the rotor. The relations between the main dimensions of the high-dynamic motor providing the maximum speedup at the given load parameters are also determined. Recommendations on the choice of electromagnetic loads of such an motor are given. In particular, the ways of increasing the linear load contributing to the growth of the electromagnetic torque of the engine are considered. This is the possibility of cryogenic cooling, allowing the use of superconducting armature windings and excitation, as well as the correction of the geometry of the core, in particular the elongation of the tooth height. The latter is confirmed by numerical calculation of the electromagnetic moment of the four-pole reactive machine at different depths of the trapezoidal groove. The dependences characterizing the reduction of the active resistance of the phase winding during liquid nitrogen cooling are also given, which contributes to the increase of the linear load. Thus, the proposed design recommendations are given taking into account the possibility of developing an engine using superconducting windings.

Research on Primary Excitation Fully Superconducting Linear Generators for Wave Energy Conversion

Linear generator-based direct-drive wave energy converters have a simple structure and high stability and provide high efficiency, thereby exhibiting great potential. However, with a low direct-driving speed, a low power density is inevitable. This paper proposes a primary excitation fully superconducting linear generator. All superconducting armature windings are located on the primaries, without motion. A cryocooler and cooling system can be simplified. Two other traditional primary excitation linear generators are compared with the proposed generator. Results show that the proposed generator has higher output voltage, lower harmonic rate, lower voltage-adjusting rate, and larger output power than the two traditional linear generators.

Design and Analysis of a Switched Reluctance Motor with Superconducting Windings and Tapering Poles

This paper presents a 6/4 switched reluctance motor with high-temperature superconducting (HTS) armature windings and tapering poles. The HTS windings are capable of carrying large current while the structure of tapering poles can improve the magnetic circuit. The cryogenic system of the stator and the mechanical configuration of the rotor are both simple and robust. The finite element analysis results confirm that the combination of the technology of HTS and tapering poles can improve the torque density of the proposed SRM by 29%.

REBCO Coils System for Axial Flux Electrical Machines Application: Manufacturing and Testing

A REBCO coil system for axial flux electrical machines applications is presented. Three couples of series-connected coils have been placed on a fiberglass disc and wired in star configuration to simulate a three-phase, ironless, and superconducting armature concentrated winding. REBCO-based commercial tape has been used to wind each of the six coils in double-pancake configuration around a fiberglass trapezoidal-shaped core. DC current-voltage curve and ac transport current losses plots of each coil have been obtained at liquid nitrogen temperature. Six copper coils having the same shape and geometric parameters of REBCO coils have been also manufactured to assemble a conventional armature concentrated winding. Laboratory testing on different armature windings has been conducted at different temperatures (77 K and 300 K) to measure the impedance (amplitude and phase) value, the back electromotive forces waveform, etc. The experimental results of both configurations (conventional and superconducting) are compared to study the feasibility of a superconducting armature winding prototype of an axial flux electric machine.

Research on Magnetic Shield Effect of AC Superconducting Armature

The superconducting motor with superconducting armature winding has simple cooling and a high running temperature of 66 K-77 K so the motor with a superconducting armature winding and excitation has been researched by many institutes. When the superconducting armature winding was adopted in the superconducting motors, the magnetic field around the coils would influence its performance. Therefore, this paper researches the distribution of the superconducting coils in the stator tooth-slot environment which is composed of ferromagnetic material. Based on the analysis of electromagnetic distribution, a new shield method is presented for decreasing the magnetic field which is around the superconducting coils when an active current flows through the coils. The shield's ferromagnetic material structure parameters are optimized by FEM (finite element method) software to seek the optimization parameters which create a minimum magnetic field. Finally, the experimental result shows the shield method can decrease the magnetic field around the superconducting coils, and the method can decrease the AC loss of the superconducting coils.

Development and testing of a 2.5 kW synchronous generator with a high temperature superconducting stator and permanent magnet rotor

High temperature superconducting (HTS) armature windings have the potential for increasing the electric loading of a synchronous generator due to their high current transport capacity, which could increase the power density of an HTS rotating machine. In this work, a novel synchronous generator prototype with an HTS stator and permanent magnet rotor has been developed. It has a basic structure of four poles and six slots. The armature winding was constructed from six double-pancake race-track coils with 44 turns each. It was designed to deliver 2.5 kW at 300 rpm. A concentrated winding configuration was proposed, to prevent interference at the ends of adjacent HTS coils. The HTS stator was pressure mounted into a hollow Dewar cooled with liquid nitrogen. The whole stator could be cooled down to around 82 K by conduction cooling. In the preliminary testing, the machine worked properly and could deliver 1.8 kW power when the armature current was 14.4 A. Ic for the HTS coils was found to be suppressed due to the influence of the temperature and the leakage field.

Analytical calculation of the flux density distribution in a superconducting reluctance machine with HTS bulks rotor

This paper deals with the analytical computation of the magnetic field distribution in a wholly superconducting reluctance motor. The rotor is made with high temperature superconductor bulks which nearly present a diamagnetic behavior under zero-field cooling. The stator consists of superconducting armature windings fed by AC currents of high amplitude. The superconducting stator winding can generate a high rotating magnetic field without the need of ferromagnetic material in the rotor. The electromagnetic torque is obtained by the interaction between the rotating magnetic field created by the superconducting stator windings and the variable reluctance due to the superconducting bulks. The proposed analytical method is based on the resolution of Laplace's equation (by the separation of variables method) for each sub-domain, i.e. rotor shaft, holes between superconducting bulks and air-gap. The global solution is obtained using boundary and continuity conditions. Magnetic field distribution and electromagnetic torque obtained by the analytical method are compared with those obtained from finite element analyses.

Electrical Properties Analysis and Test Result of Windings for a Fully Superconducting 10 HP Homopolar Motor

This paper describes the electrical analysis and performance test result of windings of a 10 HP homopolar fully superconducting synchronous motor. The homopolar motor has high temperature superconducting (HTS) armature and field coils. The stationary field coils make the cooling system simple and easy because there is no cryo-moving part. The design of the motor and the analysis of magnetization loss generated in the superconducting armature windings are performed by finite element method. The critical current test results of the 2G superconducting wire, the pancake coil for field winding and the race track coil for armature winding are reported.

Design of the HTS Permanent Magnet Motor With Superconducting Armature Winding

The superconducting motor generally adopts superconducting magnets to supply high flux density in the air-gap around the rotor. This structure can decrease the volume as a result of the high magnetic load, but it also creates cooling difficulty problems due to the cooling medium's spin sealing. Therefore, the superconducting magnets need frequent maintenance, and this structure cannot be applied in the industry field. So a novel high temperature superconducting motor with a superconducting armature coils is researched. This motor's stator uses ferromagnetic material and the superconducting windings are located in the slots. The ferromagnetic material can decrease the magnetic field in the superconducting coils, so the current carrying ability of the superconducting coils can be increased. The rotor adopts the permanent magnets as the excitation. Such design details of the motor can ensure the superconducting coils can undertake larger current and reduce AC loss. In this paper the electromagnetic field distribution of the HTS motor stator coils is analysed. Finally, the detailed HTS motor parameters are presented to analyse the output torque characteristic. From the characteristics, it can be seen that the novel HTS motor has the same power density as the traditional HTS motor.

Design of Axial and Radial Flux HTS Permanent Magnet Synchronous Motor's Rotor

Most of superconducting motors use the superconducting coils as the excitation coils which are located in the rotor. But its cool system is complex and the cool temperature is lower than 60 K. A novel axial-radial flux HTS (high temperature superconducting) motor is presented with the liquid nitrogen cool, its armature winding in stator is superconducting coils, the excitation winding in cover is superconducting coils, permanent magnets is located in the rotor. Its cool system is simple, and easy to implement. But the electromagnetic parameters are required to ensure the armature current can flow in the winding with low voltage. Through the prototype test with the copper winding instead of the superconducting winding, a novel rotor structure fit for the HTS motor with superconducting armature winding is presented.

Development and test of an axial flux type PM synchronous motor with liquid nitrogen cooled HTS armature windings

We developed an axial gap permanent magnet type superconducting synchronous motor cooled by liquid nitrogen (LN2). The motor includes 8 poles and 6 armature windings. The armature windings are made from BSCCO wire operated at the temperature level between 66K~70K. The design of the rated output is 400kW at 250rpm. Because HTS wires produce AC loss, there are few motors developed with a superconducting armature winding. In a large capacity motor, HTS windings need to be connected in parallel way. However, the parallel connection causes different current flowing to each HTS winding. To solve this problem, we connected a current distributor to the motor. As a result, not only the current difference can be suppressed, but also the current of each winding can be adjusted freely. The low frequency and less flux penetrating HTS wire because of current distributor contribute to low AC loss. This motor is an axial gap rotating-field one, the cooling parts are fixed. This directly leads to simple cooling system. The motor is also brushless. This paper presents the structure, the analysis of the motor and the tests.