The use of high-temperature superconductors in electric machines offers potentially large gains in performance compared to conventional conductors, but also comes with unique challenges. Here, the electromagnetic properties of superconducting electri
c machines with bulk HTS trapped-field magnets in the rotor and conventional copper coils in the stator are investigated. To this end, an analytical model of the electromagnetic field in radial air-gap synchronous electric machines is developed and validated, taking into account the specific difficulties that occur in the treatment of machines with bulk HTS. Using this model, the influence of pole pair number, stator winding thickness, rotor surface coverage, and air gap width on the machines Esson coefficient is calculated. In contrast to numerical simulations, the method presented here can provide results within minutes, making it particularly useful for work in early development and systems engineering, where large parameter spaces must be investigated quickly.
A trapped magnetic field of greater than 3 T has been achieved in a single grain GdBa2Cu3O7-{delta} (GdBaCuO) bulk superconductor of diameter 30 mm by employing pulsed field magnetisation (PFM). The magnet system is portable and operates at temperatu
res between 50 K and 60 K. Flux jump behaviour was observed consistently during magnetisation when the applied pulsed field, Ba, exceeded a critical value (e.g. 3.78 T at 60 K). A sharp dBa/dt is essential to this phenomenon. This flux jump behaviour enables the magnetic flux to penetrate fully to the centre of the bulk superconductor, resulting in full magnetization of the sample without requiring an applied field as large as that predicted by the Bean model. We show that this flux jump behaviour can occur over a wide range of fields and temperatures, and that it can be exploited in a practical quasi-permanent magnet system.
Direct current (DC) characterization of high temperature superconducting (HTS) coils is important for HTS applications, such as electric machines, superconducting magnetic energy storage (SMES) and transformers. In this paper, DC characterization of
a circular, epoxy-impregnated HTS coil made from YBCO coated conductor for use as a prototype axial flux HTS electric machine is presented. Multiple voltage taps were utilized within the coil during measurement to help provide further detailed information on its DC behavior as a function of length. Based on the experimental results, there exist regions of non-uniformity along the length of superconductor in the coil, resulting in non-ideal superconducting properties of the coil. By studying the current-voltage (I-V) curves across different regions, it is found that a decreasing n-value and critical current exists in the non-uniform parts of the HTS coil.
In this paper, the authors investigate a number of design and market considerations for an axial flux superconducting electric machine design that uses high temperature superconductors. This work was carried out as part of the University of Cambridge
s Centre for Entrepreneurial Learning ETECH Project programme, designed to accelerate entrepreneurship and diffusion of innovations based on early stage and potentially disruptive technologies from the University. The axial flux machine design is assumed to utilise high temperature superconductors in both wire (stator winding) and bulk (rotor field) forms, to operate over a temperature range of 65-77 K, and to have a power output in the range from 10s of kW up to 1 MW (typical for axial flux machines), with approximately 2-3 T as the peak trapped field in the bulk superconductors. The authors firstly investigate the applicability of this type of machine as a generator in small- and medium-sized wind turbines, including the current and forecasted market and pricing for conventional turbines. A study is also carried out on the machines applicability as an in-wheel hub motor for electric vehicles. Some recommendations for future applications are made based on the outcome of these two studies. Next, the cost of YBCO-based superconducting (2G HTS) wire is analysed with respect to competing wire technologies and compared with current conventional material costs. Current wire costs for both 1G and 2G HTS are still too great to be economically feasible for such superconducting devices, but round wire, so-called 3G HTS, conductors may be a promising new option with a view to the future. Finally, different cooling options are assessed for the machine design and the analysis suggests that waiting for the maturation of pulse-tube cooling technology may be the best option for this particular application.
