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Cryogenic cooling is a well-established and expanding technology. In the field of electric machines, it allows the construction of more efficient machines with a high power density. This paper addresses the main cooling technologies and their impact on cryogenic machine construction, providing perspective for their use in future electrical machines. Although cost and safety issues of cryogenic systems are still holding back the uptake of cryogenic electric motors and generators, research in this field should provide significant improvements and promote their use at different levels. [ABSTRACT FROM AUTHOR]

The second-generation high-temperature superconducting tapes (2G-HTS) such as the REBCO coated conductors are known for their strong mechanical properties and high current carrying capacity with low ac losses compared with first-generation HTS tapes as well as low-temperature superconductors. Therefore, they seem to be good candidates to assemble coils for superconducting rotating electric machines (SuREMs). In this article, to investigate the influence of a soft magnetic composite (SMC) material on the dc and ac HTS coil performances at cryogenic temperatures, GdBCO- and YBCO-based commercial tapes from different manufactures were used to assemble ten HTS coils in double-pancake configuration. These tapes were wound using two different core structures, i.e., fixed and movable cores, having the same circular shape but different diameters as well as materials, i.e., fiberglass and SMC. Critical currents and ac transport current losses of these coils were measured using the electrical method. To evaluate the parallel and perpendicular components of the profile of magnetic flux density for each assembled HTS coil, a finite element model has been developed using Maxwell 2D. Eventually, the performance of different coils for SuREM application was discussed. [ABSTRACT FROM AUTHOR]

This paper reports proof-of-principle of an optical fiber Bragg grating (FBG) operating as an electric field sensor. The system is based on a semiconductor diode laser, emitting around 1577.5 nm that interrogates the fiber sensor around its maximum reflectivity point. The application of a strong electric field causes a small variation of the optical path length of the waveguide due to electrostriction and thus perturbs the grating peak reflectivity wavelength. Calibration and dynamic range tests are performed using a parallel-plate capacitor to create a variable electric field (1–100 kV/cm). A great potential is pointed out in view of possible field applications for voltage graduation in power generators. [ABSTRACT FROM PUBLISHER]

Translator eccentricity in permanent-magnet tubular machines is practically inevitable. It results in nonuniform air-gap magnetic-field distribution, which is inherently a 3-D problem. In this paper, the effects of eccentricity on field distribution and on the resulting unbalanced radial force are analyzed. Both no-load and load conditions are presented. The proposed mathematical model only requires the knowledge of the 2-D air-gap flux density distribution when the translator is at the balanced position. The analytical expression of the unbalanced radial force is derived together with the translator deformation analysis. The information of unbalanced radial forces acting on the translator and the translator displacement can be used for machine and bearing sizing. Analytically predicted and 3-D finite-element-calculated results are compared and validated by the experimental results obtained on a specially set-up test rig. [ABSTRACT FROM PUBLISHER]

Electrical machine power density can be increased by liquid nitrogen (LN) cooling, since these machines operate at low resistivity and have a high heat transfer coefficient. This paper describes the experiments carried out on a short stroke tubular linear permanent magnet machine with copper windings and LN cooling. A cryogenic test rig for linear actuators was designed and built, and tests were carried out aimed at highlighting inherent current limiting capability, electric parameter variation in the fall from room to cryogenic temperatures, and actuator high thrust density. Shear stress reaches 17.8 \kN/m^2, and thrust-to-copper-loss ratio is 4 \N W^-1/2, which are significantly better compared to conventional actuators. [ABSTRACT FROM PUBLISHER]

This paper illustrates the potential and the convenience of copper-wound electrical machines with liquid nitrogen (LN) cooling. Natural convection heat exchange coefficients in LN are very high. Moreover, the resistivity of copper decreases remarkably at cryogenic temperature, subsequently reducing Joule loss in copper windings. Hence, cryogenic copper-wound electrical machines use less copper to carry the same current compared to air- or water-cooled machines, thereby improving the power-to-weight ratio. An experimental study is made of some practical issues related to the design of an LN-cooled machine using as an example the construction of a 220-N permanent magnet tubular linear actuator with a soft magnetic composite stator core. [ABSTRACT FROM AUTHOR]