Additional AC Loss Research Articles

Effect of a DC transport current on the AC loss in no-insulation ReBCO racetrack coils exposed to AC parallel magnetic field at 77 K and 4.2 K

ReBCO coils are developed as DC field coils in linear motor systems to increase the force density, in favor of permanent magnets. Such coils have to sustain a relatively large heat load stemming from the AC magnetic field environment in which they operate. The use of no or partial turn-to-turn insulation can make them more stable against the effects of local heating. Conversely, the radial electrical connections in no-insulation (NI) coils allow for large coupling currents, causing additional AC loss on top of the already significant heat load. Here we report on the AC loss in sub-scale NI, 4 mm wide single-tape, ReBCO racetrack coils exposed to parallel-to-the-tape magnetic field in the frequency range of 10−4 to 1 Hz at 77 K and 4.2 K, while carrying a DC transport current. AC loss is measured magnetically and electrically. The main goal of these experiments is to validate our 2D numerical model, which provides more insight into the origin of the AC loss. At low frequencies, inter-turn coupling currents are spread more or less homogeneously throughout the winding pack. Whereas at high frequencies, the skin effect causes shielding of the interior of the coil and large induced currents only occupy the coil’s outer surface.

Calculation and measurement of coupling loss in a no-insulation ReBCO racetrack coil exposed to AC magnetic field

No-insulation coils are self-protecting and can therefore generally be operated at higher current densities. However, the electrical turn-to-turn connections may cause additional AC loss when charging the coil or when it is exposed to a time-dependent magnetic field. In this work, we study the case of a no-insulation ReBCO tape racetrack coil exposed to a uniform AC field applied parallel to the tape surface. We show that an anisotropic continuum model allows to formulate analytical approximations for coupling loss in the low- and high-frequency limits. For intermediate frequencies, the continuum model needs to be evaluated numerically. The model was validated with representative measurements of AC loss in the coils, measured calorimetrically as well as magnetically using pick-up coils. The validation experiment confirms the predicted frequency dependence of the coupling loss, which is at low frequencies and at high frequencies, due to the skin effect. The transition between low- and high-frequency regimes occurs around a characteristic frequency that is directly related to the characteristic time constant associated with the current decay in (dis)charge experiments.

Hairpin Windings for Electric Vehicle Motors: Modeling and Investigation of AC Loss-Mitigating Approaches

The hairpin winding configuration has been attracting attention as a solution to increase the power density of electric vehicle motors by enhancing the slot-filling factor. However, this winding configuration brings high AC losses during high-speed operation and we require new approaches to tackle this challenge. This paper considers reducing AC losses by proposing two main methods: correct transposition of conductors in parallel paths, and enhancing the number of conductor layers in a slot. First, the proper connection of conductors in parallel paths is considered, and the essential rules for this purpose are described. Next, the paper uses a numerical approach to deal with the effect of incorrect conductor transposition in winding paths on generating additional AC losses due to circulating currents. Finally, the impact of the number of conductor layers in the mitigation of AC losses is also discussed in detail. According to the results, by increasing the number of layers, ohmic losses in the layer near the slot opening dramatically decrease. For instance, ohmic losses in the layer near the slot opening of the eight-layer setup were 82% less than the two-layer layout.

Experimental Evaluation of Current Distribution in Three-Strand Transposed Parallel Conductors Composed of REBCO Superconducting Tapes

We investigated current distribution in three-strand transposed parallel conductors consisting of REBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> (REBCO, RE = rare earth) superconducting tapes under applied AC magnetic fields. We recently derived the theoretical expression for the additional AC loss caused by the formation of three-strand transposed parallel conductors and proved its validity through experiments. However, the conditions of each strand and current distribution have not yet been discussed. Accordingly, we measured the current distribution and compared it with the calculated distribution derived using the theoretical expression. From the results, we found that the current branch ratio of the three-strand can also be predicted via the theoretical expression.

An investigation into the influence of stator tooth and conductor geometry on AC winding losses in a 365kW permanent magnet machine equipped with rectangular bar conductors

Rectangular bar conductors located in parallel stator slots with a matched geometry offer a potentially attractive solution for permanent magnet machines as they can provide an improved slot fill factor with consequent reduced loss and enhanced heat transfer compared to random wound mush type coils. However, in addition to the manufacturing challenges posed by precisely forming coils from large cross-section bar conductors, the adoption of bar conductors is often constrained by consideration of additional AC losses which are a result of induced eddy currents in the conductors. These induced currents are a result of exposure to time varying fields both from armature reaction fields and incident permanent magnet flux passing through slot opening. In this paper, a 365kW 6000rpm 6-pole permanent magnet machine equipped with various arrangements of solid bar conductors is analysed to understand and illustrate the significance of AC losses in bar conductors and to quantitatively assess the influence of various design changes to the stator. The magnitude of AC losses in a baseline model with a single series wound bar is evaluated by 2D FEA simulations and shown to be prohibitive. In order to reduce AC losses, several design features including using multiple parallel transposed conductors and further optimization of tooth and conductor geometry are considered in this paper. Simulation results show that with optimized stator tooth geometry and multiple parallel transposed conductors, AC losses can be reduced by an order of magnitude compared to a machine designed on purely static considerations.

Additional AC Loss Properties of Three-Strand Transposed Parallel Conductors Composed of REBa2Cu3O y Tapes

The additional AC loss due to forming three-strand transposed parallel conductors with REBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_y</i> (REBCO, RE = rare earth) superconducting tapes, are investigated theoretically and experimentally. In this study, supposing that the two transposition points deviate by arbitrary length from optimal ones in three-strand parallel conductor, the theoretical expression of the additional AC loss was derived. For the derivation of theoretical expressions, the voltage-current characteristics of REBCO tapes were taken into account by using a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -value model. The additional AC loss was measured via pick-up-coil method under an applied AC magnetic field with various amplitudes and frequencies. The validity of the theoretical expressions was confirmed by accordance between theory and experiment.

Experimental Validation of Theoretical Expressions on Additional AC Loss of Two-Strand Parallel Conductors Composed of REBa2Cu3O&lt;italic&gt; &lt;sub&gt;y&lt;/sub&gt; &lt;/italic&gt; Tapes via Pick-Up-Coil Method

By a pick-up-coil method, the additional ac loss due to the formation of two-strand parallel conductors consisting of REBa2Cu3O y (REBCO, RE = rare earth) superconducting tapes was experimentally investigated. We recently improved the theoretical expression of the additional ac loss assuming that the voltage–current characteristics are expressed by the n -value model. The objective of this study is to prove the validity of the improved theoretical expression by experiment. The two insulated REBCO strands were cowound into a one-layer solenoidal coil. A uniform external magnetic field was applied in the axial direction of the sample coil. The additional ac losses were measured by the pick-up-coil method in sinusoidal ac magnetic fields whose maximum amplitude and frequency are 0.1 T and 10 Hz, respectively. The observed additional ac losses corresponded with the theoretical prediction. Therefore, it was concluded that the improved theoretical expression is more accurate than the previous one. At high-frequency regions, the observed additional ac losses slightly deviated from the theoretical predictions.

Investigation on AC losses in a superconducting linear motor

A Stirling-type pulse tube cryocooler (PTC) with a superconducting linear motor for the compressor unit was developed and tested. The compressor was able to produce oscillating mass flow and pulsating pressure at low temperature environment near 80 K and transmitted compression work efficiently to the cold-end of the pulse tube cryocooler. Although he efficiency of the linear compressor excited by copper coil usually high, the superconducting linear compressor can further improve the system efficiency by eliminating the Joule heating loss of copper coil. The first attempt of superconducting linear compressor did not attain the expected high efficiency. We suspected the AC losses in the superconducting linear motor as one of the main factors to degrade the performance of the linear compressor. This paper describes the detailed investigation of AC losses in the superconducting linear compressor, primarily carried out by experiment. To extract the AC loss of the copper bobbin in the high temperature superconductor (HTS) coil assembly, we replaced the copper structure by glass fiber reinforced polymer (GFRP) bobbins and manufactured the same HTS coil. AC losses of both HTS coils were measured and compared each other. The results revealed out that the copper bobbins in the HTS coil generated 8.7 W of the additional AC loss with current of 7 Arms and operating frequency of 50 Hz. The copper structure should be replaced to improve the performance of superconducting linear compressor, and an alternative cooling method is required.

Theoretical Analysis of Additional AC Loss Properties of Two-Strand Transposed Parallel Conductors Composed of REBCO Superconducting Tapes

Superconducting tapes made of REBa2Cu3O y (REBCO; RE: rare earth, i.e., Y, Gd, or Eu) exhibit superior performance at the critical current density, even at liquid-nitrogen temperatures. However, the current capacity of these tapes must be enhanced for use in large-scale applications. We have proposed a method of constructing parallel conductors, which must be insulated and transposed to obtain uniform current sharing and low ac loss. If the transposition points deviate from the optimal ones, a shielding current is induced to maintain a constant interlinkage magnetic flux, which produces an additional ac loss. In our previous studies, the theoretical expressions for the additional ac loss were derived on the basis of a critical state model. In this study, by using an n -value model to express the I – V characteristics of REBCO superconducting tapes, we theoretically investigated the additional ac loss properties of two-strand transposed parallel conductors wound into solenoid coils. The results showed that under the nonsaturation condition, the additional ac loss increases with decreasing n value. This suggests that in parallel conductors composed of REBCO tapes that operate at liquid-nitrogen temperatures, transposition must be performed with high accuracy at the optimal points. To avoid increasing the additional ac loss, the n value of the tapes must be consistently larger than approximately 10, and the deviation of the transposition point should remain below a few percent of the conductor length.

AC Losses Measurement and Analysis for a 2G YBCO Coil in Metallic Containment Vessels

AC losses in high-temperature superconductors are an important design parameter for large-scale power applications. It is often necessary to have a metallic containment vessel to house the superconducting element in a closed-loop cryogenic system. This, however, produces additional ac losses due to induced eddy currents in the walls of the vessel. This paper focuses on the investigation and understanding of the induced eddy current loss in the containment vessel. The total ac losses of a 1-m yttrium barium copper oxide coil were experimentally measured in three different containment vessels. The predicted eddy current loss from finite-element modeling combined with the measured superconductor hysteresis loss demonstrated good agreement with the experimental results. This paper also includes the implications for practical design of commercial cryostat containment vessel for superconductor power applications to minimize ac losses.

Hysteresis Losses in Different Types of Permanent Magnets Used in PMSMs

In permanent magnet synchronous machines (PMSM), depending on the machine application, different types of permanent magnets (PM) can be used. The most common PMs are ferrite magnets, neodymium iron boron magnets (NdFeB), and samarium cobalt magnets (SmCo). The selection of a suitable magnet for a particular machine design depends on the magnet properties: remanence, conductivity, mechanical rigidity, losses, and demagnetization characteristics. Usually, the possibility of hysteresis losses in PM materials is neglected. In this paper, however, it is demonstrated that possible hysteresis losses have to be evaluated in the machine design. It is shown by measurements and simulations that in some machine designs, hysteresis losses in NdFeB, SmCo, and ferrite magnets can be a source of significant additional ac losses that may lead to too high PM operating temperatures and a reduction in the machine efficiency.

Operational Characteristics of HTS Coils With Flux Diverters in Semipersistent Mode Under Alternating Magnetic Field

This paper deals with the effect of magnetic flux diverters on the current decay behaviors of high-temperature superconducting (HTS) coils which operate in semipersistent current mode (PCM) under alternating magnetic field. In superconducting synchronous machines, the HTS field coil can be designed to operate in PCM to obtain a steady magnetic field during operation. Due to the alternating magnetic field from the armature winding, the HTS field winding experiences an external magnetic field. As a result, the additional magnetization ac loss in the field coil would be induced, and it affects the current decay characteristics. In order to improve the current decay behavior of the HTS field coil, the use of flux diverter made of magnetic material can be considered. In this paper, we experimentally investigated the current decay characteristics of HTS field coils with magnetic flux diverters under alternating magnetic field. The measurement setup, which is composed of HTS field coil and armature coil, was fabricated. The measurement test of the current decay on the HTS field coil in PCM operation was carried out as functions of the amplitude and frequency of the armature coil.

Theoretical Study on Ac Loss Properties of REBCO Superconducting Two-Strand Parallel Conductors Exposed to a Non-Uniform Magnetic Field

Abstract RE1Ba2Cu3O7-δ(RE: Rare Earth, Y Gd and so on, REBCO) superconducting tapes have great performance in critical current density, J c, property even at liquid nitrogen temperature. However, it is necessary to enhance a current capacity for practical use. We proposed the configuration of parallel conductors and adopted it to several pieces of test transformers, e.g. a 66kV-2MVA one for a power grid, 25kV-4MVA one for a Shinkansen rolling stock, and so on. The constituent strands of parallel conductors need to be insulated and transposed for the sake of uniform current distribution and low ac loss. In case the transposition points deviate from the optimum ones, shielding current is induced according to the interlinkage magnetic flux of the twisted loop enclosed by the insulated strands and the contact resistances at the terminals. It produces an additional ac loss. Up to now, we have studied in the simple situation where parallel conductors are exposed to a uniform ac magnetic field. In this study, we studied the additional ac losses in the case that two-strand parallel conductors are located in non-uniform magnetic field. We derived theoretical expressions of the additional ac losses and discussed the dependences of them on the non-uniformity in magnetic field, the deviation length of the transposition point from the optimum one, and so on.

Magnetic properties of Ni-based substrates for HTS tape

Ni–W alloys are used as the substrate in the manufacture of HTS tape via theRABiTSTM process. Theoretical work has shown that the ferromagnetism in a substrate can affect theac losses in the superconductor. This occurs in part because of the increased magneticpermeability, which alters the magnetic induction in the superconductor and therebythe loss. Magnetic hysteretic loss in the substrate is an additional contributionto overall ac loss. We report measurements of both permeability and magneticloss as a function of frequency and magnetic induction for the standard alloyNi–5W as well as composite substrates incorporating non-magnetic Ni–W alloys.The measurements are done using ac susceptibility at 77 K, at frequencies up to104 Hz and inductions in the range 1–150 mT. It is shown how to use these results to predicttotal ac loss in isolated superconducting tape and wound coils. Some important magneticparameters cannot be obtained with sufficient accuracy using a SQUID or vibrating samplemagnetometer, since the size of a sample is limited in these measurements. This makes thedemagnetizing factor large enough to degrade the accuracy of a permeabilitymeasurement.

NbTi superconductors with low ac loss: A review

Niobium titanium is still the superconductor of choice for magnets where the field changes rapidly with time. To keep the hysteretic ac losses within bounds, it must be finely subdivided. For practical handling and reliable performance in magnets, it is therefore made in the form of filamentary composite wires, with NbTi filaments embedded in a matrix of copper. Unfortunately, the copper matrix introduces eddy current coupling losses, which must be controlled by twisting and by resistive barriers. To date, the major applications have been in particle accelerators and fusion, where high operating currents are required. To achieve these high currents, the filamentary wires are made into cables. Eddy current coupling between wires within the cable can cause additional ac losses. Problems of non-uniform current distribution between the wires can prevent magnets from reaching their full current, particularly with the largest cables.

Investigation of Irregular Strand Positions Causing Additional AC Losses in CIC Conductor

AC losses with long time constants can not be simply estimated from a short sample conductor because there are many irregular loops formed by strands strongly displaced from their original positions. In our previous work, we measured trajectories of 81 strands of NbTi conductor and it was proved that strongly displaced strands produced many line contacts with other strands, and thereby caused low contact resistance and long time constants. Long loops due to the displacement of strands should also produce large AC loss because the time constant of the loss is proportional to the inductance, i.e., the length of coupling current loops. In order to investigate the long loops in practical conductors, we developed a method to estimate the strand positions over the entire length. In this method, only one cross section of the conductor is required to calculate gravities of each sub-cable. The strand trajectories are obtained in a manner that the same order sub-cables rotate around the gravity to form one order higher sub-cable. The estimated trajectories are in good agreement with the measured ones, with errors of 1 mm.

Active Damping of Inrush and DC-Currents for High Temperature Superconducting (HTS)-Transformers on Rail Vehicles

The 1-MVA-HTS-transformer made by Siemens promises a high-efficient drive component as has been reported-even if a converter is used and additional AC-losses occur. HTS-transformers on rail vehicles have to meet different demands on the energizing to the grid. Due to the small resistance, the damping of DC-components of inrush currents or due to the bouncing of pantographs is insufficient. Additional damping devices, especially a switched resistor, promise a well-working solution. On the other hand, additional costs and mounting space will be caused. But what would be possible, if we would consider the use of already implemented on-board equipment like traction converter to fulfill the given demands on the inrush current? The DC-components may be damped by an active switching of the installed converters. Investigations, results and influences on the transformers behavior of the mentioned operations by using the on-board converters to meet the demands of railroad companies will be presented here.

Long‐time constants of irregular AC losses in a large superconducting coil

AbstractA large superconducting coil wound with a Cable‐in‐Conduit (CIC) conductor caused an additional AC loss, which cannot be estimated from the short conductor sample test results. It was confirmed that the additional AC loss was generated by long current loops in the CIC conductor. Magnetic field decays of the loops with various long‐time constants were observed through Hall probes. We propose a mechanism for the formation of the long loops. The CIC conductor is composed of several staged subcables. If one strand on the surface of a subcable contacts the other strand on the surface of the adjacent subcable, the two strands must encounter each other again at the LCM (Least Common Multiplier) distance of all staged cable pitches and thereby form a pair of long loops. We numerically traced each strand in the CIC according to a method in which the subcables at all substages rotate around the center of inertia. The calculated long‐time constants of the long loops were slightly shorter than the observed ones. We labeled all strands by order in a real CIC conductor, disassembling the cable carefully after peeling the conduit. It was found that the strands in a triplex were widely displaced from their original positions, so that their contacting lengths became longer than the calculated ones. This fact makes the time constant of the loop longer and hence can explain the observed long‐time constants. The proposed mechanism is effective for estimating the long loops causing additional AC losses in the coil. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 143(1): 50–57, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10064

Theoretical analysis on ac loss properties of two-strand parallel conductors composed of superconducting multifilamentary strands

The ac loss properties of two-strand parallel conductors composed of superconducting multifilamentary strands were theoretically investigated. The constituent strands generally need to be insulated and transposed for the sake of uniform current distribution and low ac loss. In case the transposition points deviate from the optimum ones, shielding current is induced according to the interlinkage magnetic flux of the twisted loop enclosed by the insulated strands and the contact resistances at the terminals. It produces an additional ac loss. Supposing a simple situation where a two-strand parallel conductor with one-point transposition is exposed to a uniform ac magnetic field, the basic equations for the magnetic field were proposed and the theoretical expressions of the additional ac losses derived. As a result, the following features were shown. The additional ac loss in the non-saturation case, where the induced shielding current is less than the critical current of a strand, is proportional to the square of the magnetic field amplitude and the square of deviation length in transposition and has a Debye-type frequency dependence. On the other hand, in the saturation case, the additional ac loss is nearly proportional to the field amplitude and the deviation length in transposition. It has no frequency dependence at a higher frequency than the critical frequency.

Experimental study on the ac loss properties of two-strand parallel conductors composed of superconducting multifilamentary strands

The ac loss properties of two-strand superconducting parallel conductors were experimentally investigated by using NbTi multifilamentary strands with a rectangular cross section. We prepared the simple situation that was supposed in the previous theoretical study (M Iwakuma et al 2002 Supercond. Sci. Technol. 15 1525–36). The two insulated strands were sparsely co-wound into one-layer solenoidal coils. They were transposed only once and soldered at both ends. The ac losses in the sample coils were measured by a pick-up-coil method by applying uniform ac magnetic field parallel to the coil axis and comparing it with the theoretical results. The validity of the theoretical expressions derived in the previous paper was verified by the quantitative agreement with experiment in all the aspects, i.e. the dependencies on frequency, field amplitude and the deviation length in transposition in both the non-saturation and saturation cases and also the threshold condition between them. The results obtained support the theoretical prediction that the additional ac loss due to the formation of superconducting parallel conductors can be much reduced by transposition whether the induced shielding current reaches the critical current of a strand or not.