J. Ogawa

Influence of AC external magnetic field perturbation on trapped magnetic field in HTS bulk

Abstract Electric motors and actuators are most promising future applications of HTS bulks. The HTS bulks in these electric machines are exposed to AC magnetic field perturbations. The perturbations cause AC losses in the bulks and also affect the magnetic field trapped in the bulks. In a previous work we measured AC losses in a YBCO bulk subject to the AC magnetic field and found the loss characteristics well followed the Bean model. In this work we experimentally studied the influence of the AC perturbations of the external magnetic field on the trapped field in a YBCO bulk. The experimental results showed that the application of the AC magnetic field decreased the trapped magnetic field even to zero. In this paper, the experimental results are presented and a reason for the decay of the trapped magnetic field is discussed based on the Bean model.

Decay of trapped magnetic field in HTS bulk caused by application of AC magnetic field

In our previous work it was observed that trapped magnetic field in a YBCO bulk was decayed and even erased by application of AC external field whose amplitude was much smaller than the peak value of the trapped magnetic field. To study a reason for decay we investigated temperature rise of a YBCO bulk subject to the AC external magnetic field by use of thermocouples. A case was observed that the temperature rise of the bulk reached almost the critical temperature. This result shows that the decay of the trapped magnetic field was related to the temperature rise due to AC losses in the bulk.

AC losses in YBCO coated conductors carrying AC transport currents in perpendicular AC external magnetic field

The paper reports an experimental study on AC loss characteristics of a YBCO coated tape conductor carrying AC transport current in AC external magnetic field. In the experiment we electrically measured total losses that were transport current losses plus magnetization losses in AC external magnetic fields perpendicular to the tape surfaces. The transport current losses were measured by a four terminal method and the magnetization losses by a method using a pick-up coil. To measure the total losses electrically we need to pay proper attentions to prevent electromagnetic coupling between the sample currents and the external magnetic fields and to suppress inductive voltage in signals from the pick-up coil and voltage taps. The total losses were purely hysteretic for the amplitude of the external field up to 20 mT and the peak transport current up to the critical current. The loss characteristics can be explained by a theory based on the Bean model if the dependence of the critical current on the external magnetic field is taken into consideration.

Measurements of Total AC Losses in HTS Short Sample Wires By Electric and Calorimetric Methods

AC losses in a HTS tape carrying AC transport current in external AC Magnetic field were measured simultaneously by electric and calorimetric methods at the same part of the tape. The electric method uses a rectangular‐shaped pick‐up coil to measure the magnetization losses and the four terminal method with spiral voltage leads arrangement to measure the transport current losses. The calorimetric method uses a thermo‐couple to measure temperature rise of the tape due to the AC losses. The total losses from the electric method which are given by addition of the electrically measured transport and magnetization losses well agreed with the total losses measured by the calorimetric method. This result verified the validity of the both electric and calorimetric methods.

Angular dependence of AC transport current losses in biaxially aligned Ag/YBCO-123/YSZ/Hastelloy coated conductor

AC transport losses in a biaxially aligned YBCO-123 tape (10 cm in length) were measured (at 77 K) as a function of frequency and amplitude of a sinusoidally varying transport current. In the frequency range of 30-320 and the transport current losses are hysteretic in nature. The measurements of the self-field losses caused by the AC transport current as a function of the magnitude and orientation of the applied low external DC magnetic field (up to 150 mT) are also reported. The external magnetic field was rotated in the range of +120 to -120 degrees with respect to the plane of the tapes, and was in all cases perpendicular to the transport current flow. Strong anomaly in angular critical current dependence on external magnetic field was observed.

Interaction between trapped magnetic field and AC loss in HTS bulk

Dependence of AC magnetization losses in a YBCO bulk on the trapped magnetic field is experimentally investigated. The experimental results showed that the losses were hysteretic and that the losses in a bulk with trapped magnetic field were larger than those without trapped magnetic field. Distributions and profiles of the trapped magnetic field in the bulk were also measured before and after applying AC external magnetic field. It was observed that the trapped magnetic field was decreased by applying AC external magnetic field. From the trapped magnetic field profile, the full penetration magnetic field is estimated and the AC losses are calculated based on the Bean model. The calculated results well agree with the measured data.

Energy losses in YBCO-123 coated conductors carrying transport current in perpendicular external magnetic field

In the work we present experimental results for AC losses and critical currents of biaxially aligned YBCO-123 coated conductors, with Hastelloy as a substrate material. The critical current density of the tapes was of the order of 4.10 9 A/m 2 at the liquid nitrogen temperature. Magnetization losses, in sinusoidally varying external AC fields directed perpendicular to the plane of the tapes, were measured using a standard Lock-in technique. The frequency of the magnetic field was in a range of 7-64 Hz. The losses in YBCO are pure hysteretic for amplitudes up to 40 mT. Magnetization losses in the tape when carrying DC transport current are also reported. From obtained experimental data of dynamic resistance and the magnetization losses the total losses dissipated in the tape have been evaluated and compared with the critical state model predictions.

Energy loss in YBCO-123 coated conductor due to AC/DC transport current and AC external perpendicular magnetic field

Abstract In the paper we present experimental results of power losses measurements in a YBCO-123 coated conductor carrying a DC (or AC) transport current when subjected simultaneously to an external AC magnetic field. Two kinds of measurements were carried out; the first was AC transport current losses as a function of the current amplitude and the AC external magnetic field (“AC–AC” case); the second was dynamic resistance losses versus DC transport current and the AC external magnetic field (“DC–AC” case). Additionally, for both cases the magnetization losses were measured. The increase of the AC transport current loss due to the AC external magnetic field in the full saturation regime is described well with the dynamic resistance effect. This makes it possible to replace the AC transport current measurement in AC applied magnetic field with the much more simple dynamic resistance loss measurement when the total AC loss has to be determined.

Magnetization of HTS bulk array with alternate polarities for linear superconducting actuator

We developed an ex situ method for magnetization of a bulk array with alternate polarities for a full HTS linear actuator by one magnetization process. The linear actuator is composed of primary windings of Bi/Ag sheathed wires and a secondary made of YBCO bulks. The linear actuator is a synchronous type and the secondary is a bulk array with alternate polarities. For practical applications, a simple method for magnetizing the bulk array needs to develop. We demonstrated that the bulk array was alternately polarized by our method. In the paper, the method for the magnetization is explained and the results of the magnetization experiments are presented.

AC losses in a HTS coil carrying DC current in AC external magnetic field

We electrically measured AC losses in a Bi2223/Ag-sheathed pancake coil excited by a DC current in AC external magnetic field. Losses in the coil contain two kinds of loss components that are the magnetization losses and dynamic resistance losses. In the measurement, current leads to supply a current to the coil were specially arranged to suppress electromagnetic coupling between the coil current and the AC external magnetic field. A double pick-up coils method was used to suppress a large inductive voltage component contained in voltage signal for measuring the magnetization losses. It was observed that the magnetization losses were dependent on the coil current and that a peak of a curve of the loss factor vs. amplitude of the AC external magnetic field shifted to lower amplitude of the AC magnetic field as the coil current increased. This result suggests the full penetration magnetic field of the coil tape decreases as the coil current increases. The dynamic resistance losses were measured by measuring a DC voltage appearing between the coil terminals. It was observed that the DC voltage appearing in the coil subject to the AC external magnetic field was much larger than that in the coil subject to DC magnetic field.