Dynamic loss of HTS field windings in rotating electric machines

High-temperature superconducting (HTS) coated conductors (CCs) are frequently applied under complex electromagnetic fields to develop powerful, compact and efficient rotating electric machines. In such electric machines, field windings constructed by HTS CCs are adopted to increase the magnetic loading of the machines. The HTS field windings work with DC currents and due to the time-varying magnetic field environment, dynamic losses occur. In addition to the AC magnetic field, there is a large DC background field, which is caused by the self-field of the HTS field windings. This paper investigates the dynamic loss in HTS CCs using an H-formulation based numerical model for a wide range of combined DC and AC magnetic fields under various load conditions, and two different methods have been used for calculating dynamic loss. The results show that a DC background field plays a vital role to accurately predict the dynamic losses in HTS CCs. A DC background field of 75 mT can triple the dynamic loss as compared to only applying an AC magnetic field. In addition, the theoretical definition for the dynamic region for the case of solely an AC field has been found inapplicable in the case of a DC background field. Finally, a case study is done based on our double claw pole power generator to estimate the dynamic loss in an actual rotating machine, which was found to be 13.3 W. A low dynamic loss was achieved through the generator field winding design, which prevents high magnetic field fluctuations in the winding, since it is located at a distance from the air gap and armature coils. Furthermore, the rotational speed is very low and hence the resultant magnetic field frequency is low as well.