Contactless superconducting field exciter with active current control using hybrid magnet for high-temperature superconducting rotating machine

•This study deals with the design, fabrication, experiment of the novel contactless superconducting excitation device for HTS rotating machine.•The novel functions have been proposed to improve the application feasibility in various HTS rotating machine operation modes such as the initial excitation in stop mode and excitation control in forward and reverse rotation modes.•A novel structure consisting of hybrid magnets that combine the permanent magnet, AC and DC electromagnets, was proposed to replace the existing permanent magnets to generate an AMF at room temperature.•As an assist excitation source when the rotor is rotated, the DC electromagnets generate the sub- magnetic flux density to adjust the magnetic flux density from the PM or AC electromagnet during current control modes.•As the main excitation source when the rotor is in the pre-start mode, the AC electromagnets generate an AC magnetic flux density, that is, AMF, to charge the initial current even when the rotor is stationary. Rotary-type high-temperature superconducting (HTS) flux pumps using permanent magnets (PMs) are effective contactless superconducting excitation devices used for generating excitation in the field winding of HTS rotating machines. Although contactless superconducting excitation devices overcome the disadvantages of conventional contact-type excitation systems, those based on PMs are not the ideal solution because current supply to the field winding is allowed only when the HTS rotating machine is rotating in the same direction: a rapid discharge occurs when the rotation direction is reversed. They do not meet the practical operational requirements of rotating machines, viz., initial start, load fluctuation, and direction change, and alternatives to actively control excitation of field winding are essential. In this study, novel functions to improve the application feasibility in various HTS rotating machine operation modes have been proposed. Based on a numerical analysis, a prototype contactless superconducting active excitation device (CSAED) comprising PMs, alternating current (AC) and direct current (DC) electromagnets was developed using the hybrid magnet concept. The operational characteristics of active control functions in the CSAED were experimentally analysed for stationary, forward, reverse, and compensation operations of the HTS rotating machine and the qualitative effectiveness for active control of the charged current was verified. The first novel functio