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

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 function CSAED using AC electromagnet was successfully demonstrated in initial current excitation by generating the alternating magnetic field without physical rotation for a small-scale HTS magnet. However, upgrading of the AC electromagnet is required to apply for large-scale load magnet due to small value in saturation current of the CSAED. The second novel function of CSAED using DC electromagnet worked well in current-adjustment and current-conservation modes by controlling the magnetic field density using the hybrid magnet. By controlling the current of DC electromagnets without any change in the rotation speed, the saturation current was adjusted within the ranges from 61.4 A to 1.7 A, which are 86.6 and 2.4 % of critical current for load HTS magnet, respectively. In the reverse rotation, the current discharging time of CSAED using activation of DC electromagnet increased by 2474 % compared to without energizing the DC electromagnet.

Automated summary

This study proposes a novel contactless superconducting active excitation device (CSAED) to improve the application feasibility of HTS rotating machines in different operational modes. The CSAED, comprising permanent magnets, AC, and DC electromagnets, is developed using the hybrid magnet concept. Experimental analysis demonstrates the effectiveness of the active control functions in the CSAED for stationary, forward, reverse, and compensation operations.