Symmetrical Bipolar Current Control of HTS Dynamo Based on a DC Magnetic Field

As a travelling-wave flux pump, the HTS dynamo generates a DC-biased AC travelling wave on the HTS stator tape placed in air-gap, which can output large DC current into an HTS coil without the need for current leads, thus effectively reducing heat leakage and operational cost comparing with conventional contact power supplies. In this paper, we introduce a method to enable symmetric bipolar output of the HTS dynamo, based on the "four-quadrant" control originates from the theory of macroscopic magnetic coupling. This method uses N-S arrangement of permanent magnets embedded on the magnetic disk and in addition to a DC bias coil. In this work, we demonstrate the use of the DC bias coil to freely adjust the DC output current of the HTS dynamo from -40.3 A to +40.5 A, without disturbing its rotating state. We also demonstrated that the DC output current can be symmetrically reversed by reversing the direction of the DC bias field, thus enabling the HTS dynamo with bipolar output. In addition, we developed a feedback control algorithm to control the output current with an accuracy of 0.15 A, by controlling the on and off states of the DC coil. This work enables the HTS dynamo as an accurate bipolar wireless power supply, which opens its routes for various applications of HTS magnets that require accurate current control, especially for high-demanding areas such as magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR).