Optimizing Rotor Speed and Geometry for an Externally Mounted HTS Dynamo

High-temperature superconductor (HTS) dynamos provide a means of wirelessly exciting dc currents in HTS coils. By eliminating the need to connect a coil to a room-temperature power supply, an HTS dynamo of comparable performance would present significant improvements to the cost, complexity, and efficiency of HTS-based electromagnetic machines. But due to the lack of a predictive model, development of HTS dynamos must be carried out through empirical investigation. In this paper, we examine the effects of varying either the rotor speed or the number of magnets arranged symmetrically upon the rotor. Both approaches affect the frequency at which magnets cross the HTS stator wire. Here we present experimental results from an axial-flux HTS dynamo, employing an externally mounted rotor carrying m permanent magnets, where 2 <= m <= 9. We find that the open-circuit dc voltage, short-circuit dc current, and internal dc resistance are all determined solely by the magnet-crossing frequency, regardless of the number of rotor magnets and speed employed to achieve this frequency. For a fixed speed, all three parameters are found to be proportional to m across the full range of conditions studied in this work. However, for a fixed number of rotor magnets, we observe that the open-circuit voltage and short-circuit current both exhibit a similar sublinear dependence on rotor speed. Examination of the transient voltage waveform reveals that this nonlinearity is likely due to a periodic transient heat pulse absorbed by the stator wire each time the magnet traverses it at high speed.