Abstract
Linear alternators convert acoustic power produced by thermoacoustic engines into electricity. The conditions required for best acoustic-to-electric power conversion efficiency include that they operate under mechanical and electrical resonances simultaneously. When this occurs, the acoustic impedance of the linear alternator equivalent circuit becomes purely real. This work experimentally demonstrates that the effective inductance of the compact moving-magnet linear alternators typically used in thermoacoustic power converters increases with the mechanical stroke amplitude, which requires different tuning capacitors at different mechanical stroke amplitudes to achieve electric resonance and to keep the acoustic impedance real. The work explains the reasons for this behaviour and presents experimental data and a simple linear model to characterize the effective inductance as a function of the mechanical stroke amplitude. Then, the real and imaginary parts of the acoustic impedance of linear alternators are calculated with and without accounting for this effect. The effects of applying this suggested correction on the performance indices of linear alternators under thermoacoustic-power-conversion conditions are presented and discussed. By accounting for this effect, an increase in the output electric power by 9.8% is achieved, together with an increase in the mechanical stroke by 9.7% and an increase in the resonator dynamic pressure by 4.2%.