In the field of structural design, it is imperative to investigate the pertinent dynamics to facilitate the prediction of the structural response to excitations. In this study, the focus is on excitation sources that are non-ideal in nature. These sources are characterised by their inherent limitations in terms of power and inertia, which are constant parameters. Furthermore, the frequencies of these sources exhibit variability in accordance with the instantaneous state of the oscillating system. In this paper, the authors have revisited a number of publications based on the assumption that external excitations are produced by non-ideal sources (RNIS) that are of limited power supply. In the course of the present study, nonlinear phenomena such as the Sommerfeld effect and the saturation phenomenon were observed in a number of applications. These applications included fractional damping and an energy harvester, amongst others. The classical Jacobi-Anger expansion was utilised, with consideration given to the case of energy transfer between vibrating modes under external force with time-varying frequency. The following discussion will consider an example of contemporary importance with a future perspective on the engineering of research on non-ideal vibrating systems (RNIS).