Interactions between parametric, self-, and externally excited vibrations are analysed in this paper. The physical model of the vibrating system consists of a non-linear spring with periodically changing stiffness of Mathieu type and a non-linear damping described by Rayleigh's term. This system is additionally forced by a harmonic force (ideal system), or by a non-ideal energy source represented by a direct current motor with limited power supply. The model of DC motor is considered in two variants, as a classical, in Kononenko's sense model, and a complete electro-mechanical system. Quantitative and qualitative differences of the considered models are compared and discussed in the paper.