The equations of motion regarding free vibrations for nonlinear and self-excited oscillators do not differ from each other. However, the equations of motion of selfexcited oscillators contain, from a physical point of view, a “stimulating” expression so that free vibrations do not always subside like it is the case with “common” nonlinear oscillators. While the equations of motion of self-excited oscillators do not explicitly depend on time, they do for forced nonlinear vibrations. Time-dependent excitation may occur, for example, in the form of motion excitation force excitation when starting, braking, during transitional processes or even during steady-state processes (periodic excitation).