Existence of a mechanical defect in a rotating system can be interpreted as the presence of a periodic angular perturbation. Understanding the interactions between the defect and its consequences on measurable quantities representative of dynamic behavior (acceleration, speed ...) necessarily involves the implementation of numerical models. The aim of this paper is to present a model suitable for the introduction of this kind of disturbance in the case of systems operating under non-stationary speed conditions. This model corresponds to a numerical extension of approaches increasingly used to analyze experimental signals. Based on the model built, investigation of the influence of disturbance parameters on the dynamic responses of the rotating system can be performed. This model also allows comparisons of new assumptions on the origin of this perturbation. The general framework of bearing faults will be the support used to illustrate this new approach.