The development of accurate, useful, dynamical models of electromechanical systems under fault conditions for industrial applications is crucial for further development of fault diagnosis systems. Models including complex physical phenomenology have been proposed, which could have low practical applicability. In this study, we develop the dynamical model of an electromechanical system composed by an induction motor coupled to a one-stage spur gearbox considering the effects of time-varying mesh stiffness, time-varying viscous damping, transmission error, and backlash. The time-varying mesh stiffness includes faulty conditions according to different loss sizes of broken tooth. Thus, this simple yet realistic model can explain the system dynamics under different fault severity scenarios. Experimental and simulated data are compared to validate the proposed model with different levels of pinion broken tooth under constant load conditions on the gearbox. Proper results were obtained, and we conclude that our model is useful for further analysis oriented to fault diagnosis. The simplicity of the proposed model represents a good approximation of the system behavior in steady state, with less complexity than other models with a higher number of DOF.