In the theory of hidden oscillations, Sommerfeld effect is explained by the multistability and presence in the phase space of dynamical model without stationary states of two coexisting hidden attractors, one of which attracts trajectories from vicinity of zero initial data (which correspond to the typical start up of the motor), and the other attractor corresponds to the desired mode of operation with a higher frequency of rotation. Depending on the model under consideration, coexisting hidden attractors in the model may be either periodic or chaotic; such dynamical models with Sommerfeld effect are the earliest known mechanical example of a system without equilibria and with hidden attractors. For example, the Sommerfeld effect with hidden attractors can be observed in dynamic models of drilling rigs, where the electric motor may excite torsional vibrations of the drill.