The disclosed embodiments include energy harvesting devices, methods to operate the energy harvesting devices, and downhole power generating systems. In one embodiment, an energy harvesting device includes an outer stator having a first cavity formed therein. The energy harvesting device further includes a rotor disposed within the first cavity, the rotor including a second cavity, at least one radial slot, and a helical slot extending from a first end of the rotor towards a second end of the rotor. The energy harvesting device further includes an inner stator disposed within the second cavity. The energy harvesting device further includes an armature adjacent to the rotor, the armature coupled to at least one winding coupled to an electrical component to provide an electrical current to power the electrical component. The energy harvesting device further includes a housing coupled to the outer stator.