Under which condition might a rotor experience a natural frequency shift?

A rotor experiences a natural frequency shift primarily due to changes in its mass. The natural frequency of a rotor is fundamentally linked to its mass and stiffness; when either of these parameters changes, the natural frequency can be altered.

For instance, if the mass of the rotor increases (perhaps due to the addition of components or deposits), the natural frequencies typically decrease because the system becomes heavier and absorbs more energy during vibration. Conversely, if the mass decreases, the natural frequencies tend to increase. This relationship is described by the fundamental principles of dynamics, where an increase in mass leads to a lowering of natural frequency, and vice versa.

The other options focus on conditions that do not inherently lead to a change in the rotor's physical characteristics. A stable working environment, for example, helps maintain consistent operational parameters but does not impact the rotor's mass or stiffness. Low lubrication levels can lead to other performance issues but do not directly affect the rotor's natural frequency. Similarly, constant speed operation ensures that the rotor is rotating smoothly without variation in speed, which does not influence the natural frequency either. These factors may influence the operational reliability and efficiency of the rotor but do not cause fundamental changes in the natural frequency.