What are the four basic options to balance a rotor based on size?

The option stating "Narrow rotor relative to diameter and center hung" is correct because this configuration specifically addresses the principles of rotor dynamics and balance. A narrow rotor has a smaller cross-sectional area relative to its diameter, which generally helps in achieving a favorable natural frequency and reduces the mass moment of inertia. This can enhance balance characteristics. Additionally, a center-hung setup ensures that the rotor rotates around its geometric center, minimizing eccentric forces and allowing for more uniform distribution of mass, which is crucial in achieving balance during operation.

In rotor dynamics, the alignment of the rotor with its bearings and the correction of any imbalances directly impact performance and longevity. With the combination of a narrow design and center-hung positioning, the rotor is better suited to maintain stability and efficiency during operation, even at high speeds.

The other options do not align with these principles as clearly. A wide rotor may introduce additional complexities in balancing and can lead to greater inertial effects, while an overhung rotor configuration might pose challenges relating to stiffness and stability under dynamic conditions. The option regarding "No phase readings available" does not present a viable strategy for rotor balance and does not fit within the context of common rotor balancing options.