Critical speed in turbine operation is best described as a condition where the machine experiences what?

• A. Turbine will stall
• B. Turbine will overheat
• C. Turbine will accelerate uncontrollably
• D. Turbine will vibrate violently

Answer: (D)

Critical speed is the rotor’s resonance point—when the rotating shaft excites one of its natural vibration modes. At this speed, the forcing from rotation matches the rotor’s natural frequency, causing the vibration amplitude to surge. That dramatic, sustained vibration is why this condition is described as the turbine vibrating violently. Such resonance can lead to bearing wear, shaft fatigue, misalignment, and even failure if not managed, so designs and operation aim to avoid running at or near these speeds.

Stalling relates to flow issues and pressure drop, overheating is a thermal problem, and uncontrolled acceleration isn’t the defining result of hitting a critical speed—those issues don’t inherently describe the resonance-induced vibration that characterizes critical speed.