Which outcome is associated with critical speed in rotating machinery like turbines?

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

Answer: (C)

Critical speed is the rotor speed at which the natural bending vibration of the rotor-bearing system lines up with the rotation, causing resonance. When a turbine rotor reaches that speed, the vibration amplitude can grow dramatically because the rotating energy efficiently excites the rotor’s vibration mode. If damping isn’t enough to suppress it, the rotor can start to vibrate violently, which is the direct consequence of crossing a critical speed. Other issues like overheating, aerodynamic stall, or uncontrolled acceleration come from different mechanisms (friction, cooling, flow conditions, or control faults) and are not the result of resonance at the critical speed.