Which of the following describes the effect of critical speed on a steam turbine?

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

Answer: (B)

When a rotor’s natural bending frequency lines up with the engine’s rotational speed, resonance occurs. Steam turbine rotors are flexible and have several natural frequencies. If the turbine speed passes through or sits near one of these frequencies, the forces acting on the rotor repeatedly reinforce the vibration instead of damping it. That amplification makes the rotor vibrate much more than usual, often violently, which is why the correct description is a violent vibration when operating at critical speed.

This isn’t about acceleration from steam flow, overheating, or stall. Those conditions come from other causes (heat transfer, lubrication, or flow dynamics) and aren’t the result of hitting the rotor’s critical speed.