When a steam turbine reaches critical speed, the immediate mechanical response is typically?

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

Answer: (A)

When a steam turbine reaches a speed that matches a natural vibration mode of the rotor and its supports, the system goes into resonance. At that critical speed, the unbalance forces and steam excitations are applied in sync with the rotor’s natural frequency, so the energy of the motion is transferred efficiently into that vibrational mode. The result is a large, rapid oscillation of the rotor relative to its supports, i.e., a violent vibration. This immediate response happens because damping isn’t enough to quickly dissipate the absorbed energy once resonance occurs, so the vibration amplitude rises sharply.

Stalling, overheating, or runaway acceleration aren’t the immediate mechanical consequence of hitting a critical speed. Stalling is unrelated to rotor resonance, overheating is a thermal issue from sustained operation, and uncontrollable acceleration isn’t the typical immediate reaction of the rotor to resonance, which is dominated by the sudden, large vibration of the rotating assembly.