What does critical speed mean?

• A. Speed where it vibrates excessively and causes turbine failure
• B. Speed at which energy efficiency is maximized
• C. Speed at which steam is best evaporated
• D. Speed that is never reached in practice

Answer: (A)

Critical speed is the rotational speed at which a rotor’s natural vibration frequency is excited, causing resonance. When rotation reaches this speed, the system can vibrate excessively because the input force from rotation aligns with a natural mode of the rotor. In a turbine, these large vibrations stress bearings and other components and can lead to fatigue or failure if not managed. This concept isn’t about energy efficiency, steam evaporation, or an unattainable speed—it's about resonance-induced vibration risks. So, the description “speed where it vibrates excessively and causes turbine failure” captures the essential idea.