During the start and stop of the unit, when the speed increases or decreases to a certain value, the vibration of the unit increases suddenly. When the speed continues to increase or decrease, the vibration decreases. This speed that causes the vibration to increase suddenly is called the critical speed .
The rotor of a steam turbine is an elastic body with a certain free vibration frequency. In the manufacturing process of the rotor, because the center of the shaft and the center of gravity of the rotor cannot completely coincide, there is always a certain eccentricity. When the rotor rotates, centrifugal force is generated, and the centrifugal force causes the forced vibration of the rotor. When they are the same or proportional, resonance will occur, causing the amplitude to suddenly increase, and the speed at this time is the critical speed.