Turbine shaft material has its own natural frequency, when
turbine rotates on such a speed that frequency of shaft
become close to its natural frequency, machine causes noise
& high vibrations because of resonance due to matching of
frequency. Running of Steam "TURBINE" on this speed is
avoided & this is called Critical speed. A turbine may have
more then one critical speed, which may depend upon number
of couplings.

Turbine shaft material has its own natural frequency, when
turbine rotates on such a speed that frequency of shaft
become close to its natural frequency, machine causes noise
& high vibrations because of resonance due to matching of
frequency. Running of Steam "TURBINE" on this speed is
avoided & this is called Critical speed. A turbine may have
more then one critical speed, which may depend upon number
of couplings.

actually the critical speed of turbine should not based
entirely on resonance effect....the critical speed is based
on the uniform speed of the generator .....and we ensured
this should not equal to natural frequeny

The angular speed of rotating machinery at which excessive vibration is produced; at this speed the periodic disturbing force coincides with a mechanical resonance of the shaft and/or of the machinery or its supports.this effect in turbine is called critical speed of turbine

Turbine shaft material has its own natural frequency, when
turbine rotates on such a speed that frequency of shaft
become close to its natural frequency, machine causes noise
& high vibrations because of resonance due to matching of
frequency. Running of Steam "TURBINE" on this speed is
avoided & this is called Critical speed. A turbine may have
more then one critical speed, which may depend upon number
of couplings.............

when the normal rpm of turbine coinside with the natural frequency of tranverse vibration of the shaft.at this the vibration tends to infinity and causes of deflection of shaft.

it is the phase of speed at which the natural frequency of the turbine shaft matches with the frequency at the speed. during this period more vibration and noise will come. to avoid this,(in fact we can not avoid) acceleration will be increased so that less time to cross the critical speed. thanking you.