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when natural and forced frequency become equal,,,then critical speed achieved. it will cause by some imbalance of rotor.
now the frequency according to your question.
When frequency of turbine(forced frequency) equal to natural frequency of turbine(frequency of atoms of metal by which turbine is made of) become equal imbalance of rotor occur (if turbine rotate on this rpm for a while). vibration of turbine on critical speed increases...which ultiamtely effect on thermal expansion of turbine rotor and body(joins,nut,bolts)
