What will happen to Turbine(whose rated speed 3000rpm) if it runs in low rpm(say 2850) at its full load ?

Answer 1

for every rpm there is different load possible.
if the maximum load is at 3000 rpm then usually the max load at 2850 rpm is lower.
if you put too much load the rpm will drop (if the load will decrease it will stop dropping and even may increase, if not it will continue dropping until it will stop). since the controller will try to give more fuel (to increase rpm) it may overheat and break.

Answer 2

3000 rpm 2 pole generator produces 50Hz
2850 rpm produces 47.5 Hz (5% lower freq) this would also produce %5 less inductive reactance in motor windings resulting in higher current draw.

If you have a 100A circuit main breaker from the generator an it is drawing 100A then it is fully loaded and the circuit breaker should not trip even though it is at the limit (100%)
If however the 100% loading referres to equipment normally operated by the generator then the %5 reduced speed causes the %reduced impeadance and results in a %5 increase in current draw. the 100A draw would increase to 105A the 100A circuit breaker would then be running at 105% of its rated current and will trip. When depends on the settings, there is s thermal overload which has an inverse current-time relationship. This means a small overload takes longer for the breaker to trip, while a larger overload takes less time. AND there is an instataneous trip level as well.

In terms of this question the fulled loaded generator will eventually trip its circuit breaker and go off line if no adjustments are made. If the load is reduced, or voltage output raised or speed restored the current draw will be reduced and if it returns to 100% of the CB rating then the generator will remain on line. In otherwords it is possible to adjust the voltage settings to compensate for an incorrect speed and run the generator. Though I don't recommend doing this for a prolonged period.


As for mechanical loading, turbines use journal bearings which are supplied with oil. The spinning shaft causes a "hydro dynamic" wedge of oil to be formed which supports the shaft and centers (positions) the shaft at its normal operating point. Since a generator runs at a steady speed compared to a propulsion turbine driving a ship, the generator bearings have a tighter tolerance.

Running the generator at a different speed will cause the oil wedge to have a different shape and results in a different operating position of the shaft within the bearing. Whether this difference is significant enough to cause a I am not sure. But if so this mis-alignment due to low speed can negatively effect the bearings, reduction gear alignment and in an extream case reduce the clearance between the stator and rotor of the generator. At best the slight misalignment (from distorted oil wedge) will cause a change in the oil flow through the bearings, this can cause bearings to run warmer and/or overheat resulting in physical damage.