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When i look at the output shaft torque curve of a free shaft gas turbine i can see that they are almost like an electric motor. It makes peak torque at very low rpm and then torque keeps gradually dropping. I know that in electric motors the torque drops due to back EMF as RPMs increase. In free shaft gas turbines, what causes this torque decrease as RPMs increase? For example the curve for AGT1500 enter image description here

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Let's start with this example:

Start up the turbine with no load connected to the shaft, then advance the fuel flow rate to the maximum allowable value. The output shaft will achieve some (very high) RPM after a short while, at which the speed of the turbine blade tips starts getting close to the speed of the gas flowing through the turbine disc. In this condition, the ability of the blades to extract useful work from the gas flow falls off towards zero, and so the torque output of the turbine disc falls off towards zero too.

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