Counter torque in alternator supplying reactive power
So you have a three phase (or single phase AC generator) running a reactive load
The reactive part of the load will make no torque on the shaft of the generator
it will appear mechanically as increased rotational innertia
You will likely still have some torque caused by resistive losses in the wiring ans the generator's stator windings, and if the alternator output is tapped to provide field current that will present itself as a mechanical load too, also magnetic losses, eddy currents etc...