I have been asked several times what type of behavior an induction motor may
have in a hydro where the RPM is higher than the rated motor RPM.
Induction motors as generators may be used well and the voltage output can
be varied if the frequency is not required to be stable.
If the RPM is kept constant, one can vary the Biasing capacitors to vary the
output voltage if we increase the capacitance the voltage increases and vice
versa if the capacitance is reduced -- so adding capacitors via relay or
solid state devices one can vary the output voltage though one needs to
realize that the motor will have additional heating losses.
Also, if the motor is allowed to run at higher RPM the output voltage will
increase -- I do not specify such changes because there are too many motors
with diverse designs and the changes is best to measure them to attain what
is desired or needed.