In some applications the rotor operates within power and speed limits on its natural
characteristic, e.g. with a centrifugal pump as load. Power limitation in rotors with
rigid blade angle may be effected by passive acting measures such as spoilers or
braking flaps. For machines of small rating also passive pitching mechanisms, e.g.
by means of pre-loaded springs, are known. Speed limiting methods acting on the
load side such as feeding energy into a dump resistor are also restricted to small
ratings.