AC induction motors and synchronous motors are simple and low maintenance, but are awkward to apply for traction motors because of their fixed speed characteristic. An AC induction motor only generates useful amounts of power over a narrow speed range determined by its construction and the frequency of the AC power supply. The advent of power semiconductors has made it possible to fit a variable frequency drive on a locomotive; this allows a wide range of speeds, AC power transmission, and rugged induction motors without wearing parts like brushes and commutators.