the motor should only rotate in a rated speed,if the speed is more than rated speed then the motor will be damaged........to control the speed of motor a rheostat should be connected to the field...
in some cases we need a desired speed, like in cassette stereo system we need the proper speed so that we can listen the song in a proper manner so we need to control the speed of motor. more...
the speed of ac motor can be controlled by controlling the synchronous speed (Ns=120f/p). and synchronous speed can be varried or controlled by changing frequency or by changing poles. but practicaly...