Why there is a requirement of variable frequency? All Questions › Category: Power Electronics › Why there is a requirement of variable frequency? 0 Vote Up Vote Down chetan shidling asked 4 years ago I need short information. 1 Answers 0 Vote Up Vote Down chetan shidling answered 4 years ago The speed and frequency are always proportional. The motto of variable frequency is to control the speed of the AC motor. (N = 120f/p). By varying the frequency the speed of the motor is controlled.