Speed Control of DC shunt motor can be possible by providing resistance in the armature and field circuits and varying the resistors speed changes can be carried out over a wide range. On the other hand speed control of induction motor is costlier and bit tedious.
Speed of Induction motors = (120x f x (1-s))/p.
Therefore for speed control either supply frequency should change or number of poles should change and other methods can be employed. Care should be taken that in the process efficiency and power factor should be maintained.