A variable speed drive can be achieved by variable-frequency control
The emf induced in the stator of the induction motor is given by:
If supply is changed E1 will also change to maintain the same air gap flux.If the stator voltage drop is neglected the terminal voltage is equal to E1.But in order to avoid saturation and to minimize losses,motor is operated at rated air gap flux by varying frequency in such a way such that V/f ratio remains constant.
The variable frequency is obtained using a)Voiltage source inverter b)current source inverter c)cycloconverter
soft starter generally make use of thyristor with a varying firing angle