VFD

To change the frequency of supply to an induction motor, thereby enabling speed change

Slight correction:

To change the frequency and voltage of supply simultaneously going to an induction motor, thereby enabling speed change

Agreed!

The purpose is to reduce operating costs, by doing what electricalexpert65 said.

Maybe there are other reasons, but this isn't my field.

By varying the speed of the Motors we should able to control the power consumption.

"By varying the speed of the Motors we should able to control the power consumption."

Not really, this is a common misrepresentation. Motors use the amount of power that the load requires of them, plus any inefficiencies in the motor and controller. If variation in the output is necessary, using a VFD typically has less losses than most other types of variation, but if variation is unnecessary, then a VFD can actually introduce more losses than just running DOL and permanently trimming the output to match the load requirement.