Sasikumar, why don't you fit an inverter?
The initial cost may be more but you will soon(?) pay that back as the problem that I see is purely start up requirement. Using an inverter you can
(1) set the start up to whatever time you need to.
(2) adjust the running speed to accurately give you required airflow (this can be by pressure or mass airflow monitoring, this in time will give you payback on your inverter costs through reduced power consumption.
(3) possibly over time find that you can fit a smaller motor (but that would be for another time)
Also there are/were grants towards these purchases, worth considering. Try ABB/Hitatchi/SEW/Seimens/Omron etc.
Hope this is of some help.