hi ,
why it is perticularly stated that certain pwm controllar support upto say 2.2kW whereas the higher igbt can be connected using high rated drivers to the same controllar?

Not sure I completely understand your question. But it may be related to how VFDs are constructed now. Smaller drives use a power topology that is all integrated: the rectifier, inverter, braking chopper and firing circuits are all mass produced on a single substrate called an IPM, short for Integrated (or Intelligent) Power Module. It is one solid unit roughly the size of a credit card and the only thing you need to do is tie in your TTL signals directly into it. Above that size, you tend to see the component topologies broken down into smaller pieces so the firing circuits, for example, need to be separate.