jayshah, I'd advise a little care in acting on jraef's comments above. I'm thinking he's been spending a bit too much time around paper tensioning systems or other very accurate speed control systems.
His comments are as always, true and well intended but, you really don't need to switch to vector control if you can stand a little error in the speed readout. For example, if you have a motor with 3% nameplate slip (say 1455rpm full load) and your motor loading varies from no load to full load (very few loads actually swing this far), then your readout would vary within about 3% of actual speed. And the error would all be negative.
On the other hand, if you pick a premium efficient motor with only 1% slip (1485rpm full load, for example) and your load swings only from full load to half load, then your display error would be around .5%.
If you need better than that, follow jraef's good advice. You can get down in the range of .01% no load to full load with a good vector system.