Is there any problem if Ahu's of 7.5 TR copper connection done with 3/4 inch copper pipe. will it effect cooling if yes please expand?

Yes, for 7.5 TR AHU, 1/2" copper pipe is the right size. If 3/4" pipe introduced instead of 1/2" pipe, the following effects will be entertained;

1. Velocity will be reduced and hence the colling effect will be comparatively low

2. Sound will be less because of the larger pipe size