Mr. Rupert is correct! 
Mr. Ken, you are asking a complex question. I can answer it only partially.
When you fix the power to the motor and then replace the original 6" driver pulley with 3" driver pulley than the torque on the driven top 3" pulley will be two times larger.
It is because the torque on the driven pulley is inversely proportional to the driver pulley diameter.
Jan
Mr. Ken, you are asking a complex question. I can answer it only partially.
When you fix the power to the motor and then replace the original 6" driver pulley with 3" driver pulley than the torque on the driven top 3" pulley will be two times larger.
It is because the torque on the driven pulley is inversely proportional to the driver pulley diameter.
Jan