33.Why induction motor rotates at a lower speed than speed of rotating field (synchronous speed)

To produce torque the rotor must be carrying a current. In induction motor this current arises due to the rotating magnetic field cut across and generate a voltage in them. The closer the rotor speed approaches field speed, the slower the rate of cutting rotor conductors so a less current induced in rotor a smaller torque produced. 

If both speed became equal then no current would be flowing in rotor conductor so no torque is produced.