The real action in an AC drive system takes place in the motor.  This is really where it all happens.  In order to fully understand an AC drive application, it is of the utmost importance to grasp how motors use electric power. 

All loads moved by electric motors are really moved by magnetism.  The purpose of every component in a motor is to help harness, control, and use magnetic force.  When applying an AC drive system it helps to remember you are actually applying magnets to move a load.  To move a load fast does not require more magnets, you just need to move the magnets fast.  To move a heavier load or to decrease acceleration time (accelerate faster), more magnets (more torque) are needed.  This is the basis for all motor applications.  You can add magnets (and torque) to a drive system by using a motor with a core that is either longer, larger in cross-sectional diameter, or some combination of both.