AC drive controllers, commonly referred to as inverters, are generally more complex than its DC drive controller counterparts.  This is due to the fact that AC drive controllers must perform two power section functions.  The first of these is the conversion of the AC line power source to DC, and secondly an inverter change from the DC to a coordinated adjustable frequency and voltage output to the AC motor.  The reason why AC drives is so popular is based on the simplicity and reliability of AC drive motors, which has no brushes, commutator or other parts that require routine maintenance.  In a way this compensates for the complexity of the AC drive controller.  AC motors are desirable for a variety of uses due to their robust construction and low cost.  AC drives have the ability to make an existing standard constant speed AC motor an adjustable speed device simply by the addition of an adjustable frequency controller.