AC motors use alternating current to generate a huge range of mechanical output power, from fractional horsepower all the way to hundreds of kW. They are the most widely varied electric motor, as they are simple in design and operation, and are applicable to many different uses. However, they cannot produce these results without some form of control.

AC motor controllers solve this problem by electrically regulating the speed of these useful machines. Their adjustable output allows the motor speed to be precisely controlled which is often difficult with most AC motors. Source