AC Drives are also known as Inverters, Variable Speed Drives (VSDs), Variable Frequency Drives (VFDs), and Frequency Converters used to control AC Induction motors.  If an AC motor is connected directly to the main supply, the motor will accelerate quickly and then rotate at a fixed speed which is dependent upon the electrical supply frequency and the motor design.

Control is achieved by either estimating the motor speed, known as open loop control, or by measuring the speed, known as closed loop control. 

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