Abstract: An AC drive or variable frequency drive (VFD) is a device that is used to control the speed of an electrical motor. The speed of is controlled by changing the frequency of electrical supply of the motor. It converts the frequency of the network to anything between 0 to 300Hz or even higher and thus controls the speed of motor proportional to the frequency. The benefit of an AC drive is that it controls are normally operating at constant speed. It enables the user to control the speed of motor proportionally that gives him various benefits in terms of process control, system stress and energy savings. PLC or programmable logic controller is a digital computer used for the automation of various electromechanical processes in industries. It consists of microprocessor which is programmed using the computer language. This proposed paper discusses various applications of AC drives and controlling mechanism of PLC.

Keywords: PLC, variable frequency drives, v/f method, AC drives, speed controller, frequency based controller.