Adjustment of the speed of the motor is the application,  which is required in all the process industries & machinery  Mfg.’s. Variable Frequency drive is used as energy saving device for some of the machines to save power during no load or it is used as final control element of the closed loop control application where the speed of the motor is changed to control one parameter. Some of the application is provided with manual speed control of the motors. These are the open loop control applications