Any change in input supply appears at o/p which is sensed by control circuit. The circuit generates a signal according to the variation in input. This signal operates the motor in desired direction to achieve the set output voltage. The motor with the gear assembly moves chain system which moves carbon roller assembly on the coil. As rollers move on both sides of coils simultaneously, the potential difference between the two rollers changes with the movement of rollers. This change in P.D changes the voltage input to primary of series transformers which induces required voltage in secondary to give constant output voltage.