Porter Governor is a modification of Watt Governor with central load attached to the sleeve. This load moves up and down the central spindle. The additional force increases the speed of revolution required to enable the balls to rise to any predetermined level.
Porter governors has two fly balls which are attached to the arms of the porter governor . These two arms are pivoted to the top of the spindle. This spindle is driven by the engine. The parts of the arms just above the fly balls are connected to the central sleeve. This sleeve moves up and down according to the movement of the balls. There are stoppers place in the spindle to limit the vertical movement of the spindle. This sleeve is carries a heavy central load. The movement of the sleeve controls opening and closing of the throttle valve.
When the load of engine decreases, there will be sudden increase in speed of engine and spindle speed will also increase. As the spindle speed increases two fly balls also start rotating around the spindle fast. The centrifugal force will push the balls outward making the balls move in upward direction. Since the ball moves upward , the arms also moves upward and the sleeve connected to the lower part of the arm also moves upward. Upward movement of sleeve actuates the throttle valve via a mechanism connected to the sleeve to decrease the fuel supply to the engine. The decreases in fuel supply decreases the speed. Hence speed is maintained.
In the other case, when the load of the engine increases speed of the engine decreases. As the engine speed decreases , speed of the spindle also decreases and centrifugal force in the balls. Hence the balls come down with the arms. As the arms comes down , the sleeve connected to the arm also comes down and it actuates the throttle valve which increase the fuel supply. Due to increase in fuel supply, speed of engine also increases.