Hydraulic turbines are usually coupled to an electric generator and the generator must run at constant speed to maintain frequency of supply constant. The speed of generator N in rev/min, the frequency of supply (f) in Hertz and number of poles of the generator P are related by the equation:
f = NP/120
The peripheral velocity u of turbine wheel must remain constant as speed is constant. The velocity u and speed N are connected by the formula:
u = pi DN/60
where D is mean diameter of the wheel.
It is also desirable to run turbine at maximum efficiency and therefore speed ratio u/VI must remain same which means the jet velocity must not change as head available H is constant. The only way to adjust the load is to change hydraulic power input given by p= yQH
As y, specific weight of water and H are constant, the only variable factor is Q volume flow rate of water entering the turbine. The flow rate Q is Q = Area of nozzle x velocity of jet
Thus flow rate will change by changing the area of the jet or more closely the diameter of the jet. This is accomplished by a spear valve and deflector plate shown in figure below.
The spear alters the cross-sectioned area ofthe jet. The position of spear is controlled by a servo mechanism that senses the load change. For a sudden loss of load when the turbine is shut down, a deflector plate rises to remove the jet totally from the buckets and to allow time for the spear to move to new position.
As seen in the figure for high load the spear valve has moved out, then increasing the area of the jet, at low loads the spear has moved in, decreasing the area of the jet. Deflector plate in normal position and fully deflected jet are also seen in the figures.
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