Pelton turbines are suitable for power extraction when water energy is available at high head and low flow rate
Working a Pelton turbine is simple
High speed water jet is ejected through a nozzle
When this jet hits buckets of Pelton wheel
it induces an impulsive force
the rotating shaft runs a generator and produces electricity
but demand and power may fluctuate over time
a governing mechanism which controls position of sphere head meets this requirement
with lowering power demand, sphere headed water in lead nozzle is moved in so that water flow rate is reduced
if power demand increases, sphere head is moved out. This will increase flow rate
This in turn makes sure that turbine will rotate at a constant specified RPM
which conforms to power supply frequency
so this mechanism acts as speed governing mechanism for Pelton wheel
A cut is provided on bottom portion of buckets.
This makes sure that water jet will not get interfered by other incoming buckets.
One of the most important parameter of Pelton turbine design is number of buckets on a disc.
If number if buckets is inadequate, this will result in loss of water jet.
With lowering number of buckets, at some point of operation, complete water jet might be lost.
This will decrease efficiency of the Pelton Turbine drastically/
So there should be an appropriate number of buckets which will make sure no water is lost.
Most vital component of Pelton wheel is its bucket.
Buckets are cast as single solid piece in order to avoid fatigue failure.
Water jet is split into two equal components with help of a splitter.
The special shape of the bucket makes the jet turn almost 180 degree.
This produces an impulsive force on bucket.
Force produced can be easily derived from newton's second law of motion.
blade out lead angle close to angle 180 degrees is usually used in order to maximize impulsive force.
Absolute velocity of fluid will have huge drop from in lead to exit of bucket.
However pressure remains same.
So Pelton turbine gains mechanical energy purely due to change in kinetic energy of jet.
not due to change.
Which means Pelton turbine is a pure impulse machine.
Pelton turbine design is always aimed at extracting maximum power from water jet
or maximizing efficiency.
Power extraction is maximum when product of impulsive force and bucket velocity is maximum.
If Pelton wheel bucket are held stationary
even if there is huge impulsive force, power extraction will be zero.
Similarly if buckets are moving with same speed of jet,
water jet won't be able to hit the bucket which leads to zero impulse force .
Again power extraction will be zero.
In short, power extraction is zero, both have zero bucket speed and when bucket speed is same as jet speed.
So optimum power extraction happens in between.
It can be shown using Euler's terminal machinery equation that maximum power extraction happens when bucket speed is half of jet velocity.
So it is always desirable to operate Pelton wheel with this condition.
Pelton turbines can give efficiency as high as 90% at optimum working conditions.
Hope you got a good understanding on working of Pelton turbines. Thank you.