The turbines used in hydro electric power plants as prime movers of alternator can be classified in following ways :
(a) According to the type of flow of water.
(b) According to the action on fluid.
(a) According to the type of flow of water, consider the three axes.
- Axis along the shaft of the machine : The machines having flow of water along this axis are called axial flow turbines.
- Axis along the radius : The machines in which the water flows along the radius are called as radial flow turbines.
- Axis along the tangential directions : The turbines using this type of flow are called tangential flow turbines.
(b) According to action on fluid the turbines are divided into the following categories :
- Impulse turbines : In these type of turbines, the whole of pressure energy available in water is first converted into kinetic energy and the water at atmospheric pressure impinges the blades of the turbine. This type of turbine is mostly used in a Pelton wheel type.
- Reaction turbines : In these type of turbines, part of the available pressure energy of water is converted into kinetic energy in the guide apparatus, before it strikes the blades of the turbine. The pressure at all stages of the reaction turbines is not atmospheric.
These type of turbines, used at present, are Francis, Kaplan and Propeller type. We will restrict our discussion and study in brief about Pelton wheel, Francis and Kaplan type of turbines which are most commonly used in hydro-electric plants as prime movers.