In case of power plants we generally use Synchronous generators, which are also known as Alternators, they are named as synchronous as they rotate at synchronous speed or we can say that they rotate in synchronisim with the stator field being magnetically locked. The reason we are using AC generators instead of DC generators is because our transformers operate only in AC and conversion of DC supply to AC will be very expensive .The alternator works on the principle of Faraday’s law of Electro Magnetic Induction.
(600MW Generator at KMPCL Unit#3)
Faraday’s law of Electro Magnetic Induction: According to this law whenever there is a relative space or time variation between the magnetic field and the conductor, an EMF is be generated in the conductor.
If the emf generated in the conductor due to induction gets a path then the current flows. The major difference between the AC and DC generators is that in case of AC machine the armature winding is placed in the Stator and the field winding is placed in the rotor winding.
Why Armature winding is stationary?
- For large generators the current carrying capacity is very high so it will be easy to collect the current instead of using slips as the slip loss will be very high comparatively
- Armature winding insulation can be done easily as the voltage is very high, whenever we deal with higher voltages, insulation is a major concern.
- Rotor tooth strength increases as the rotor slots will be of lesser size.
In the alternator we provide DC supply to the field windings for creating a constant magnetic field by various methods which we have already explained in “Various Types of excitation systems”. After giving the excitation to the rotor then we rotate the rotor by the help of Turbine as the turbine rotates the rotor of the alternator rotates and a rotating magnetic field is created inside the alternator. When this rotating magnetic flux cuts the stator conductors then according to the Faraday’s law of Electro Magnetic Induction an emf is generated in the conductors and by the help of conductors this power generated is supplied to the grid. Generally the power supplied from the alternator is a 3-phase supply(R-Y-B).
