ELECTRIC GENERATOR

An electric generator :- which is the motor that produces electricity. A coal-fired power plant uses steam to turn the turbine blades; whereas a hydroelectric plant uses falling water to turn the turbine. The results are the same.The theory is to build a dam on a large river that has a large drop in elevation (there are not many hydroelectric plants in Kansas or Florida). The dam stores lots of water behind it in the reservoir. Near the bottom of the dam wall there is the water intake. Gravity causes it to fall through the penstock inside the dam. At the end of the penstock there is a turbine propeller, which is turned by the moving water. The shaft from the turbine goes up into the generator, which produces the power. Power lines are connected to the generator that carry electricity to your home and mine. The water continues past the propeller through the tailrace into the river past the dam. By the way, it is not a good idea to be playing in the water right below a dam when water is released .
Generator Theory :- A hydraulic turbine converts the energy of flowing water into mechanical energy. A hydroelectric generator converts this mechanical energy into electricity. The operation of a generator is based on the principles discovered by Faraday. He found that when a magnet is moved past a conductor, it causes electricity to flow. In a large generator, electromagnets are made by circulating direct current through loops of wire wound around stacks of magnetic steel laminations. These are called field poles, and are mounted on the perimeter of the rotor. The rotor is attached to the turbine shaft, and rotates at a fixed speed. When the rotor turns, it causes the field poles (the electromagnets) to move past the conductors mounted in the stator. This, in turn, causes electricity to flow and a voltage to develop at the generator output terminals.
Rotational Speed :- The rotational speed of the generator is usually the same as the speed of the turbine, because they are directly connected. Some plant designs incorporate a speed-increasing gearbox between the turbine and generator. The speed of the turbine is determined by the design and hydraulic conditions. Speeds for hydrogenerators are typically in the range of 50 and 600 revolutions per minute (rpm). For a generator operating in a 60-Hz system, the rotational speed (in rpm) times the number of field poles on the rotor is always 7,200.
The Stator :- The stator is a donut-shaped structure surrounding the rotor. The stator is made up of a steel frame supporting stacked steel laminations. The conductors, called the stator winding, are recessed in slots in this lamination structure. The stator winding is arranged so that when the rotor turns, the field poles pass only a fraction of an inch from it. The movement of the magnet next to the conductor causes electricity to flow in the conductor.Stator windings of larger generators are made up of individual stator coils. Each coil is made of multiple strands of copper. In a modern insulation system, the copper is insulated with mica held in place with an epoxy or polyester resin. The individual coils are connected to each other through jumpers and then to a ring bus. The ring bus is connected to the generator leads, which in turn are connected to a power step-up transformer. Finally, the transformer is connected to electrical power grid. Some generator designs use half-coils (bars) in lieu of full coils.
Stator Laminations :- The stator core is made up of steel laminations that are assembled in a stationary steel framework. The laminations are punched, or laser-cut, from electrical grade sheet-steel, approximately 0.014 inch thick. A thin coat of insulating varnish is baked on each lamination to insulate it from the other laminations to reduce eddy current losses. Notches are cut into the inner radius of the lamination, so that after assembly the stacked lamination notches align to form stator slots. The laminations are stacked, one at a time and clamped in a framework to form a rigid structure. After approximately 2 inches of the core is stacked, ¼-inch spacers are placed to provide air ducts for cooling air circulation.
Exciter :- The exciter supplies the direct current used to create the rotating magnetic field necessary for generator action. The exciter may be a rotating type that is directly connected to the generator shaft or a modern static system utilizing solid-state devices fed from a high-voltage bus. The excitation system is equipped with a voltage regulator, which is used to vary and control the generator voltage.