A generator is a device that converts mechanical energy to electrical energy. The energy generated is used to power our everyday activities in homes, factories, ships. These generators are driven by prime movers such as steam turbines, gas turbines, internal combustion engines or hand cranks which drive the generators to produce electricity. Generators provide nearly all the power for electric power grids. The reverse conversion of electrical power to mechanical power is done by an electric motor. Many motors can be mechanically driven to produce electricity.
Generators are generally classed into two types:
Dynamos or direct current generators were the first generators used to generate electricity. In these generators the alternating current is converted to direct current with a commutator, a set of rotating switch contacts on the armature shaft. The commutator reversed the connection of the armature winding to the circuit every 180 degree rotation of the shaft, creating a pulsing DC current. The dynamo employed self powering electro- magnetic field coils rather than permanent magnets to create the stator field. The use of electromagnets rather than permanent magnets greatly increased the power output of a dynamo and enabled electricity generation on a large scale which could be used for industrial applications. Large power generation plants rarely use dynamos in recent times to generate electricity due to the widespread use of alternators which generate alternating current for power distribution.
Alternators or alternately current generators are devices used to generate alternating current widely used for power distribution. Alternating current is now widely used today than direct current because it can be easily transformed from from low to high voltages to allow low energy losses over long distances and the overall maintenance cost is lower compared to DC generators. Alternators also called synchronous generators are directly connected to the power grid and must be properly synchronized during starting. They are also excited with special control to enhance stability of power system. AC generators consist of two permanent magnets of the North and South poles placed opposite to each other creating a magnetic field. There is a coil which is rectangular in shape and is the armature. These coils are attached to slip rings and attached to them are carbon brushes. The slip rings are made of metal and insulated from each other. One end of the brushes are connected to the slip rings and the other to the circuit. The coils rotate about an axis which is perpendicular to the magnetic field. There is also a shaft which rotates rapidly.
Working of Alternator
When the armature coils rotate between the poles of the magnet upon an axis perpendicular to a magnetic field, flux is produced which changes continuously, hence the word alternating current. Due to this an emf is induced in the armature. This produces electric current through the galvanometer, slip rings and brushes. The galvanometer swings between positive and negative values, indicating the flow of alternating current.
An induction generator or asynchronous generator is a type of alternating current electric generator that uses the principles of induction motors to produce electrical power. Induction generators operate by mechanically turning their rotors faster than synchronous speed. A regular AC induction motor can be used as a generator without any internal modifications. They are used in mini hydro power plants, wind turbines or in reducing high pressure gas streams to lower pressure because they can recover energy with relatively simple controls.
Principle of Operation.
An induction generator produces electric power when its rotor is turned faster than synchronous speed. For a four pole motor operating at 60Hz the synchronous speed is 1800rpm, and the same four pole motor at 50Hz will operate at a synchronous speed of 1500rpm. The motor normally turns slightly slower than synchronous speed and the difference between synchronous and operating speed is known as slip. In generator operation the prime mover (turbine or engine) drives the rotor above the synchronous speed (negative slip) the stator flux still induces current in the rotor but since the opposing rotor flux is now cutting the stator coils an active current is produced in the stator coils and the motor now operates as a generator sending power back to the electrical grid.