An induction generator is a type of AC or electrical generator which uses the induction principles to produce power. The generator is operated by turning the rotor mechanically in the generator mode which gives a negative slip. In many cases, the AC asynchronous motor acts as a generator, without undergoing any internal modifications. Induction generators as well as motors produce electrical power as their rotor is turned fast, faster than the synchronous frequency. In normal operations of the motor, the stator flux rotation moves much faster than rotor rotation. This leads to the stator flux inducing rotor currents. This process creates a rotor flux that has magnetic polarity that is opposite to stator.
To operate the generator, a turbine or engine that is the prime mover drives the rotor at high speed. While the stator flux will still induce the rotor currents, the opposing rotor flux cuts the stator coils producing an active current. The motor is now operating as a generator, and it sends power back to the electrical grid.
Induction generators can often be found in wind turbines as well as a number of micro and mini hydro installations. This is due to the fact that they are able to produce power at different rotor speeds. The generators are electrically and mechanically simpler than other types of generators. They do not require brushes or commutators. They are very suitable and are commonly used in wind generating stations where speed is a variable factor. One of the major advantages of the induction generator is the frequency regulation. The power system regulates the volts and output frequency, which are independent of variations in speed. The self-regulation mechanism minimizes complexity in the control system.
The generator is increasingly being used as a way to recover energy that would normally be lost. The power that is generated can be used on site or it can be sold to the utility system that supplies the site. Water and wind driven generators are used to convert the recovered energy into electrical energy. The generator has a number of benefits for micro and mini power systems. The first is the low cost of materials through the use of electromagnets rather than permanent magnets. The cost of labor required is also substantially lower than that of other types of generators.
The quality of the generator power is quite reasonable and constant. The AC voltage produced is reasonably sinusoidal and the voltage can easily be rectified to produce a DC voltage that is constant. When there is an internal failure in the induction generator it is safely shut down through de-excitation, within a few seconds. This is safe for those working in the area as hazardous situations can be prevented.