Generator Blog

An induction generator is a type of electrical generator that is mechanically and electrically similar to a polyphase induction motor. Induction generators produce electrical power when their shaft is rotated faster than the synchronous frequency of the equivalent induction motor. An electric voltage (electromotive force) is induced in a conducting loop (or coil) when there is a change in the number of magnetic field lines (or magnetic flux) passing through the loop. When the loop is closed by connecting the ends through an external load, the induced voltage will cause an electric current to flow through the loop and load. Thus rotational energy is converted into electrical energy.

Induction generators are often used in wind turbines and some micro hydro installations due to their ability to produce useful power at varying rotor speeds. Induction generators are mechanically and electrically simpler than other generator types. They are also more rugged, requiring no brushes or commutators.

Induction generators are not self-exciting, meaning they require an external supply to produce a rotating magnetic flux. The external supply can be supplied from the electrical grid or from the generator itself, once it starts producing power. The rotating magnetic flux from the stator induces currents in the rotor, which also produces a magnetic field. If the rotor turns slower than the rate of the rotating flux, the machine acts like an induction motor. If the rotor is turned faster, it acts like a generator, producing power at the synchronous frequency.

In fact, an induction generator may operate as a motor or a generator. For instance, a standard, 3 phases, AC motor may be powered from the 50 Hz grid, with the motor speed “slipping” at less than for 50 Hz synchronism. If this motor is itself forced to rotate at more than for 50Hz synchronism by a rotating power source, (e.g. a diesel engine or wind turbine), while connected to the grid, it delivers current to the grid as a generator. The current flow is proportional to the slip, i.e. the small difference, 3%, between synchronised rpm and the actual rpm. This slip is too small to notice as a speed change of a wind turbine rotor, so induction generators are classed, somewhat erroneously, as fixed-speed generators. This type of generator is very simple, rugged, and relatively cheap. Usually it is “excited” into operation.

In induction generators the magnetizing flux is established by a capacitor bank connected to the machine in case of stand alone system and in case of grid connection it draws magnetizing current from the grid. It is mostly suitable for wind generating stations as in this case speed is always a variable factor.