An induction generator, also called an asynchronous generator, is a type of alternating current electrical generator. The generator's rotor is placed within a rotating magnetic field, and the rotor is then spun by an external source of mechanical energy so that it rotates more rapidly than the magnetic field. The rotating shaft begins dragging the magnetic field field forward, sending electricity flowing into the generator's coils. Induction generators are less complex and more rugged than other forms of generators and can continue effectively producing power if their rotor speed changes. An induction generator needs an external supply of electricity to create its rotating magnetic field and start operating, but once it has started generating power it can continue running on its own, provided it has a source of mechanical energy.
Induction generators are commonly used in wind turbines, which use wind to provide the mechanical energy to move the generator's rotor. The generator's ability to function at varying speeds allows the turbine to remain in operation in varying wind conditions. Small hydroelectric power sources, sometimes called micro hydro generators, also use induction generators. These generators are equipped with a device called an induction generator controller, which prevents the induction generator from damage and allows it to keep functioning during variations in water flow. Owing to the simplicity of their design, very small induction generators capable of powering household appliances can be built with readily available parts, such as the motors of washing machines.
Wind turbines often use a design called a doubly-fed induction generator, in which the rotor windings are connected to an electronics converter that can import or export reactive power to or from the generator as needed. This allows the generator to stay synchronized with the power grid during variations in wind speed. It also makes the power system as a whole more stable by allowing wind turbines to continue running and providing power to the grid uninterrupted in the event of a voltage dip in the grid, a capability called low-voltage ride through.
Induction generators are distinguished from synchronous generators, in which the rotor and the magnetic field rotate at the same rate. Synchronous generators can produce electricity more efficiently than induction generators, but need to be powered at a constant rate. The underlying principles of the induction generator can be applied in reverse to create an induction motor, in which the rotor is made to rotate more slowly than the magnetic field in order to convert electricity into mechanical energy.