Asynchronous Induction Generator

An induction machine, normally used as a motor, can also generate electricity. The same squirrel-cage machine that turns electrical power into rotation will feed power back into the grid when it is driven mechanically faster than its synchronous speed. In motoring operation the rotor turns slightly slower than the rotating magnetic field of the stator, a difference called slip. When an external prime mover pushes the rotor above synchronous speed, the slip reverses sign and the machine acts as a generator.

This mode of operation relies on the stator being connected to a source of alternating current that supplies the magnetising current needed to set up the rotating field. For that reason a standard asynchronous generator usually requires connection to an existing grid, or a bank of capacitors to provide the reactive power, before it can produce output. The rugged squirrel-cage rotor has no brushes or slip rings, making the machine simple, robust and inexpensive to maintain.

Asynchronous generators are common in smaller and medium wind turbines because they tolerate variations in driving speed and connect to the grid relatively easily. Their main limitations are the need for an external source of reactive power and a power output that depends on how far above synchronous speed the rotor is pushed. Synchronous generators are generally preferred where independent voltage and frequency control is required, but the induction generator remains a practical choice for many distributed and renewable installations.

Frequently asked questions

How does an induction motor become a generator?

When a prime mover drives the rotor faster than the stator's synchronous speed, the slip reverses and the machine delivers power instead of consuming it.

Why does an asynchronous generator usually need a grid connection?

It requires an external source of reactive power to create its rotating magnetic field; a grid or a capacitor bank supplies this magnetising current.

Where are asynchronous generators commonly used?

They are widely used in wind turbines because they are rugged, low-cost and connect easily to the electrical grid.