Invented by Nikola Tesla and Mikhail Dolivo-Dobrovolsky in the late 19th century, the Squirrel Cage Induction Machine can be successfully used as generator in small scale hydro applications. Simple, robust and compact, it is ranked as the most reliable type of electric generator, mainly due to the small number of components. The power and speed range are standardized, making the induction generator available as on-the-shelf component in the power range of up to several hundred kW.
Small scale Wind stations uses several induction generator types:
SQIN – squirrel cage;
WRIN – wounded rotor, with brushes and slip-rings;
CEIN – capacitor excited.
If connected to the national grid (DOL, Y/D or with soft starter) it imposes a cvasi-constant speed operated turbine, proper for constant head and flow sites. If driven by reaction type turbines, as for efficiency reasons, variable head and flow conditions can be dealt by constant speed induction generators combined with adjustable stator guide vanes and rotor blades, or by variable speed induction generators combined with fixed stator guide and rotor blades. The second solution simplifies the mechanical equipment, but requires regenerative 4Q frequency converters, offering excellent results.
In island operated applications, the squirrel cage induction generator has to be excited through external capacitors. This configuration works up to 15 kW, at constant speed, thus external hydraulic or ballast load controllers have to be deployed.
Several constructive variants (1/3-phase, low or medium voltage, horizontal or vertical, open-drip proof, etc.) of induction generators are available.