Induction generator

Because they can recover energy with relatively simple controls, induction generators are useful in applications such as mini hydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure.

Induction generators have an AC rotor and cannot bootstrap using residual magnetization to black start a de-energized distribution system as synchronous machines do.

Power factor correcting capacitors can be added externally to neutralize a constant amount of the variable reactive excitation current.

After starting, an induction generator can use a capacitor bank to produce reactive excitation current, but the isolated power system's voltage and frequency are not self-regulating and destabilize readily.

In generator operation, a prime mover (turbine or engine) drives the rotor above the synchronous speed (negative slip).

The generating mode for induction motors is complicated by the need to excite the rotor, which being induced by an alternating current is demagnetized at shutdown with no residual magnetization to bootstrap a cold start.

The generator is able to supply current out of phase with the voltage requiring more external equipment to build a functional isolated power system.

If the prime mover is unable to produce enough power to fully drive the generator, speed will remain somewhere between 1800 and 1860 RPM range.

In induction generators, the reactive power required to establish the air gap magnetic flux is provided by a capacitor bank connected to the machine in case of stand-alone system and in case of grid connection it draws reactive power from the grid to maintain its air gap flux.

For stand-alone systems, frequency and voltage are complex function of machine parameters, capacitance used for excitation, and load value and type.

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.

The load must be removed and the induction generator restarted with either an external DC motor or if present, residual magnetism in the core.

Minimum capacitance per phase: If the load also absorbs reactive power, capacitor bank must be increased in size to compensate.