Take an ordinary induction motor and spin it faster than its synchronous speed — it turns into a generator! This is simply the idea behind the induction generator: its simplicity and tolerance of variable speeds made it the first choice for early wind farms, while a single fundamental drawback limited its spread.
The Idea
The induction generator is similar in concept to an induction motor: its rotor typically does not contain a controlled magnetic field as in the synchronous generator; instead, it relies on mutual induction between the stator and the rotor. When the rotor is driven faster than synchronous speed, the direction of power flow reverses, and the generator injects real power into the grid.
Why Was It Suited to Early Wind Power?
- Simple and rugged: a squirrel-cage rotor with no brushes, no slip rings, and no excitation system — minimal maintenance in towers that are hard to access.
- Tolerant of speed variation: wind is variable, and the induction generator can tolerate slip around the synchronous speed without losing synchronism — unlike the strict synchronous type.
- For these reasons, it was used in some wind applications, especially variable-speed ones.
Its Fundamental Drawback
The induction generator needs reactive power from the grid or from compensation devices in order to operate efficiently — it draws its magnetization from an external source since it has no self-excitation. While the synchronous generator supports grid voltage, the induction generator burdens it or requires accompanying compensation capacitors.
And What Happened Next?
With the development of power electronics, many modern wind applications shifted toward solutions based on synchronous generators combined with electronic conversion and control systems: the generator rotates freely at the variable wind speed, while electronic converters deliver a regulated frequency and voltage to the grid — combining speed flexibility with control quality. Details are covered in the wind turbine.
Sample answer: The synchronous generator carries a controlled field in its rotor via the excitation current, giving it precise control over its voltage and frequency and allowing it to produce both real and reactive power together — which is why it is the standard for power plants. The induction generator has no controlled field in its rotor and instead operates via mutual induction between the stator and rotor, making it simpler and more rugged and tolerant of speed variation (which is why it was used in early wind applications), but it consumes reactive power from the grid or requires compensation, and its control over voltage and frequency is limited.
Treating the induction generator as 'a cheaper equivalent to the synchronous generator.' The difference is qualitative, not just a matter of price: the induction generator cannot produce reactive power or regulate voltage — which is why it cannot serve alone as the backbone of a grid, and it remained limited to specific roles.
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