Introduction
An induction generator, also called an asynchronous generator, is a type of AC generator that converts mechanical energy into electrical energy when driven above its synchronous speed. Unlike synchronous generators, it requires an external power source or capacitor bank for excitation. These generators are valued for their rugged design, low maintenance, and cost-effectiveness, making them ideal for renewable energy systems, remote power setups, and regenerative braking.
Key Overview
- Definition: Induction generator operates as a generator when the rotor speed exceeds synchronous speed.
- Operating Principle: Power generation occurs under negative slip conditions (s < 0).
- Excitation: Requires external reactive power from the grid or capacitors.
- Self-Excited Type: Uses capacitor banks for excitation in off-grid systems.
- Applications: Commonly used in wind turbines, braking systems, and rural electrification.
Working of Induction Generator
The induction generator initially functions as a motor, drawing lagging reactive power from the supply. When the prime mover increases its speed beyond synchronous speed, the rotor slip becomes negative, and the machine begins to deliver active power back to the grid.
Key Conditions for Generation:
- Slip (s) must be negative.
- Prime mover torque must exceed electromagnetic torque.
- Generating torque opposes rotor rotation.
Torque-Speed Characteristics:
- Generating region lies in the negative slip zone.
- Pushover torque: Maximum torque limit before over-speeding.
- Efficiency decreases if pushover torque is exceeded.
Mathematical Relation:
Total reactive power from capacitor = reactive power needed by generator + load
Excitation Requirement
- Induction generators are not self-excited under normal grid-connected operation.
- Excitation is provided by:
- Grid (externally excited generators)
- Capacitor banks (self-excited generators)
Types of Induction Generators
1. Externally Excited Generator
- Requires grid connection for excitation.
- Widely used in regenerative braking systems.
2. Self-Excited Induction Generator (SEIG)
- Uses capacitors to supply magnetizing current.
- Suitable for off-grid applications like wind energy and rural electrification.
Advantages of Induction Generator
- Simple and rugged construction.
- Low cost and easily available motors (squirrel cage type can be used).
- No synchronization with grid required.
- Can operate in parallel without hunting.
- Inherent fault protection – does not supply short-circuit current.
- Compact for a given power output.
Disadvantages of Induction Generator
- Cannot generate reactive power on its own.
- Requires external excitation from grid or capacitors.
- Efficiency drops at higher speeds beyond pushover torque.
- Reactive volt-ampere demand can be high.
Applications of Induction Generator
- Wind turbines and renewable energy plants
- Regenerative braking in electric locomotives and cranes
- Remote and rural power generation (mini hydro, biogas setups)
- Standalone or weak grid environments
Conclusion
The Induction Generator, or Asynchronous Generator, offers a cost-effective and durable solution for distributed and renewable power generation. While it requires reactive power for excitation, its robustness, ease of operation, and low cost make it highly suitable for systems like wind turbines, micro-hydro stations, and remote energy setups. Understanding its operation—especially the role of negative slip and reactive power—is essential for successful deployment in real-world electrical engineering applications.