What Are Induction Generators?

Induction generators are alternating current electrical machines that convert mechanical power into electrical power by operating an induction machine above its synchronous speed. Unlike synchronous generators, which require a separate DC excitation source to establish their rotor magnetic field, induction generators derive their excitation from reactive power supplied by an external AC source or shunt capacitor bank. They are the same physical machines as induction motors and can operate in either mode depending on whether the rotor spins below or above the synchronous speed of the stator's rotating magnetic field. The induction generator's simplicity, mechanical robustness, and tolerance for variable-speed operation have made it a preferred choice in wind power installations and small-scale hydroelectric generation.

Operating Principles and Slip

The behavior of an induction machine is governed by slip, defined as the fractional difference between synchronous speed and actual rotor speed. When a mechanical prime mover drives the rotor above synchronous speed, slip becomes negative: the rotor conductors cut the rotating magnetic field in the opposite sense, reversing the direction of induced rotor currents and causing the machine to deliver rather than absorb electrical power. The magnitude of real power transferred to the grid is proportional to negative slip, and the machine simultaneously draws reactive power to maintain its air-gap flux. An IEEE publication on modeling wind farms in load flow analysis discusses how induction generators' slip-dependent power characteristics are represented in power system studies, including their interaction with network voltage and frequency. The simplicity of this mechanism, requiring no brushes, slip rings, or excitation controller for the squirrel-cage type, contributes directly to the machine's reliability.

Doubly-Fed Induction Generators

The doubly-fed induction generator (DFIG) extends the basic squirrel-cage concept by connecting the rotor windings to the grid through a back-to-back power converter, rather than shorting the rotor circuit internally. This arrangement allows the converter to inject or absorb current at slip frequency, giving the system independent control over real and reactive power output and enabling variable-speed operation over a wide range, typically 30 percent below to 30 percent above synchronous speed. An IEEE paper on induction machine-based wind generators with maximum power point tracking examines neural-network-based control schemes that extract maximum power from variable wind speeds while minimizing system losses. DFIGs dominate utility-scale wind turbine installations because variable-speed operation reduces mechanical stress on the drivetrain and allows the turbine to follow the optimal aerodynamic torque curve across a range of wind speeds.

Reactive Power and Excitation

A fundamental constraint of induction generators is that they cannot self-excite without an external reactive power source. Grid-connected machines draw the reactive current needed to magnetize the stator from the power system, imposing reactive power demands on the network. In islanded or standalone applications such as remote micro-hydro plants, shunt capacitor banks are connected at the machine terminals to supply the required excitation reactive power. Capacitor sizing must account for the expected range of load power factor and speed variation; insufficient capacitance prevents voltage buildup, while excessive capacitance can cause self-excitation at no load and lead to overvoltage. IntechOpen's chapter on induction generators in wind power systems details the capacitor sizing procedures and voltage regulation strategies used in self-excited induction generator installations.

Applications

Induction generators have applications in several areas of power generation and distribution, including:

• Wind turbine generators in both fixed-speed and variable-speed doubly-fed configurations
• Small-scale run-of-river hydroelectric plants and micro-hydro installations
• Industrial regenerative drives that return braking energy to the supply grid
• Distributed generation systems connected to low-voltage distribution networks