- Induction Generator Definition: An induction generator (also known as an asynchronous generator) is defined as an induction machine used to generate electricity.
- Operating Principle: Induction generators work when the slip is negative, achieved by increasing the prime mover speed beyond synchronous speed.
- Magnetizing Current Requirement: They require external sources for magnetizing current and reactive power, often provided by supply mains or other generators.
- Self-Excited Generators: With a capacitor bank, induction generators can operate in isolation by providing their own reactive power.
- Application of Induction Generator: Induction generators are used in regenerative braking systems and windmills for converting wind energy into electrical energy.
An induction machine operates as an induction generator, or asynchronous generator, when mechanical input drives it into its generating region. Its stator then delivers active electrical power while the machine still needs magnetising reactive power.
Using the conventional slip definition, the generating conditions are:
- Slip becomes negative because rotor speed exceeds synchronous speed. The rotor current and electromagnetic torque reverse relative to motoring operation.
- The prime mover supplies mechanical torque while electromagnetic torque opposes rotation.
Couple the shaft to a controllable prime mover. When it drives the rotor above the synchronous speed set by stator frequency and pole count, slip becomes negative and active power flows from the shaft through the stator to the electrical system.
The permitted generating-speed range depends on the machine design, current limit, cooling and connected system. Excess mechanical torque increases negative slip and current until a protection or thermal limit is reached. Because the rotor does not run exactly at synchronous speed, this machine is asynchronous. It must not be confused with a synchronous generator.
A grid-connected induction generator draws magnetising current and reactive power to establish its rotating magnetic field. The grid, another generator, a converter or local capacitors can provide this reactive-power demand.
A plain induction generator cannot sustain an isolated terminal voltage without a magnetising source. A suitably selected capacitor bank can support self-excitation when residual flux, rotor speed and load conditions allow voltage to build. The result is an isolated induction generator.
Isolated Induction Generator
This arrangement, also called a self excited generator, connects a capacitor bank across the stator terminals. The capacitors supply leading reactive power for machine magnetisation and the load’s reactive demand.
The machine and an inductive load consume lagging reactive power. The capacitor bank supplies leading reactive power, so its output must cover the magnetising demand plus the relevant load demand at the intended voltage and frequency. Too little capacitance may prevent voltage build-up; too much can cause excessive voltage.
Residual magnetism produces the small terminal voltage oa when the rotor reaches a suitable speed. This voltage drives capacitor current ob. The resulting magnetising current increases air-gap flux, which raises the terminal voltage and capacitor current in a cumulative build-up process.
Voltage builds until the machine magnetisation curve intersects the capacitor line at an operating point, marked f in the figure. The intersection depends on capacitance, rotor speed, magnetic saturation and load. If the conditions do not provide a stable intersection, self-excitation will not build or hold the required voltage.
Application of Induction Generator
One established application of induction generator operation is regenerative braking. A grid-connected induction generator can return energy when a descending hoist or overhauling load drives three phase induction motors above synchronous speed. Modern variable-speed drives can also return energy through a regenerative converter.
Induction generators have been used in wind turbines and small isolated generation systems to convert mechanical energy into electrical energy. Fixed-speed grid-connected machines use a narrow slip range, while variable-speed systems commonly add rotor or full-scale power converters.
Important limits of a directly grid-connected induction generator include:
- Efficiency depends on machine size, loading, speed and loss design; it is not inherently poor in every application.
- A directly connected machine normally absorbs reactive power and operates at a lagging power factor unless capacitors or a converter provide compensation.
- Magnetising reactive-power demand increases stator current and network losses, so the installation may require local compensation.
Advantages of Induction Generators
- A squirrel-cage rotor has robust construction and no brushes or separate rotor excitation.
- A standard induction-machine design can be economical and widely available.
- Power density can be favourable, although size per kW depends on speed, cooling and rating.
- A grid-connected unit does not exhibit synchronous-machine rotor-angle hunting.
- It does not require rotor-angle synchronisation like a synchronous generator, but connection still needs controlled switching, protection and acceptable speed and voltage conditions.
Disadvantages of Induction Generators
- A basic induction generator cannot regulate its own reactive-power demand. The grid, capacitors or a power converter must supply magnetising voltamperes.
