Induction Generator | Application of Induction Generator

Induction machines can function as generators, known as induction generators or asynchronous generators. But under what conditions do they work as generators?

An induction machine will behave as an induction generator when:

• Slip becomes negative due to this the rotor current and rotor emf attains negative value.
• The prime mover torque becomes opposite to electric torque.

Now let us discuss how we can achieve these conditions. Suppose that an induction machine is coupled with the prime mover whose speed can be controlled. If the speed of the prime mover is increased such that the slip becomes negative (i.e. speed of the prime mover becomes greater than the synchronous speed).

This setup allows the machine to function as an induction generator. If the prime mover’s speed increases too much, the generator’s efficiency drops. Since the speed during operation isn’t synchronous, it’s also known as a synchronous generator.

An induction generator isn’t self-excited and needs magnetizing current and reactive power to create a rotating magnetic field. This power usually comes from supply mains or another generator.

An induction generator can’t work in isolation because it continuously requires reactive power from the supply system. However, we can have a self-excited or isolated induction generation if we use a capacitor bank for reactive power supply instead of an AC supply system. We’ll now discuss isolated induction generators in detail.

Isolated Induction Generator

This type, also known as a self excited generator, uses a capacitor bank connected across its stator terminals to provide necessary reactive power.

The function of the capacitor bank is to provide the lagging reactive power to the induction generator as well as load. So mathematically we can write total reactive power provided by the capacitor bank is equals to the summation of the reactive power consumed by the induction generator as well as the load.

There is generation of small terminal voltage oa (as in figure given below) across the stator terminal due the residual magnetism when the rotor of the induction machine runs at the required speed. Due to this voltage oa the capacitor current ob is produced. The current bc sends current od which generates the voltage de.

The cumulative process of voltage generation continues till the saturation curve of the induction generator cuts the capacitor load line at some point. This point is marked as f in the given curve.

Application of Induction Generator

Let us discuss application of induction generator: We have two types of induction generator let us discuss the application of each type of generator separately: Externally excited generators are widely used for regenerative breaking of hoists driven by the three phase induction motors.

Self-excited generators are used in the wind mills. Thus this type of generator helps in converting the unconventional sources of energy into electrical energy.
Now let us discuss some disadvantages of externally excited generator:

• The efficiency of the externally excited generator is not so good.
• We cannot use externally excited generator at lagging power factor which major drawback of this type of generator.
• The amount of reactive power used to run these types of generator required is quite large.

Advantages of Induction Generators

• It has robust construction requiring less maintenance
• Relatively cheaper
• Small size per kW output power (i.e. high energy density)
• It runs in parallel without hunting
• No synchronization to the supply line is required like a synchronous generator

Disadvantages of Induction Generators

• It cannot generate reactive voltamperes. It requires reactive voltamperes from the supply line to furnish its excitation.