Induction generator

An induction generator is also known as an asynchronous generator. Like a DC machine, the induction machine works both as a motor and generator.

What is an induction generator?

An induction generator is an induction machine that works as a generator when the slip becomes negative. And then it starts supplying energy back to the main supply.

The operation of the induction machine depends on the value of the slip. When the slip is positive, the machine operates as an induction motor. And for the negative value of the slip, the machine works as an induction generator.

The slip becomes negative when the rotor achieves a speed above the synchronous speed.

S = (Ns-Nr)/Nr

Where,

Ns = synchronous speed

Nr = rotor speed

When the slip is positive i.e. rotor speed is less than the synchronous speed. The induction machine operates as an induction motor.

But normally the induction motor runs below the synchronous speed. So it requires an external force to achieve speed above the synchronous speed. 

That’s where a prime mover is used to exert an external force which helps in achieving a speed greater than the synchronous speed.

How does it work?

Diagram of induction generator.
A generator is connected to the prime mover. The generator draws reactive power from the main supply and supply, active power to the supply.

When the stator winding of the induction machine is connected to the AC supply. It induces a revolving field in the stator which rotates at synchronous speed. This induces a current in the rotor.

The interaction of stator and rotor fields produces torque. Due to this, the rotor starts rotating. Rotor speed is less than the synchronous speed i.e. the slip becomes positive.

Therefore, initially, the induction machine starts operating as an induction motor.

If the rotor speed increased slightly by the external prime mover. The rotor speed matches the synchronous speed. Therefore the slip becomes zero and the rotor produces no current. So, at this position, the machine neither takes current nor supply it to the main supply.

On the further increase, the rotor will achieve a speed greater than the synchronous speed. Due to this, the slip becomes negative and the rotor conductors start generating current in the opposite direction.

The rotor current produces a rotating magnetic field in the rotor which applies torque on the stator in opposite direction. this results in an emf in the stator winding.

The magnitude of the emf in stator winding is greater than the applied voltage and also the emf is in opposite direction. this pushes the current to flow out of the machine, thus the machine starts operating as an induction generator.

This shows that the induction generator is not a self-starting machine. It requires magnetizing current and draws reactive volt-amperes from the main supply and at the same time supply active volt-amperes to the main supply.

The output of the generator depends upon the magnitude of the slip i.e. how much active power the generator supplies depends on how much the slip is above the synchronous speed.

Torque-speed characteristics of induction generator

The induction machine works both as a motor and a generator. The behaviour of the induction machine through a torque-slip plot is shown and explained below:

The plot is divided into 3 categories braking region, motoring region, and generating region.

The induction machine operates as a motor for a positive slip and for a negative slip the machine starts operating as a generator.

Here pt.1 indicates the maximum torque of a 3-φ induction machine. And the pt.3 indicates the maximum torque of the induction generator also known as pushover torque.

If by any chance the torque value exceeds the maximum value of torque. The induction generator will Overspeed and the generating effect vanishes.

According to the torque-slip characteristics, the operating range of the induction generator is limited to the value of pushover torque.

Self-excited or Isolated induction generator

The induction machine works as a motor and a generator which is not self-starting.

But the isolated induction generator does not require an existing ac supply for reactive power.

A capacitor bank connected across the terminal provides reactive power to the generator as well as the load. It is shown in the figure below:

isolated induction generator. an additional capacitor bank is connected to the generator.
ISOLATED INDUCTION GENERATOR

But how does it work?

An initial residual flux is required. To produce the initial residual flux the induction machine starts as a motor till the speed reaches above the synchronous speed.

The generated flux induces a small terminal voltage, which produces the current. The current produces more flux that assists the residual flux, which increases net flux, which in turn causes a net increase in the voltage.

This process of voltage build-up continues till the magnetization curve intersects the voltage-current characteristics of the capacitor bank.

At the point of intersection, the reactive power supplied by the generator is equal to the reactive power supplied by the capacitor bank.

Advantages

  • It can protect itself. If a short circuit occurs on its terminals, then the machine stops generation.
  • Due to its robust construction, it requires less maintenance.
  • It has a small size per kW output power.
  • It runs in parallel without hunting.
  • The need for auxiliary equipment is very less.

Disadvantages

  • For lagging P.F. load the voltage collapses rapidly.
  • It requires reactive power from the main supply.

Applications

These generators are used in wind turbines, mini-hydropower plants.

Quick summary

  • The induction machine works as a motor as well as a generator.
  • The induction machine starts operating as an induction generator when the slip becomes negative i.e. the rotor speed becomes larger than the synchronous speed.
  • It is not a self-starting machine.
  • A capacitor bank is connected to the stator terminal of the generator to make it self starting, known as an isolated induction generator.
  • It is used in windmills, mini-hydro power plants.

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