**What
is an Induction Generator?**

Like
other electric generators, an induction generator converts mechanical energy
into electrical energy. As the name suggests, it is a type of induction machine
and works on Faraday's Law of Electromagnetic Induction.

An
**induction machine** can work as either a motor or a generator, meaning an
induction motor can be operated as a generator. As generators, induction
machines are not so popular because of their **reactive power** demand. A source of
reactive power is required all the time at the stator side to establish its
magnetic field (discussed further in this article).

Induction
generators are commonly known as “asynchronous generators” because they never
run at synchronous speed. In fact, they generate electricity when running at a
speed greater than synchronous speed, which is typically achieved with the help
of a prime mover.

**Construction
/ Parts of Induction Generators**

Like
any other electrical rotating machines, an induction generator consists of two
major parts: stator (stationary part) and rotor (rotatory part). Generally, a
**squirrel cage rotor** is used. The construction of an induction machine is
identical for both induction motors and generators. (refer **construction of induction motors**)

**Working
of Induction Motor as a Generator **

As
discussed above, the basic working principle for both the induction machine
i.e. induction generator and the induction motor are common, which is based on
Faraday's Law of Electromagnetic Induction.

We
have already published an article on the working of induction motors. We
recommend you to first go through that article, so that you gain a solid
understanding of **how an induction motor works**, which will help in easily
understanding the concept discussed here.

**In
induction motors**, we explained that the induction motor runs at
a speed N which is always slightly less than the synchronous speed Ns (N < N_{s}),
because if the motor runs at synchronous speed Ns (N = N_{s}), then the
relative speed between the rotor and synchronously rotate magnetic field would
be zero, resulting in no torque developed in the motor. Therefore, the **slip in induction motors** is always positive (can be observed with the help of the given
expression of slip).

If
the speed of the induction motor (N) is increased to super synchronous speed (N
> N_{s}) while it remains connected to AC mains, **then the
induction motor works as an induction generator**. In this condition, it
generates active power to mains while drawing the reactive power from the AC
mains.

The
super synchronous speed for the induction motor can be achieved with the help
of a prime mover. Due to the super synchronous speed of the rotor (N > N_{s}),
the operating slip of induction generators is negative (can be verified with
the help of the expression of slip given above). Generally, the operating slip
for the induction generators is (-0.01 to -0.05).

The reactive power burden of induction generators on the grid can be relieved by installing shunt capacitors (connected in delta configuration) across the terminals of induction generators. These shunt capacitors deliver the lagging vars or reactive power demand by the generators and the rest reactive power demand by the load is fulfilled by the transmission line.

Since, they absorb reactive power or lagging vars, in other words, we can say that
they deliver leading vars and in case of generators, the operating power factor
is what they deliver. Therefore, an induction generator operates at a leading
power factor.

The
frequency of active power supplied is equal to the frequency of the reactive
power drawn by it from the AC mains.

**Torque
Slip Characteristics of Induction Generators**

The
**torque slip characteristics of the induction motor** is shown in the given
figure. In these characteristics, the region in which N > N_{s} or
slip is negative is the generating zone, representing the torque slip
characteristics of induction generators.

In the generating zone, the torque slip curve is linear in
between the zero slip (s=0) and the slip (s_{m}) at which maximum
torque (pushout torque) occurs. This is the recommended operating region for
the induction generators, as discussed above the operating slip for induction
generators is (-0.01 to -0.05).

**Advantages
& Applications of Induction Generators**

A
major advantage of the induction generator is its frequency regulation, meaning
in induction generators the frequency of output voltage is independent of speed
variation unlike synchronous generators, where the speed has to be tightly
controlled to match the synchronous speed, so that efficiency of output voltage
does not deviate from the line frequency. As we discussed above the frequency
of the active power supply will depend on the frequency of the reactive power
source.

This
self-regulation property makes induction generators best suitable for variable
speed power generation like wind-mills, micro hydro power plants etc. when
connected to an infinite bus.

**Disadvantages of using Induction Generators**

Induction
generators require a reactive power source all the time to generate active
power. Hence, they will act as a reactive power burden on the existing system.
This is the major drawback of using induction generators.

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