How does an induction generator works ?

Electrical Generator means an electrical equipment which is used in power plant to generate electricity by rotating the generator shaft with prime mover. There are different types of generator which can be used in power plant such as DC generator, 3 phase Induction generator, 3 phase synchronous generator. Generally, in small type of power plant or in remote area, 3 phase induction generator is used for power production. We have already discuss about 3phase induction motor. Here we are going to discuss about 3 phase induction generator, how does an induction generator work, grid connected & stand-alone/isolated types, advantages, limitation & application.

Induction generator || What is induction generator?

Induction generator is an AC generator which will produce 3 phase AC electricity by the help of prime mover ( like turbine) that can be further converted into different form of voltage for the use of different places like domestic, commercial etc. It is also known as asynchronous speed. Actually, induction generator is also a 3 phase induction motor and it acts like motor when it is started. At that time, it draws lagging reactive power ( Voltamperes) from the supply. A 3 phase induction motor will act like induction generator only when it rotates more than synchronous speed of that motor. For example, A 4 pole 3 phase induction motor with 50 Hz frequency then, its synchronous speed is 1500 rpm ( 120 x 50 / 4 ) then, if that machine runs more faster than 1500 rpm it will act like generator i.e. this is called induction generator. For that rotation, it need extra prime mover like turbine or motor which shaft is connected to that induction generator to rotate it in same direction of rotating field produced by the stator winding for the production of electricity.

On generating mode, this will produce generating torque which is opposite of rotation of rotor and the slip will also negative. The output electricity generated by induction generator is depend on magnitude of the slip value ( negative slip ) which also means, how fast the rotor will rotate above the synchronous speed.

Types of induction generator (IG)

1. Grid connected IG

The rotor of IG need to run  beyond the synchronous speed by prime mover and for excitation, reactive power need to supplied by grid or drawn from grid.

2. Stand-alone IG

The rotor will run more than synchronous speed and for excitation, there is capacitor (star or delta connected).

How does an induction generator works ?

There are two types of IG which work on same principle but on operation, there is quite small difference.

As we know, IG is not self excited generator so it is excited by external medium. In grid connected IG, that excitation is provided by the grid system. The rotating magnetic filed is produced in the stator winding while exciting current is supplied from the line or grid. The IG will start rotating by external prime mover and it run beyond the synchronous speed. All the time of rotation in starting phase (before and after the synchronous speed), the supply of reactive power from grid need to be done for excitation at rated voltage and frequency.

Also, the IG can work without supply from grid for that a capacitor bank is needed which is known as stand-alone IG. The capacitor is connected across the terminal of IG for excitation. The capacitor bank requires the presence of residual flux for initial excitation and if there is no residual flux, the machine need to run from the supply as induction motor for while to create residual flux. Now, the IG will run slightly beyond the synchronous speed by prime mover at no load. The residual flux in the magnetic circuit induces small voltage in the stator at a frequency proportional to the rotor's speed. This voltage across connected at its terminals gives leading current through the capacitor bank or lagging magnetizing current through the stator terminal which cause increase in air gap flux and this leads to increase in voltage at the stator terminal. This voltage cause increase in lagging magnetizing current or leading current through the capacitor bank and again voltage will increase. This process will go on until final voltage met i.e. at that point where the magnetizing characteristic of IG and voltage-current (V-I) characteristic of capacitor bank are intersected.

In the given figure above, C1, C2, C3, C4 are different value of the capacitor which are used in the capacitor bank for excitation in stand-alone IG. E1 and E2 are the induced voltage that will generate at final in this process and IC1 and IC2 are the leading current through the capacitor. Here C1, C2, C3, C4 are in decreasing form where C1 and C2 are capable of producing emf and C3 is critical capacitor where no emf is induced. Below the value of critical capacitor, voltage will not induce so it is necessary to use a good value of capacitor according to magnetizing characteristic and V-I characteristic. For the value of capacitor in capacitor bank, our engineering calculator is there for that calculation.

Induction generator exciter (capacitor size)

On stand-alone IG, it produced active power only but in a system there are inductive as well as resistive load so for reactive power, capacitor will help to produce reactive power.

Advantages of induction generator

1. An induction motor can be used for induction generator so it is cheap and require less maintenance.

2. It is not necessary to be synchronized for parallel operation and runs in parallel without hunting.

3. It requires less auxiliary equipment, and has small size.

4. Self-protection feature in short-circuit condition.

5. Speed variation of prime mover is less important.

Disadvantage or limitation of induction generator

1. It is not self stating so it requires supply from grid or capacitor bank needed for excitation.

2. It doesn't produce reactive power so grid or capacitor bank is responsible to balance power factor.

Application of IG

It is generally used in wind mill or wind energy recovery system as alternative energy source. Also, it is used in remote area for additional power supply as the power supplied by transmission line is weak.