INDUCTION GENERATOR

Induction generator is especially used in wind power plants for producing electricity.

Why induction generator?

• Because, due to its ability to produce useful power at varying rotor speeds.

• Induction generator are mechanically and electrically simpler than other generator types. And it is rugged, requiring no brushes and commutator.

Working:

• Induction motor itself works as a induction generator but with some paramater constraints. Squirrel cage induction motor is widely used for many application. One of its application is in wind power plant.

• When a three phase AC supply is applied to stator, rotating magnetic field(RMF) is created which rotates in synchronous speed.
• According to electromagnetic induction, these RMF cuts the stationary rotor conductor bars. As a result, emf get induces in the bar and due to the short circuit of bars, current suppose to flow in the rotor bars and creates its own magnetic field.
• Due to the interaction of two magnetic fields( rotor and stator) rotor starts to rotate. But the speed of the rotor is lesser than that of the synchronous speed.

Why rotor speed is less than synchronous speed?

• Because, there is no cutting of flux in the rotor copper bars if the rotor runs in synchronous speed. 
• For example, if synchronous speed is 1500 RPM. Then the speed of rotor is 1470 RPM.

How induction motor work as induction generator?

• By changing the slip parameter, we make it to work as induction generator.  
• Slip is nothing but the ratio of difference between synchronous speed and rotor speed to the synchronous speed.

• From the figure, if the slip value is negative(s<0) the motor acts as generator.

When slip is negative?

• If we rotote the rotor greater than the synchronous speed, the slip become negative. In this instant, the stator starts to produce electricity i.e acts as generator.

Standalone self excited induction generator(SEIG) for wind mill:

• Standalone means off grid operation. That is, the rotor is having certain residual magnetism.
• Once the rotor rotates, the flux in the rotor cuts the stationary stator conductor and thus voltage is induced. 

• But the voltage induced is less. In order to boost up the voltage, capacitors are connected to the three phase winding of the stator.
• The voltage regulation of the this type of generator is poor. Because, if speed increases suddenly the voltage also increases suddenly and lower the speed ,the induced voltage is very small.
•  For example, if rotor speed is 600 RPM, the induced voltage is 20 volts. If rotor speed is 1200 RPM, the induced voltage is 120 volts. This regulation can be maintained  by the static VAR compensator.