Induction Generator : Part2

1.1 Circle Diagram of Induction Generator
Using circle diagram, the induction generators can also be analysed. Fig. 1

As the antiphase component of current reverses, direction of current also changes. It will be below horizontal shown by OE. E is the operating point. As seen from circle diagram.
AB = Rotor Cu loss.
BC = Stator Cu loss
CD = Constant losses.
DE = Generator output.
BE = Rotor input.
The slip is given by,

Similarly other required quantities can be obtained from the circle diagram.
1.2 Comparison of Induction Generator and Synchronous Generator
The distinct features of induction generator compared to synchronous generators are as follows :
i) It will not require d.c. excitation.
ii) It is not self excited but external a.c. supply of fixed frequency is required.
iii) The frequency of induction generator is decided by the frequency of the excitation voltage which is supplying current to it.
iv) Synchronization of generator is not required as no emf is generated until it is connected to the line.
The following are the advantages of induction generator.
i) Synchronization for induction generator is required.
ii) The construction is rugged for rotating parts.
iii) Unlike in synchronous machine, there is no danger of hunting or drop out of synchronism for induction generators.
iv) When it short circuited, it delivers small power as the excitation quickly reduces to zero.
v) Induction generators are more suitable for high speeds.
vi) With the help of excitation supply and frequency, the voltage and frequency of induction generator are controlled.
Although induction generators are having above mentioned advantages, it has following advanatges.
i) It must be run in parallel with the synchronous machine.
ii) The load is not deciding the power factor of induction generator but the power factor depends on slip.
1.5 Applications
Because of distinct superiority of the synchronous generator, induction generators are rarely used to supply commercial power.
One application of induction generator is in railway for braking purposes. When the train is moving down a gradient, the induction generators runs above synchronism. As the torque in this region is negative, the braking action is achieved in the train. In addition to this the energy generated by induction generator is given to the line so that the load on main generating station is somewhat relieved. In this case no complicated control apparatus is required.