What is Induction Generator (Asynchronous Generator) ?
An induction machine is sometimes used as a generator. The complete torque-speed characteristics of a 3-phase induction machine for all ranges of speed.
Initially, the induction machine is started as motor. It draws lagging reactive voltamperes from the main supply. Then the speed of the machine is increased above the synchronous speed by an external prime mover in the same direction as the rotating field produced by the stator winding.
Then the induction machine will operate as an induction generator, and will produce a generating torque. This generation torque is opposite to the rotation of the rotor (or opposite to the rotating field produced by the rotor). Under these circumstances, the slip is negative and the induction generator delivers electrical energy to the supply mains.
In the equivalent circuit of an induction motor shown in fig below
The mechanical shaft load has been replaced by a resistor of value given by
Rmech =R2 ÷s ×(1–s)
In an induction generator, the slip s is negative and, therefore, the load resistance no longer absorbs power, but acts as a source of power. In other words, the induction generator supplies electrical energy to the supply mains to which it is connected.
The output of the induction generator depends upon the magnitude of the negative slip, or on how fast above synchronous speed the rotor is driven in the same direction or rotating that occurred when it operated as an induction motor.
As seen by torque-speed characteristics, there is a maximum possible induced torque in the Generating mode. This torque is called pushover torque of the generator. If the prime mover applies a torque greater than the pushover torque, the generator will overs speed.
The rotating magnetic field in the polyphase induction motor is produced due to the exciting current supplied to the stator winding from the supply line. The supply must continue to be available even if the machine is driven above synchronous speed. In other words, an induction generator is not a self-excited generator. It is necessary to excite the stator with an external polyphase source at all times at rated voltage and frequency and driving at a speed above the synchronous speed set by the supply frequency. Since the speed of the induction generator is different from the synchronous speed. It is known as asynchronous generator.
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| Induction generator (Asynchronous Generator) |
Initially, the induction machine is started as motor. It draws lagging reactive voltamperes from the main supply. Then the speed of the machine is increased above the synchronous speed by an external prime mover in the same direction as the rotating field produced by the stator winding.
 |
| Torque speed characteristics of an indication machine |
Then the induction machine will operate as an induction generator, and will produce a generating torque. This generation torque is opposite to the rotation of the rotor (or opposite to the rotating field produced by the rotor). Under these circumstances, the slip is negative and the induction generator delivers electrical energy to the supply mains.
In the equivalent circuit of an induction motor shown in fig below
 |
| Equivalent circuit of an induction motor |
The mechanical shaft load has been replaced by a resistor of value given by
Rmech =R2 ÷s ×(1–s)
In an induction generator, the slip s is negative and, therefore, the load resistance no longer absorbs power, but acts as a source of power. In other words, the induction generator supplies electrical energy to the supply mains to which it is connected.
The output of the induction generator depends upon the magnitude of the negative slip, or on how fast above synchronous speed the rotor is driven in the same direction or rotating that occurred when it operated as an induction motor.
As seen by torque-speed characteristics, there is a maximum possible induced torque in the Generating mode. This torque is called pushover torque of the generator. If the prime mover applies a torque greater than the pushover torque, the generator will overs speed.
The rotating magnetic field in the polyphase induction motor is produced due to the exciting current supplied to the stator winding from the supply line. The supply must continue to be available even if the machine is driven above synchronous speed. In other words, an induction generator is not a self-excited generator. It is necessary to excite the stator with an external polyphase source at all times at rated voltage and frequency and driving at a speed above the synchronous speed set by the supply frequency. Since the speed of the induction generator is different from the synchronous speed. It is known as asynchronous generator.
Advantages of Induction Generator:
- An induction generator has a robust construction requiring less maintenance. An ordinary cage motor may be used as an induction generator. Therefore, it is relatively cheap.
- It has a small size per kW output power.
- It does not have to be synthesized to the supply line as does a synchronous generator.
- It runs in parallel without hunting.
- Speed variation of prime mover is less important.
- An induction generator needs little auxiliary equipment.
- It has a self-protective feature. If a short circuit fault occurs on it's terminals, the excitation fails and the machine stops generation.
Limitations of Induction Generator :
An induction generator can't generate reactive voltamperes. Actually, it required reactive voltamperes from the supply line to the furnish it's excitation, since it has no means for establishing air gap flux with the stator open circuited.
Operation of an induction generator requires synchronous machine, whether generators or motors, on the line to supply the induction generator with its needed reactive voltamperes. It is this limitation of reactive voltamperes requirements which restricts the use of induction generators to a few rather unusual application.
Application of Induction Generator :
Induction generator have been used since early in the twentieth century, but by the 1960s and 1970s they had largely disappeared from use. However, with the increasing oil prices since 1973, its use started again.
Induction generators are used principally with alternative energy sources, such as wind mills, or with energy recovery systems in industrial processes. They are often used to supply additional power to a load in a remote area that is being supplied by a weak transmission line.
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