Why do Induction Generator need capacitor bank?

[I_am_learning]

I can't understand why Induction Generator when diving stand-alone load must be provided with capacitance bank. In fact I am not sure as how induction generator works at all.
Here is my attempt.

When the rotor rotates, then due to residual flux initially some voltage will induce in the rotor, so current flows and that creates rotating magnetic field relative to the rotor. This in turn will create stator emf and hence stator current will flow that will again create rotating magnetic field relative to stator. These two field re-in force and the phenomena rises in magnitude.

If the above crude description is correct where has the capacitor to come?

 

[Fish4Fun]

The short answer is Power Factor Correction, you can read about it here:

http://en.wikipedia.org/wiki/Power_factor

for a general idea about the concept of PF.

Fish

 

[I_am_learning]

--For power factor correction/ To provide Reactive power -- To provide magnetizing current ...
I have heard all of this.

Could somebody explain step by step the process of voltage build up in Induction generator. Then perhaps, in some steps we could see the need for capacitors.

 

[SirAskalot]

For the stator winding to set up a magnetic field it need reactive power.(delivered form capacitors)

Remanence in rotor starts to induce a voltage in the stator. Causing a current flow in the capacitor bank. The capacitors deliver reactive power to the stator windings making it able to set up a magnetic field. This magnetic field induces a voltage and hence a current in the rotor making voltage build up in stator windings.

Without the capacitors the stator windings could never induce a magnetic field.

 

[Fish4Fun]

removed double post

 

[Fish4Fun]

For a detailed step-by-step:

http://en.wikipedia.org/wiki/Induction_generator

You already stated above:

When the rotor rotates, then due to residual flux initially some voltage will induce in the rotor, so current flows and that creates rotating magnetic field relative to the rotor.

Assuming the residual flux is initially in the rotor it induces a voltage in the stator coils. Assuming some load on the stator coils, the change in flux in the stator coils creates an EMF as current flows. This induced EMF is magnetically linked to the rotor, increasing the magnetic field in the rotor. As the rotor moves to the next stator coil, it has an increased flux and produces a slightly larger voltage in this stator coil. @ 1800 RPM this "ramping" effect is happening @ 60 times per second per pole.

Power Factor and Power factor Correction are more important concepts.

Fish

 

[I_am_learning]

And one more thing, when driving standalone load -- Does it make sense to say, when the speed is above synchronous speed...
There is no synchronous speed because it is not connected to the grid. Then don't it generate power at even the slowest speed?

 

[Fish4Fun]

And one more thing, when driving standalone load -- Does it make sense to say, when the speed is above synchronous speed...
There is no synchronous speed because it is not connected to the grid. Then don't it generate power at even the slowest speed?

This statement is not as clear cut as you might assume. The rotor and the stator are designed to operate at a particular frequency (typically 50hz or 60hz). Frequencies above or below the design frequency cause dramatic efficiency losses. This fact is independent of function (generator/motor). Moving more than 5% above or below design frequency can introduce large losses. There are some induction motors designed to operate @ 50hz-60hz, but typically they are less efficient than their counter parts designed for a particular frequency.

So, in short, in the case of a generator, while under no-load the operation is technically "synchronous" (at any speed), and the voltage output of the generator might be a fairly linear function of RPM, any load at all applied to the generator will demonstrate a very non-linear power curve.

Fish

 

[m.s.j]

For the stator winding to set up a magnetic field it need reactive power.(delivered form capacitors)

Remanence in rotor starts to induce a voltage in the stator. Causing a current flow in the capacitor bank. The capacitors deliver reactive power to the stator windings making it able to set up a magnetic field. This magnetic field induces a voltage and hence a current in the rotor making voltage build up in stator windings.

Without the capacitors the stator windings could never induce a magnetic field.

It is excellent reply, but one point shall be clear more. The initial emf of machine cannot produce with capacitor bank, indeed it is produced by core residual magnetizing flux and increased continuously tile rated emf by capacitive load current.
When an induction generator first starts to turn, the residual magnetism in its field circuit produces a small voltage. That small voltage produces a capacitive current flow, which increases the voltage, further increasing the capacitive current, and so forth until the voltage is fully built up. If no residual flux is present in the induction generator 's rotor, then its voltage will not build up, and it must be magnetized by momentarily running it as a motor.
The most serious problem with an induction generator is that its voltage varies wildly with changes in load, especially reactive load. Typical terminal characteristics of an induction generator operate alone with a constant parallel capacitance. Notice that , in the case of inductive loading, the voltage collapses very rapidly. This happens because the fixed capacitors must supply all the reactive power needed by both the load and the generator, and any reactive power diverted to the load moves the generator back along its magnetization curve, causing a major drop in generator voltage. It is therefore very difficult to start an induction motor on a power system supplied by an induction generator-special techniques must be employed to increase the effective capacitance during starting and then decrease it during normal operation.

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