Consider an induction generator with its stator connected to a source of reactive power. This should help create a rotating magnetic flux in its stator, right?
As the rotor is driven by a source of mechanical power, a voltage due to change in stator flux gets induced in it. This voltage drives a current and that sets up a rotor flux.
This flux opposes the change in stator flux and induces a voltage in the stator coils to oppose that flux which sets up the stator current.

A couple of questions in this regard:

1. Is my understanding of the basic power generation correct?

2. How does the reactive power source (capacitor bank/STATCOM) set up the initial flux?

3. What is direction of the rotor flux vector?

 

this is being lightly touched on in this thread;
http://forum.allaboutcircuits.com/showthread.php?t=71830

your excitation must slightly lead generated power, in order to set up the magnetics prior to sweeping the windings. These leading vars can be supplied with capacitors. I'm not sure I can answer your direction question other than these leading vars give rise to generation through all quadrants.

 

Let's look at the operation of an induction generator in transition from a motor to a generator.

An AC voltage applied to the stator of an induction generator sets up a rotating magnetic field which induces current in the rotor proportional to the slip speed frequency. This generates a corresponding field that pulls the rotor with the rotating field (it's acting as a motor).

If you drive the shaft, the slip frequency is reduced and becomes zero at synchronous speed thus reducing the induced rotor current to zero (at which point there is no net torque). If you continue driving the rotor above synchronous speed, there now will be slip in the opposite direction, generating rotor current which creates a rotating magnetic field that pushes on the stator field rather than being pulled by it. This creates a stator voltage the pushes current out of the stator windings against the applied voltage. The device is now acting as a generator.

Note that a pure induction generator must have a source of AC voltage to work against at all times since that source determines the frequency and voltage of the output. The generator is always operating above synchronous speed and has no specific output frequency of its own.

An interesting feature of the induction generator is that is doesn't require speed regulation to synchronize with the line frequency. As long as the generator is above synchronous speed it will output power in synchronism with the line voltage and absorb all the shaft power applied to it.

 

this is being lightly touched on in this thread;
http://forum.allaboutcircuits.com/showthread.php?t=71830

your excitation must slightly lead generated power, in order to set up the magnetics prior to sweeping the windings. These leading vars can be supplied with capacitors. I'm not sure I can answer your direction question other than these leading vars give rise to generation through all quadrants.

Thanks, that discussion was quite useful in answering some of my queries too.

However, it is this excitation that I am a bit confused about. You provide leading vars to the stator and that sets up a magnetic flux which sweeps over the rotor windings, which are being rotated. Is that correct?

Sort of like a dc generator with current-excited poles on the stator/residual magnetism effect helping the current production?

 

crutschow's post is a good explanation of operation. You would do well to reread it several times.

 

This creates a stator voltage the pushes current out of the stator windings against the applied voltage. The device is now acting as a generator.

Note that a pure induction generator must have a source of AC voltage to work against at all times since that source determines the frequency and voltage of the output. The generator is always operating above synchronous speed and has no specific output frequency of its own.

After you create a rotor flux that opposes the stator's why do we continue to provide the real power excitation to the stator? Just to fix the frequency of the opposing stator output voltage?

Can we not start an induction generator on its own? Without the need to start it as a motor first?

I read that if you connect an induction generator to a cap. bank for the reactive var supply and run it above synchronous speed it would start producing power. But again the frequency of the active power it produces is not fixed. Also, I am not clear as to how the capacitors would set up a "rotating" flux in the stator.

I am sorry if I don't make much sense. It is just that I am really confused with the basic principles of an IG which I need to work out before I start with doubly-fed IGs for my project.

Thanks!

 

EDIT: Just realized that the caps work in the same way to produce a rotating field as they might do in a single phase machine. But the understanding the rest of the working eludes me as before

 

After you create a rotor flux that opposes the stator's why do we continue to provide the real power excitation to the stator? Just to fix the frequency of the opposing stator output voltage?

Can we not start an induction generator on its own? Without the need to start it as a motor first?

I read that if you connect an induction generator to a cap. bank for the reactive var supply and run it above synchronous speed it would start producing power. But again the frequency of the active power it produces is not fixed. Also, I am not clear as to how the capacitors would set up a "rotating" flux in the stator.

.................................

You need a continuous stator voltage to provide the desired output frequency since an induction generator operates above synchronous speed by some slip ratio depending upon the amount of power the generator is providing. If you removed the voltage then the generator output voltage and frequency would vary depending upon the generator speed and the output load.

Yes you can rev up the generator to above synchronous speed before you apply the stator voltage so it doesn't act as a motor first.

I don't see how a capacitor would allow an induction generator to operate without a stator voltage source. Even if it worked, the output voltage and frequency would still be variable.

 

Here is one of the sources I read the cap thing from.

http://www.qsl.net/ns8o/Induction_Generator.html

Alright. On its own the IG has a speed dependent frequency which can be fixed on connecting it to a fixed voltage source.

 

I am not clear as to how the capacitors would set up a "rotating" flux in the stator.

as mentioned, caps are capable of creating leading vars, which setup the rotor magnetics ahead of the sweeping stator windings.

The previous discussions assume feeding into an 'infinite bus' where the frequency reference is external. This is an application of the generator, but in no means, preclude the device from functioning in a standalone application.

 

I'm not 100% sure if I'm understanding the questions properly, but it sounds like
part of what you are looking for is the understanding of how the induction machine can start without first:

1. Being operated as a motor with the stator connected to the grid

2. Having some sort of energy present in either the rotor or stator.

In the past, this has puzzled me as well.

While I've not been able to find an extremely detailed step by step explanation, this helped:

http://www.generatorguide.net/howgeneratorworks.html

Look toward the bottom of the page.

After you've digested the info from the link, maybe you would care to read further.

While it is popular for responses in these forums to include statements such as "capacitors provide vars to the generator". This has always confused me.
So, perhaps it would help you (as it has done me) to think about the following when you consider connecting capacitors to the output terminals of your induction machine. (whether single phase or polyphase)



1. Capacitors are loads to an AC source, the same as a resistor or inductor, but of course the current they draw is leading the voltage (reference) sine wave.

2. When capacitors are connected to the output terminals of the induction machine,
do they not negate some of the inductive reactance of the stator windings?
If so, what effect does this have on the machine in particular at startup?

I'll stop here, hopefully before I drive everyone crazy..........

 

Post #9 was caught up in moderation because of the link. I have approved it.