Electrical Engineering - Yelp!!! (Induction Generators...)

Author

Yelp!!! (Induction Generators...)

simplyagam

2007-02-10, 9:25 am

Hi!
New to Usenet Groups so plz don't mind the inhibitions... the thing's
this... I was trying to do some study in Induction Generators so I go
to all these web pages, Wind Power... captive generation... etc etc...
ok cool... but what about the principles... so I download a few IEEE
papers, read a thesis or two... yaa they have the mathematical model
(dq0 or per-phase etc etc)... the simulation even works... wonderful,

But all this time, the thing which's being irritating me is the basic
principle... what I could find till now is dis... just like in
Induction Motors, the rotor follows the synchronously rotating mmf, in
IGs its the opposite... the rotor is ahead and the mmf of the
generated electric field is following it. This is fine, and can be
visualized when we connect an IG to grid and rotate the prime mover
wid speed greater than the grid frequency... BUT... the problem I have
is in self-excited IGs,

They say that u require a reactive power source (usually a charged
capacitor or smthing)... dis is not ENUF!!! How does one explain the
building up of voltage... I mean how can I visualize it? There is no
external excitation to rotor... so how's the induction phenomenon
causing the rotating rotor to induce voltage back into the stator
windings.... it just doesnt make any sense....

I mean in synchronous generators, we can justify that yes.. dere is a
rotating magnetic field (due to excited rotor) viz. cutting the stator
coils and emf's being induced... In DC generators, its pretty much the
same thing as excitation source (or atleast remenant magnetic field)
is there helping the field windings... but what the heck is going on
in IGs....
Smone having anyy idea.... plz plz plz help me..... my project's
dependent on it.

daestrom

2007-02-11, 8:25 pm

"simplyagam" <simplyagam@gmail.com> wrote in message
news:1171116853.615812.152410@v45g2000cwv.googlegroups.com...
> Hi!
> New to Usenet Groups so plz don't mind the inhibitions... the thing's
> this... I was trying to do some study in Induction Generators so I go
> to all these web pages, Wind Power... captive generation... etc etc...
> ok cool... but what about the principles... so I download a few IEEE
> papers, read a thesis or two... yaa they have the mathematical model
> (dq0 or per-phase etc etc)... the simulation even works... wonderful,
>
> But all this time, the thing which's being irritating me is the basic
> principle... what I could find till now is dis... just like in
> Induction Motors, the rotor follows the synchronously rotating mmf, in
> IGs its the opposite... the rotor is ahead and the mmf of the
> generated electric field is following it. This is fine, and can be
> visualized when we connect an IG to grid and rotate the prime mover
> wid speed greater than the grid frequency... BUT... the problem I have
> is in self-excited IGs,
>
> They say that u require a reactive power source (usually a charged
> capacitor or smthing)... dis is not ENUF!!! How does one explain the
> building up of voltage... I mean how can I visualize it? There is no
> external excitation to rotor... so how's the induction phenomenon
> causing the rotating rotor to induce voltage back into the stator
> windings.... it just doesnt make any sense....
>
> I mean in synchronous generators, we can justify that yes.. dere is a
> rotating magnetic field (due to excited rotor) viz. cutting the stator
> coils and emf's being induced... In DC generators, its pretty much the
> same thing as excitation source (or atleast remenant magnetic field)
> is there helping the field windings... but what the heck is going on
> in IGs....
> Smone having anyy idea.... plz plz plz help me..... my project's
> dependent on it.

Induction generators with no 'grid connection' are pretty lousy. The thing
is, the amount of slip between the rotor speed and that of the rotating MMF
is highly variable. So with no other frequency base, the output frequency
of the IG varies greatly with load.

As far as voltage buildup, first you start with a cap. It discharges
through a phase of the stator. This temporary magnetic field induces a
current in the rotor. The rotor field induces a voltage in the stator.
*IF* there is a load connected, then some more current flows in the stator
winding and the process builds. If there is no load, then the capacitor
alone *may* be charged slightly and the process may still build. But this
will depend a lot on the size of the cap versus the reactance of the
machine.

Suffice to say, the build up is very problematic. A load that carries
enough current in the stator, but not too much. The frequency of the stator
will be a function of the total impedance (internal as well as load) and the
speed of the rotor.

I can't really see how an isolated IG would be of any use to anyone.
Voltage output highly uncontrollable, frequency output also uncontrollable.
Thus, power output very uncontrollable. Depending on the prime mover being
used, a lack of control on loading may result in overspeed.

daestrom

>

John Rye

2007-02-12, 9:25 am

Hello

You have already had several useful suggestions/explanations.

The best paper of which I am aware describing what goes on is :-

The process of self excitation in induction generators by J M Elder et al IEE
Proc Vol 130 Part B No 2 March 1983 pages 103-108.

This mentions the point that has been missed so far "residual magnetism".
This can provide the initial kick to get things started.

My main interest in this area was in understanding what happened when an
induction generator running in parallel with the public supply system became
separated from it at a point remote from the generator, with load connected
between the generator and the break.

John

--
John Rye
Hadleigh IPSWICH England
<http://web.ukonline.co.uk/jrye/index.html>
---< On Line using an Acorn StrongArm RiscPC >---

Don Kelly

2007-02-13, 3:25 am

"John Rye" <jrye@ukonline.co.uk> wrote in message
news:4eb3f4f1d0jrye@ukonline.co.uk...
> Hello
>
> You have already had several useful suggestions/explanations.
>
> The best paper of which I am aware describing what goes on is :-
>
> The process of self excitation in induction generators by J M Elder et al
> IEE
> Proc Vol 130 Part B No 2 March 1983 pages 103-108.
>
> This mentions the point that has been missed so far "residual magnetism".
> This can provide the initial kick to get things started.
>
> My main interest in this area was in understanding what happened when an
> induction generator running in parallel with the public supply system
> became
> separated from it at a point remote from the generator, with load
> connected
> between the generator and the break.
>
> John
>
> --
> John Rye
> Hadleigh IPSWICH England
> <http://web.ukonline.co.uk/jrye/index.html>
> ---< On Line using an Acorn StrongArm RiscPC >---
----

In that case, you lose the excitation source and the voltage collapses so
generation ceases and you are in the dark.
An induction generator, unlike a synchronous generator, cannot supply its
own excitation and must be supplied with reactive either from the system
(simplest) or from capacitors (messier). --

Don Kelly dhky@shawcross.ca
remove the X to answer
----------------------------

John Rye

2007-02-13, 1:25 pm

Hello Don

In article <XHaAh.961917$R63.102060@pd7urf1no>,
Don Kelly <dhky@shaw.ca> wrote:
> "John Rye" <jrye@ukonline.co.uk> wrote in message
> news:4eb3f4f1d0jrye@ukonline.co.uk...
> ----

> In that case, you lose the excitation source and the voltage collapses so
> generation ceases and you are in the dark.
> An induction generator, unlike a synchronous generator, cannot supply its
> own excitation and must be supplied with reactive either from the system
> (simplest) or from capacitors (messier). --

Sorry I did not make the situation that I was describing clear enough. One
normally has capacitors adjacent to the generator for power factor
correction. These and the system capacitance can mean that if the connection
to the main system opens several miles away, the generator can continue to
generate at an undefined voltage and frequency. There can be customers
connected to the system between the point it is open and the generator, and
they may not appreciate the voltage and frequency that they receive.

John

--
John Rye
Hadleigh IPSWICH England
<http://web.ukonline.co.uk/jrye/index.html>
---< On Line using an Acorn StrongArm RiscPC >---

John Gilmer

2007-02-13, 1:25 pm

> Sorry I did not make the situation that I was describing clear enough. One
> normally has capacitors adjacent to the generator for power factor
> correction. These and the system capacitance can mean that if the
connection
> to the main system opens several miles away, the generator can continue to
> generate at an undefined voltage and frequency. There can be customers
> connected to the system between the point it is open and the generator,
and
> they may not appreciate the voltage and frequency that they receive.

"Seems to me" that a UPS could be used with an IG. The UPS would provide
some "reactive power" but the IG would provide the "real power." A portion
of the "real" power would be use to provide replacement juice for the
batteries in the UPS.

Some years ago I was told that larger aircraft use IGs with inverters for
frequency stabilization. The IGs are rotated by a "roughly" constant
output speed hydraulic "transmission."

Also "seems to me" that IGs would be a natural for "supplemental" power
while on the grid. Basically, they would slow down your meter when the wind
is blowing. If you could count on the IG to stop generating when the grid
goes down you would not need to protect against "back feeding."

When it comes to commercial aircraft to a good approximation, cost isn't
much of a problem. But (pulling numbers out of you know where) paying $20k
for a "box" that flies is down in the noise. But more than $2k for a home
use "box" may be way too much.

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