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Posted by harry on July 29, 2009, 5:29 pm
 
On Jul 29, 1:44 am, cl...@snyder.on.ca wrote:

No capacitors needed.  There are a couple of errors in the Wikipedia
thing too.
Why would you need capacitors?
Three phase, single phase makes no difference.
This a thing I did at college years ago on a motor test set. It was
part of the course. It's not a big deal.

Posted by Ulysses on July 30, 2009, 1:13 am
 


I don't know why you need capacitors but every induction generator I've
worked on had one.  Why *don't* you need one?  Heck, I don't even know what
the capacitor does in this case.  Are you saying all you need to do is hook
up a load and flash it by connecting a battery across the output leads
(neutral and hot)?  Do all induction motors need to be flashed if they don't
have a capacitor, or does it even matter?  Will it automatically establish
itself at 60 Hz (assuming it's a 60 Hz motor) if ran at a little over the
rated speed?  Do they need to achieve a specific speed (rpm) in order to
start producing electricity?  Is that why Bart's wind generator didn't work?
Is Bart still with us?

Now I have to go take something apart so I can hook up an old washing
machine motor...



Posted by harry on July 30, 2009, 5:54 pm
 
There is no DC anywhere in an induction motor/generator. Go back &
read the post carefully.
Only single phase induction motors have a capacitor in series with the
start winding. This shifts the phase angle slighly so that the motor
will self-start.
Three phase induction motor stator produces a revolving flux that
makes the motor self starting.
Induction motors do not run at synchronous speed, they run slightly
slower.
Again read the post.
Get read up on the theory of induction motors.
Start with this:- http://en.wikipedia.org/wiki/Induction_motor

Posted by Ulysses on July 31, 2009, 12:35 am
 

connected

induction

establish

What I thought you said was that they needed to be started at a slower speed
and then the speed was increased once it started producing power.  I think
I'm not understanding what you mean by "synchronous speed."  Are you
refering only to grid-connections?


Well, I read it three times so far and it didn't sink in so maybe a few more
times....



Posted by harry on August 3, 2009, 6:49 pm
 
If you are in the USA, and it was a single (pair of) pole machine,
synchronous speed would be 3600 rpm. If it was a two (pairs of) pole
machine, synchronous speed would be 1800 rpm, three poles 1200 rpm.
and so forth.
The frequency of your AC mains electricity in the USA is 60cycles/
second.
Here in the UK it's 50 c/s (or Herz as we say)

A synchronous motor runs at synchronous speed. An induction motor runs
at slightly less.
The stator of a (say) three phase induction motor makes a revolving
flux revolving at synchronous speed divided by the number of poles
(the way it's wound that is).
The rotor revolves in the same direction.  As there is no physical
connection, the current in the rotor is induced by the revolving flux
in exactly the same way as a transformer.  However if it were
revolving at exactly the same speed as the magnetic flux no current
would be induce because the rotor conductors would not cut any flux.
So it has to go a bit slower.

All transformer maths applies to the induction motor.  Except that the
frequency in the "secondary" side (ie the rotor) is very low, only 3
or 4 c/s

A synchronous motor has a magnetic field in the rotor which is not
induced, it iis an electro magnet (though lately some have permanent
magnets)

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