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"re-exciting" a dead generator

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Posted by no spam on March 13, 2007, 5:56 pm
 
I just read the generators can lose their magnetism (or some sort) if
allowed to sit for long times.  This prevents them working.  In the article
it was suggested that you could fix the problem by starting the generator
and running electricity into one of the outlets.

1) Is it true that this can happen?

2) Will the 'fix' suggested work?
    2a) any chance it will fry the genny if done wrong?

3) Is there a better/safer way to fix the problem?



Posted by Ulysses on March 13, 2007, 7:38 pm
 


article

So I've heard.



From what I have heard, yes.


From what I have heard, yes.


I'm no expert but I'm pretty sure most modern generators (at least the
small, homeowner type) have an additional "excitation" coil which, if I'm
not mistaken, is activated when you start the generator.  Maybe it somehow
got disconnected.  You can also get loss of output or at least low output
from a shorted or open diode.  There are generally two diodes on the rotor
which can be destroyed by such things as short circuits.  I imagine trying
to excite the generator incorrectly could also damage the diodes.  In order
to check the diodes I've always unsoldered them.  The stationary coil
usually needs to be removed in order to check the diodes.  This can, in some
cases, be a bit involved.  I would also check the circuit breakers for
continuity.


There was a discussion on how to reexcite a generator a while back but since
I did not completely understand precisely where to apply the external
voltage I am, and will remain, inexperienced in the procudure (until I found
out for sure).  I was unable to find all the needed information with a
Google search but maybe you'll have better luck.



Posted by Neon John on March 13, 2007, 7:50 pm
 You don't run the generator, normally.  Just apply 12 volts DC from a
battery or a charger or something to the power outlet for a second.
This re-magnetizes the stator and rotor and should make it come back
up normally.

On the odd chance that this doesn't do it, a more complex procedure
does involve running the engine.  With the engine running, momentarily
apply 12 volts to the output with a 300 or more watt lamp in series
with the 12 volts.  The lamp is to protect the 12 volt source from the
120 VAC when the generator comes up, as it will practically instantly.

having to do procedure two is extremely rare.  In fact, I can't think
of an instance in the last 20 years where that was necessary.  For
that matter, I can't recall having to "flash the field" on a consumer
type generator in that length of time.  If the generator doesn't come
up immediately when cranked then there is probably something wrong
with it.

As with anything involving electricity, yes you can fry something if
you really botch the procedure.  Little chance of that if you follow
my procedure.

John



John De Armond
See my website for my current email address
http://www.neon-john.com
Cleveland, Occupied TN
Don't let your schooling interfere with your education-Mark Twain

Posted by Eric Sears on March 13, 2007, 7:59 pm
 

I too have heard so. The only "alternator" I have ever "re-excited"
was an ordinary induction motor used in a microhydro system.

  It MIGHT - but there are a number of "ifs".

 A very good chance I would say - especially if the alternator is
still connected to an internal combustion engine of some sort.

You could try "zapping" it with a small gell battery (which is what I
did with the induction motor". But be careful!! If you do it with the
motor running, you may very well suddenly find yourself holding on to
two live wires at high voltage. As they say "keep one hand in your
pocket at all times!!"
If you don't know what you are doing - consult an expert (not me!!).

Eric Sears


Posted by scott on March 14, 2007, 12:15 am
 

  1) yes, it does happen, but not often and usually to very old generators.
I have seen some new generators that actually have a small permanent magnet
in the armature (rotor) to start them, and some welding generators add a
very small winding on the engine charging system or the ignition armature
to "kick start" the generator units.

2) That will work if you limit the current somehow, the way some service
manuals suggest is to make up a harness with two male plugs and two 40 watt
120 v light bulbs in sockets, so the current flows in series through the
two light bulbs and to the generator. The bulbs limit the current and the
sockets are to replace the bulbs when they burn out, they will take 240
when in series for a while, but momentary overvoltages can occur when the
regulator comes on . If you only need to do it once leave out the sockets
but use the bulbs as when your generator comes on it will not be in phase
with the grid socket and could burn out the generator and worse the house
circuit as well. With the setup described I can't see you damaging
anything, but a generator with an electronic regulator is more susceptible
to damage (Honda says to remove the regulator and put it in another room
before welding or applying current to the generator!).
  
3) A safer way is to use a strong magnet to induce a current in the rotor
by bringing it near while running, but I have not tried this method. Some
generators with regulators can be revived by removing the wires from the
brush leads and applying 12 volts through a 1156 light bulb (to limit
current) to the slip rings for a moment and then reconnecting the regulator
and starting . If it does not work, reverse the polarity and try again .
Whenever you get a generator working this way be sure to apply a heavy load
to fully remagnetise the rotor , don't rely on it until it had been loaded
at least half of its capacity and starts to generate again after sitting
overnight.

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