Diagnosing a generator problem - HELP!

 
Cutting a long story short. On my Trace 3024 inverter, there is a
tolerance setting for the voltage of an incoming AC source. I noticed
that my generator was under 240v when idoling and would take about 5
minutes of warming up before the inverter would start to load it. So,
I made the range really wide, like 180-260v (240v system)

I think I blew a capacitor in the generator because it's not
outputting anything...

I noticed that before when the tolerance was much narrrower (and
closer to 240) that the inverter would nicely load the generator
progressively to to the limit set by me (around 10amps AC) but that
when I widened the range the inverter seemed to 'dump' the load
directly on the generator.

either way the generator now seems to be dead. The engine runs,
there's no burning smell from the alternator. The last time the
generator stopped working in this manner I had blown a capacitor.

Does anyone know where to start wit hte generator? It's got a quality
italian alternator on it and wouldn't have been drawing anything like
a full load. I did try a spare capacitor I had but that didn't make
any difference. (I don't know however if the capacitor I had was ok)


Q. Anyone know how I can dianose the fault. (The repairers say they
can't fix until after xmas) on the generator?
Q. Does anyone know how I can test a capacitor so see whether or not
it's blown?
Q. Do you think setting the tolerance that wide on my generator would
have caused the fault?


Any help would be much appreciated.

Cheers
Steve

  wrote:


By "Italian alternator", are you referring to the ones that Harbor Freight and
Northern Tool sell?  The ones with the extruded aluminum frame, no slip-rings and
with a capacitor hooked to an aux winding?  The rest of this post assumes that
you
are.

This type of generator is known as a "harmonically excited" generator.  The
capacitor
resonates the associated winding at the third harmonic of 60hz.  This coil then
excites the rotor field via transformer action.  The field has a diode across the
coil that half-wave rectifies this current.  Clever design that is fairly new -
the
patent is less than 20 years old.

The value of the capacitance controls the voltage regulation point.  More
capacitance
== more voltage.  Ergo, the capacitance value matters.

Assuming the blue smoke hasn't leaked out of the windings, about the only things
that
can go wrong are the capacitor and the diode on the rotor.

The first thing I'd look at is the capacitor.  The replacement must be exact in
the
capacity (microfarads) and at least the same voltage rating.  Higher voltage is
OK.

How you test the diode depends on what equipment you have.  Do you have a source
of
60 hz magnetic field.  A tape degausser, for example?  If you do then hold it
next to
a pole piece on the rotor and look at the voltage across the diode.  If you have
a
scope, you'll see a half-wave rectified waveform.  If you have only a voltmeter
then
you will see some value of DC voltage.  If you see no voltage, try the AC scale.
 If
you see AC voltage then the diode is probably open.  No voltage either way and
the
diode is probably shorted.


A gross test is to first test it with an ohmmeter for short.  If the cap isn't
shorted, you should see a momentary upscale reading on an analog meter or a
momentary
indication of resistance on the megohm scale of a DVM before both meters return
to
infinity.

Then hook it in series with about a 40 watt lightbulb to 120vac.  The bulb should
light at least a little.  You could compute the reactance of the capacitor using
1/2(pi)F*C where pi is 3.1417, F is the frequency (60 hz) and C is the
capacitance in
farads.  If you know the resistance of the bulb (120 volts/40 watts = 3 ohms)
then
you can quadratically add the values sqrt(R^2+Z^) to compute the composite
impedance.
From the 120vac you can compute line voltage and with an AC ammeter, check the
current.  Use ohm's law to compute the resistance necessary to pass that much
current.  This resistance should equal the composite impedance you just
calculated.
If they differ by much then the capacitor value is something other than its
marking.

A capacitor checker, of course, makes the task trivially easy.  Many DVMs now
have
the ability to measure capacity directly.


No.  The poor voltage regulation was an indication of an unhappy generator.  If
the
replacement capacitor contained a built-in bleeder resistor (most motor run
capacitors do anymore) then that may be at the root of the initial problem,
depending
on the value of the bleeder.  The bleeder would lower the Q of the resonant
circuit
and that might not properly saturate the rotor.  An unsaturated rotor would
exhibit
poor voltage regulation.  Try to find a replacement cap that does not contain a
bleeder.  They've available, you just have to look and ask.

If you want more info on how this type of generator works, you might want to
look up
the patent.  I don't recall the number but the title contains the words "harmonic
voltage regulation".  Hit one of the patent databases and search around.

Oh, one last thing.  Make sure your speed is spot-on.  No more than 61 hz
unloaded.
If the speed is off-spec very much then the harmonic circuit operates
off-resonance
and the field isn't properly excited.  Set the speed with a tach or a frequency
meter
- either method is equally effective.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
I don't suffer from insanity, I enjoy every minute of it.

 Thanks John...

I'd say that it's a pretty decent alternator, not the worlds best, but
decent enough. The cost of the alternator is around $900 AUD.

What I can say is that it's definitely brushless. Outside of that, I'm
not particularly technical.


But just getting to the capacitor. People have been suggesting to
stick a screwdriver across the contacts before measuring with a DVM
(with an insulated screwdriver) does this seem ok??????


It's a pretty big capacitor!

 wrote:



The capacitor is unlikely to be charged since the resonant winding is a DC short
across the cap, but if it is, shorting it with a screwdriver will burn pits in
the
driver and, with internally fused caps, sometimes burn out the fuse, rendering
the
cap junk.  I much prefer to keep a lightbulb screwed into a pigtail socket handy
on
the bench for such tasks.  A 100 watt lightbulb will discharge any reasonable
sized
cap in just an instant.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
You have a magnetic personality... That must be why all your mental floppies are
blank.

 

 360 ohms, and then, only when the lamp is being lit by its rated voltage.

 Quite simply a 40 watt 120 volt lamp uses one third of an amp and
resistance in ohms is given by voltage divided by current. In this
example, dividing anything by one third is the same as multiplying by 3,
hence 360 ohms.

 At a much reduced voltage, a tungsten GLS lamp filament will have a
much lower resistance value than normal.


Regards, John.

 Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.