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What's Wrong with my Diodes?

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Posted by Ulysses on May 29, 2009, 3:06 pm
Hi guys.  I attached the coils and rotor (the entire PMA) from a Honda
eu2000 generator to a 4 HP engine--direct drive, just like it originally was
on it's original engine.  This is a 3-phase alternator in a Y configuration
so I connected two diodes in series to each output and connected them in
parallel with each of three outputs attached between each pair of diodes.  I
attached a 200 volt electrolytic capacitor, somewhere in the 600 uF range.
I attached a voltmeter set for the highest voltage.  I started the engine at
a slow speed and the voltage reading started out at about 190 VDC and
quickly climbed until it was over 600 and I shut it down.  Things started

Did I somehow make a voltage multiplier?  I'll test the output again (two
wires, AC, and then do the square root of 3 thing) but when I tested it in
the past it was reading just over 200 VAC at high RPMs.  I also noticed that
I could not start the engine with the diodes and capacitor connected.  There
was too much load.  The final output was not connected to anything.  Should
there be a load with only the capacitor and diodes connected?  I tested each
diode and they all checked out OK.

Posted by vaughn on May 29, 2009, 5:01 pm

   It sounds to me like you have connected everything corectly.  You did not
mention the PIV of the diodes you are using.  Given the high output of those
diodes, you should be using 1000-volt diodes.  Diiodes in series do not
necessarily divide the reverse voltage evenly between themselves, so simply
connecting lower voltage diodes in series is usually not a good way to go.

   Assuming that your diodes are rated at a high enough voltage and still
check good, I would connect the phases one at a time and spin up the
generator so see if you get equal voltage.  You may find a bad diode or a
bad winding.  If you happen to have a shorted winding, you are screwed; but
then you would have loading and overhesting even with no wires connected,
which does not fit your description.



Posted by Scott on May 30, 2009, 12:51 am
 On Fri, 29 May 2009 08:06:06 -0700, in alt.energy.homepower, "Ulysses"

Isn't the EU2000 an inverter type generator?  I've read that many of those
generate relatively high voltage internally, using the inverter to convert
it to the 120v AC power you want.  Supposed to be more efficient that way.
IIRC the manual for my RV's 3400W generator says that it feeds 400 volts to
the inverter, so I wouldn't be surprised if you saw an open circuit voltage
of 600 or more.

Posted by Tim Jackson on May 30, 2009, 9:26 am
 Scott wrote:

I don't know about he design of this generator, but a couple of things
spring to mind.  One is that the OP said he used a 200V capacitor and
got 600V output.  That would start things smoking pretty quickly once
the capacitor collapsed.  The other is that small alternators generally
operate at several hundred Hz: generating 50/60Hz directly is
inefficient at small scales (even a 4-pole machine would be turning at
only 750/900rpm).  Were the diodes used appropriate for this service?
If you use regular power diodes at this sort of frequency, they will get
pretty hot too, the long turn-off time means they are an effective short
circuit for a while during commutation.  It is important to use
high-speed diodes.

Tim Jackson

Posted by m II on June 1, 2009, 4:00 pm
 Tim Jackson wrote:

It's my firm belief that a four pole machine would be turning around
1800 rpm for a 60hz output.

These people share my delusions:



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