Author Topic: Leroy Somer 6KV generator repair  (Read 2927 times)

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Opera House

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Leroy Somer 6KV generator repair
« on: December 17, 2009, 10:49:02 AM »
I'm working on a Leroy Somer MA1326 6KV single phase brushless alternator.  I searched the web for technical information and repair tips and found nothing.  So I thought others might be interested in hoe these work.  This was a construction light tower so it has seen a lot of the environment. I was told this this would work for about 10 minutes and the lights would then fade.  I can't get it to produce more than 10 volts.


After removing the covers and blowing out the mud bee nests and digging into it I find there are two sections alternator and exciter.  These comprise three circuits.  The first is the regulator.





The diodes and mov are on this board and the resistor is this





This takes the actual output power, rectifies it to reduce the magnetic field of the exciter section. Shorting the 45 ohm resistor reduced the AC output from 10 to 5V.  That verified that section was working.  Resistance tests showed the wiper on the resistor was loose so the generator likely experienced some over voltage at times.

The next circuit takes voltage from the alternator stator, rectifies it  and powers the exciter.  I know I drew the opposite,it was late and this is the only way it makes sense.





There is a diode on each side.  These tested good.  One heatsink will have to be removed to get to the bridge on the shaft.


Third circuit is rotor with rotating rectifier.  




This has a 25A potted metal bridge rectifier mounted to the shaft.  Since this is the only thing left not tested it is likely the problem.  I will remove one wire from the AC terminal of the bridge and apply battery current to it testing both directions in the next couple of days.  Visually looks fine except for the varnish I have to remove.

« Last Edit: December 17, 2009, 10:49:02 AM by (unknown) »

Opera House

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Re: Leroy Somer 6KV generator repair
« Reply #1 on: December 17, 2009, 11:12:35 AM »
Corrected stator wiring and regulator diode board.
« Last Edit: December 17, 2009, 11:12:35 AM by (unknown) »

Flux

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Re: Leroy Somer 6KV generator repair
« Reply #2 on: December 18, 2009, 04:36:02 AM »
This is about as simple and crude as you can get. This is the first brushless machine I have seen with a single phase rotating rectifier. It costs so little more to have made it 3 phase but I have noticed before that Leroy Somer aren't inclined to over engineer things.


I suspect you are right in that the rotating bridge has failed. One thing you might be able to do to prove this is to separately excite the exciter field coil from a battery  and see if the main volts builds up.Don't push it for too long in this mode unless the volts come right up or you may cook the exciter rotor feeding a shorted bridge. If the volts come up fairly well then you will soon feel a hot spot on the stator if it has shorted turns.


It will be a bridge fault or an exciter rotor or main stator fault in all probability. Much less likely is an exciter field short or a main rotor field short but any of these would prevent build up.


Flux

« Last Edit: December 18, 2009, 04:36:02 AM by (unknown) »

Flux

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Re: Leroy Somer 6KV generator repair
« Reply #3 on: December 18, 2009, 04:41:40 AM »
I assume you have tried it without the MOV just in case that has shorted out, it should run without if for tests but don't try motor starting loads.


Flux

« Last Edit: December 18, 2009, 04:41:40 AM by (unknown) »

Opera House

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Re: Leroy Somer 6KV generator repair
« Reply #4 on: December 18, 2009, 10:39:49 AM »
I am a little troubled by the fact that shorting the resistor in the feedback loop reduced the voltage.  That indicates at least some path in the rotor circuit is working.  Could still be one open or shorted diode in the rotating bridge.  It will be Monday before I can work on it again.  I didn't have anything to power the field at the time. MOV definitely isn't shorted.  Can't figure out why I can't get this picture to post, size is smaller than the other ones.  

« Last Edit: December 18, 2009, 10:39:49 AM by (unknown) »

Flux

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Re: Leroy Somer 6KV generator repair
« Reply #5 on: December 18, 2009, 11:21:01 AM »
"I am a little troubled by the fact that shorting the resistor in the feedback loop reduced the voltage."


I think that is normal, you are just measuring residual and as you load it more the voltage drops. This happens with shunt dc generators but normally you reach a critical resistance where the residual is reinforced by the small field current and it builds up.


If your machine has a fault it will take more current into the exciter field to cause build up. If the fault is minor you may still get it to build up with increase of speed but any serious fault will need a field current greater than you can produce from the residual.


Shorted turns in dc fields ( exciter stator or main rotor don't have a great effect but shorted turns in ac windings such as exciter rotor or main stator will need a very large flux to get volts to rise and you will have serious heating if it does build up.


An open bridge diode may delay build up but it may happen at increased speed. A shorted bridge diode has similar effects to a shorted exciter winding and it will never build up and will cook if self excited.


I suspect you will find a faulty main rectifier but if that checks ok then separately excite it and you will soon find the faulty winding as it will get hot very quickly.


Flux

« Last Edit: December 18, 2009, 11:21:01 AM by (unknown) »

Opera House

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Re: Leroy Somer 6KV generator repair
« Reply #6 on: December 18, 2009, 10:58:12 PM »
I'm a strong believer in evidence.  They never touched anything electrical, but previous owner did.  The junction box cover was loose and missing one screw.  This had some connectors for the feedback circuit that I checked, but I couldn't see in with a dental mirror to look at what other wiring there was.


I had checked the exciter stator diodes and they were ok.  Wires were removed from one side of the diode because the two exciter fields did short out the diode.  Found the exciter fields in series with a diode clamping out each field in the opposite direction.  Thought this was a novel approach.   I did not check alternator coil continuity or if the two exciter windings actually connected to each other.   This is where I bet the problem is.  It just seems natural that a thermal or some other protective fuse would be placed here.  Manufacturer must have something to prevent the alternator from destroying itself.  I bet I find an open when continuity is checked.

« Last Edit: December 18, 2009, 10:58:12 PM by (unknown) »

Opera House

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Re: Leroy Somer 6KV generator repair
« Reply #7 on: December 23, 2009, 05:27:18 AM »
I traced out the wiring and found much of what I expected.  Rotating rectifier from the exciter to the alternator and AC power output is rectified to subtract magnetic field of the exciter to regulate output voltage. The stator wiring was not at all what I originally thought.  What bothers me is the alternator output depends totally on residual magnetism.  The exciter's stator field is built up only when current is drawn from the alternator.  I would have expected some electrical path from the alternator section to the exciter.  OR...at least a capacitor on the output to build up some field.


Multiple attempts cleaning connections never got the output over 10V. Shorting out the 45 ohm feedback resistor would drop the voltage to 5V.  This at least indicated the rotor electrical path was somewhat working.  I put 8VAC @3A on the rotor coil to check the  diode bridge.  Wouldn't call that a real flash, but after putting everything back the alternator worked. Attached is the schematic for the exciter stator field.  I can't get over that it relies on

residual magnetism with no way to build up the field until current is drawn.  

« Last Edit: December 23, 2009, 05:27:18 AM by (unknown) »

Opera House

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Re: Leroy Somer 6KV generator repair
« Reply #8 on: December 24, 2009, 09:58:07 PM »
"I am a little troubled by the fact that shorting the resistor in the feedback loop reduced the voltage."


I just reread that and i can see the intended meaning is not clear.   I expected the voltage to drop as a result of increased feedback that lowered magnetic field to the exciter.  This also indicated that the rotating diode was passing some current.  It was looking like everything would test good and there would still be no output.  Indeed, that is what happened.  Flashing the rotor with some current seems to have corrected the problem.

« Last Edit: December 24, 2009, 09:58:07 PM by (unknown) »