Need a bit of 'excitement

[dogshocker]

I have an old Lister, two cylinder, diesel generator. I don't know the model (pics 1,2,3) but, until recently, it ran perfectly. The engine still does but the generator part does not offer power any more. I placed a meter across the outlet wires and I had a reading of no more than 3 volts. I am told that it needs "exciting". Since then, I have had a few different people have a look at it but nobody seems to actually know what they are doing... certainly, no more than I do... and I'm a gardener not an electrical engineer.

   I have attached some photos in the hope that somebody can tell me the correct model and, if possible explain how to re-excite the generator part. The unit has a 'start-o-matic' system but I do not use it. I use the hand crank. Most folk who've come to look have always looked inside the start-o-matic box (pic 5) for an answer. I am no expert but I think the problem lies inside the top box (pic4), that is mounted on top of the generator unit. The parts inside look ancient and I'm not even sure where to get replacements or even if they are available any more.

   Any help would be much appreciated, thanks.

[AdeV]

I have a vague handle on the theory, but have never had excite a generator in practice...

Basically, the "magnets" are actually electromagnets; and they need a certain amount of power to actually do anything. This is normally provided by a DC circuit. Now... I think (but I may be wrong - someone more knowledgeable please step in) that when the generator first starts, it needs "a bit" of residual magnetism (from last time it was run) in order to generate enough power to start to generate - some of that power is then bled off into the exciter circuit to build the output up. Hence self-exciting. Except if it's lost that residual magnetism, it can't start up.

The solution is to "flash" the field coils with some DC to rebuild the residual magnetism, at which point the generator should work again. Although you do probably want to figure out why it lost its residual magnetism in the first place (I'm guessing it wasn't because it sat for decades between uses). Can't help you at all with that...

tl;dr: Find out which are the field coils, and give them a burst of DC. Google may help you with the amounts, but I'm guessing 12v or 24v depending on the starter battery spec voltage.  Things may be complicated by the fact i'm guessing it's a combined starter/generator; if it has a separate automotive style starter, then ignore that, it'll just be a generator: Much of the gubbins in the S-o-M box will be for load detection and the motor start sequence (i.e. operate fuel/decompression solenoid, motor the generator to spin the engine up, wait for voltage to rise, switch to generator mode, watch for no load, de-operate fuel solenoid. All done with a few simple components and bimetallic strips - really rather clever.

[dogshocker]

I have a vague handle on the theory, but have never had excite a generator in practice...

Basically, the "magnets" are actually electromagnets; and they need a certain amount of power to actually do anything. This is normally provided by a DC circuit. Now... I think (but I may be wrong - someone more knowledgeable please step in) that when the generator first starts, it needs "a bit" of residual magnetism (from last time it was run) in order to generate enough power to start to generate - some of that power is then bled off into the exciter circuit to build the output up. Hence self-exciting. Except if it's lost that residual magnetism, it can't start up.

The solution is to "flash" the field coils with some DC to rebuild the residual magnetism, at which point the generator should work again. Although you do probably want to figure out why it lost its residual magnetism in the first place (I'm guessing it wasn't because it sat for decades between uses). Can't help you at all with that...

tl;dr: Find out which are the field coils, and give them a burst of DC. Google may help you with the amounts, but I'm guessing 12v or 24v depending on the starter battery spec voltage.  Things may be complicated by the fact i'm guessing it's a combined starter/generator; if it has a separate automotive style starter, then ignore that, it'll just be a generator: Much of the gubbins in the S-o-M box will be for load detection and the motor start sequence (i.e. operate fuel/decompression solenoid, motor the generator to spin the engine up, wait for voltage to rise, switch to generator mode, watch for no load, de-operate fuel solenoid. All done with a few simple components and bimetallic strips - really rather clever.

Okay Ade, I'll give this a go. Many thanks for offering some help. In truth, I don't know a capacitor from a fuse so I'm guessing with most of anything that I try. Cheers for your help.

[AdeV]

In truth, I don't know a capacitor from a fuse so I'm guessing with most of anything that I try.

Ah, that one's easy: A capacitor mostly has both legs coming out of the same end! Although not always.... but if it does, it's definitely a capacitor! Or a transistor.... dang.

[dax021]

And, your engine is a SR2

[dogshocker]

And, your engine is a SR2

Thank you so much for letting me know. I'm learning all the time.

[Powdermonkey]

Lord Almighty....This is fundamentally easy.  Just understand how an electric motor or generator works.  FIRST, there is a "rotor".  It's the part that rotates.  INSIDE the motor, there are some "brushes" that rub against a bunch of copper colored AND segmented rings.  In other words, there are rings that goes around the central shaft, and they're CUT....maybe....24 times or so (just a guess as to the number).

Now....forced against THESE rings are carbon "brushes", that look like black rectangular....stubs.  THESE brushes FEED a modest amount of DC voltage to this ring....AND therefore TO the rotating windings. 

OCCASIONALLY, these rotating winding lose their magnetism.  And when they do, you get no generator output. 

SIMPLY follow the wires that attach DIRECTLY to those brushes...up into your "dog house".  You'll see them terminate at a screw-on junction.  Okay....

Grab ye a 6V dry cell battery, or....virtually anything else for that matter.  Even a 12 V battery, if applied VERY QUICKLY, would work.  Using some alligator clips and wire, temporarily touch the battery terminals TO those screw-on junctions that go directly to the brushes. 

Some dispute whether you should do this WHILE the engine is turning the generator, or not.  Some feel it best to "flash the field" while the motor is NOT turning.  Me, I've done it BOTH ways.  Static, and under rotation.  Matters not to me.  Both work.

You'll induce "juice" into the rotating winding, and ergo...magnetism.  And in making magnetism, all the tiny elves will starting running through the stator wires, and create AC voltage.  The world will rejoice, and people will again live in harmony....

(Unless of course you've got a broken winding in there....and then....your day has gone to shit.  )

[dogshocker]

Thanks Powder monkey. This is a great explanation for a person that knows as little as me. A very useful walkthrough. I'm going to follow your instructions and, as you mention at the end, if I'm lucky, my day will not have gone to sh1t, lol. Cheers mate... have a great Christmas and New Year.

[dax021]

Thanks Powder monkey. This is a great explanation for a person that knows as little as me. A very useful walkthrough. I'm going to follow your instructions and, as you mention at the end, if I'm lucky, my day will not have gone to sh1t, lol. Cheers mate... have a great Christmas and New Year.

I second this and might have to use this trick when I dire up my genny, which has been standing for about 3 yrs now

[AdeV]

Lord Almighty....This is fundamentally easy.

Yeah, you say that.... but then when you find yourself peering into a 3"x2" (that's 75mm x 50mm for those who speak French) box with 14 black wires - all covered in something that is almost, but not quite exactly, like grease and are seemingly joined in mysterious and irresponsible ways, and wondering "Just what in hell does each wire do, and why are there FOURTEEN of them?!?!" Added complication, an original Lister generator head was often the starter motor too, so everything was doing dual duty. Even if my original Start-o-matic gen head hadn't been dropped on its head at some point (bending the shaft quite badly), it had burnt out windings too - starter windings, I think, unless the starter windings were also the excitation windings. I never got to the bottom of it, I put an ST head on & threw the old Lister unit away when I ran out of storage space.

Electronics, I can do. Mains, I can do. The theory of generators, I can do. In practice? Nah mate.

[Powdermonkey]

Lord Almighty....This is fundamentally easy.

Yeah, you say that.... but then when you find yourself peering into a 3"x2" (that's 75mm x 50mm for those who speak French) box with 14 black wires - all covered in something that is almost, but not quite exactly, like grease and are seemingly joined in mysterious and irresponsible ways, and wondering "Just what in hell does each wire do, and why are there FOURTEEN of them?!?!" Added complication, an original Lister generator head was often the starter motor too, so everything was doing dual duty. Even if my original Start-o-matic gen head hadn't been dropped on its head at some point (bending the shaft quite badly), it had burnt out windings too - starter windings, I think, unless the starter windings were also the excitation windings. I never got to the bottom of it, I put an ST head on & threw the old Lister unit away when I ran out of storage space.

Electronics, I can do. Mains, I can do. The theory of generators, I can do. In practice? Nah mate.

Okay....well, in THEORY, you could "jump" your own battery wires DIRECLTY to the brushes, rather than to the wires that go TO the brushes.  And...you COULD try this BEFORE starting the engine and rotating assembly.  It IS possible to "flash" the field in a static environment.  Then, once flashed, you could immediately start the engine and see if the residual magnetism provides enough flux to again self-generate DC voltage....which in "turn" (get it?) would generate AC voltage.