Dynamo help

[electrajohnt]

Hello, some advice please.
Lucas 6v dynamo. Does not appear to be charging.
It will motor without prompting when F & D are bridged and the 6v battery connected. Positive to dynamo body and negative to the bridge. It motors fairly slowly but starts without hesitation.
The bike is positive earth.
I have taken it apart, cleaned the commutator where the brushes run. That is very nice and clean, segments have clear gap. Virtually 0ohms between each segment.
Cleaned the brushes, they are in good condition.
Put it on a drill with tester set at DC connected to bridge and dynamo body it shows about 18v.
Put a 21w 6v bulb in parallel (I think that’s what it is called if it’s connected to the same connections as the meter.
The bulb does not light and the meter shows about 2v.
If I connect the bulb on its own without the meter connected, that does not light.
What is wrong with the dynamo, what can I test.
Photo maybe attached to show setup.
JohnT

05E94FCF-C01F-49FF-8121-48AE2A76DE3B.jpeg

[SPRIDDLER]

Zero or reduced output under the load of a bulb is usually due to a break in one or more of the armature windings.
Have a look at this excellent website here for test procedures:

https://www.matchlessclueless.com/elect ... as-dynamo/

Daft question but is the bulb a good 'un?
It's not clear from your pic but could your soldered (?) connection to the bulb bottom pip be shorting to the bulb casing?

[electrajohnt]

Thank you Spriddler.
Done all the tests in the Matchless Clueless article. What a brilliant piece of writing that is. All my readings came out ok but still no good when load is put on the dynamo.
One last whirl is to flash it as suggested but I doubt that will cure it.
Has anyone got any other tests I can do to try and determine what needs replacing, Maybe if I try and get second hand field or commutator windings is an option, I don’t know, not been down this road before.
JohnT

[clive]

Fitting field windings is specialist job needing a special tool to force it into place. If you have not tried flashing over do so as this really is necessary. I have also had a dynamo which will motor but steadfastly refused to produce charge. Cannot help with certainty about what needs replacing but I would suspect the windings every time rather than the field coil.

[Groily]

Before consigning any of it to the dustbin of dyns that don't, and after confirming that polarity and direction of rotation haven't been upset, have you tried hooking the bulb up only AFTER the meter starts to show you a decent output? Ie, after cut-in has been achieved?
Sometimes (often), the current demands of a cold bulb, with no battery in the circuit to warm it up, makes the thing hard to light, even a 21Watter. Alternatively, suggest trying with a 12v bulb hooked up instead, which will light more easily.
If you hook the bulb on when the meter's showing umpteen volts, and the output take a massive nosedive and the bulb still won't light, then I'd suspect the armature . . . but I'd also give the brushes a gentle squeeze to enforce contact with the commutator while the thing's running to see if that helps, it sometimes does. A squirt of contact cleaner even . . . can work wonders . . .
The tests we do by bridging D and F are fairly brutal, in the sense that the real life of a dynamo with a battery and regulator in circuit is gentler. Because in that real life, the dynamo is NOT being asked to provide the surge current to light cold bulbs - the battery obliges. This brutality makes the tests good ones though, because if the dyn can get illuminations out of a cold 6v bulb, it's doing OK! It should be noted also, though, that we shouldn't run the dynamo for long at high revs with F and D bridged . . . don't want to 'overfeed' it!

Sounds as if the field coil is OK, but as Clive says, the armature is the more likely suspect if there is a problem.

[electrajohnt]

Thank you all for your advice and comments. Is there a test I can do to test if it’s field coil or armature.
When I tried to flash it I was expecting a “sizzle” for the second or so contact was made. All I got was a pretty limp spark, is that indicative of anything other than my misplaced perception of what I would see.
JohnT

[clive]

a limp spark is probably a fair description. Still no success now its been flashed over then?

[electrajohnt]

Hello Clive.
No, as I say, looks like I am after second hand dynamo or parts of one. Anyone help please.
JohnT.

[Groily]

You can test the field coil by measuring resistance between the ends of the wires - around 3.5 to 4 ohms on a short Lucas E3N or H.
To test an armature detached from the dynamo you need a 'growler' or similar.
The field coil seems to work. You wouldn't be seeing it motor, or your 18v at revs without loads, if it didn't have continuity. The armature is far more likely to be the culprit.
Usually if an armature is dead the thing will have 'flat spots' when you motor it, if it will motor at all that is. For part of each rev it will be weaker than the rest. You'll feel it stuttering if you lightly press on the pinion or shaft with your finger while it's turning.

I am not so far convinced that the thing isn't capable of working.
Typically, with a dying armature you'll see a couple of volts odd with no load on, if that, not a straight climb with revs to double digits as you report. Of course, anything's possible because these things can be very trying as all are agreed. But have you tried applying loads after it's cut in, rather than hooking one up with the meter from rest? (Wasn't clear to me from your original post - but it is something you need to try for the reasons given.) And applied a little extra pressure to the brushes while it's running?

The idea of fitting 2nd hand bits is only any good if you are 100% certain of their being in a proven good state - and if they are dirt cheap. By the time you've had to change or replace the bearing and so on, there's a bit of work, and on the basis that doing things once beats constant revisits hands down, if it really does need an armature, I'd get a new (UK-made) one from any of the usual suspects. I'm as much of a 'make do and mend' cheapskate as anyone, but false economies can end up costing more, plus the added exasperation!

[clive]

I am with Groily on fitting a new armature rather than seeking a secondhand one, however if you are after them please use the forum area put aside for wanted adverts as they are not allowed in the open forum areas. Better still place an advert and it will go in the mag which reaches many more people.
When I first got my 49 G80 it was difficult to get new armatures and I did not have access to the sort of test data that is now available. The only method I had to check if the Dynamo was working was to install it and see. With the rear mounted magneto this entailed completely removing the entire primary side so the Dynamo could be removed. The only alternative was to remove the timing side inner and outer cases which often results in resetting the cams as well as retiming. What a pain when you have done it two or three times! As a result I now run a G3L with an AJS engine.
As Groily seems to be suggesting your test process may not reflect the Dynamo in real use and if you can wiggle it into the bike without major disassembly I would be inclined to install it and see it it works when connected to a regulator. Producing 18volts suggests to me this is worth a try. If that is a fail I would go for a replacement new armature.