Reduced charging, why?

[roadrocket]

Hello

What would cause indicated charging to reduce? When I rebuilt my dynamo a couple of years ago, the charge would be positive with full lights on at a bit before 2000 rpm. Now I have to go past 3000 to see a positive charge. I have only gone as far as to remove the end cover to see if all looked well, and it did. Though I could blow a serious amount of black dust out with compressed air. Could the commutator be partly fouled by worn brush material? Any usual suspects?

Cheers, Otto

[a10 gf]

I'd include the regulator in the search and tests, and the battery.

[muskrat]

G'day Otto.
The first thing I'd check is that the drive sprocket (the big one) isn't slipping on the taper.
Do a search on dynamo testing, quite a few posts. Also https://www.a7a10.net/BSA/lucasmain.htm and http://matchlessclueless.com/electrical/
Cheers

[duTch]

 

What would cause indicated charging to reduce.......

 Unless I'm missing something (*edit*- it sounds kinda normal?) , it will naturally 'reduce' to '0' as the battery charges to full....Sorry if I'm stating the obvious, the way I see it is if the lights/brake light are on and the ammeter goes to '-ve' it's not charging (low revs), but when you rev it will go through '0' to '+' as the depleted charge is replaced (the more '+' is good and shows a hefty discharge had happened) and progressively 'reduce' again to '0' as the battery is fully charged....If the light is brighter when you rev the engine shows the Genny is working....

     Did that make sense? or is there something else happening ?

 NB - 1 hour later- just added an edit
 

[roadrocket]

I'll check the battery and see if the regulator cuts off as it should. Other than that I guess it has to be the dynamo. There is no doubt that the charge is a lot less than usual, so something must be amiss.

Thanks!

[PatM20]

If there was a load of dust on the commutator, give the grooves a little clean out too.

[roadrocket]

Hello all

To round off this inquiry I'll report success! I eventually took the dynamo of and apart. It turned out that one brush was of bad quality, and was worn exccessively, leaving a black glazing on the commutator. I cleaned the commutator with emery cloth and renewed both brushes, and full charge was restored. Hooray!

But one thing was odd: I am quite sure I didn't swap the brush leads round, but the dynamo was motoring the wrong way round at first. Can polarity change, or did I really manage to wire the brushes wrongly first?

Cheers, Otto

[berger]

NEVER! use emery on a commutator it is a conductor and can get stuck between the bars on the com, use  fine sand paper and then gently scrape out between the bars this is my two penneth worth , ps PUB TOMMOROW

[groily]

If you also disconnected the field coil wires that emerge from behind the brush plate, you might have reconnected them the opposite way round maybe? (They are usually hard to tell apart.) That reverses the direction of rotation, same as swapping the brushes.
Polarity should not have been affected unless you motored it with the battery connected with the earth and live reversed from how it is on the bike. The fact it works again indicates 'no problem' there though. So sounds like 'success'!

[duTch]

 

.....But one thing was odd: I am quite sure I didn't swap the brush leads round,......& groily; "If you also disconnected the field coil wires that emerge from behind the brush plate, you might have reconnected them the opposite way round maybe? "............

When I was trying to figure mine out, I decided that a 'rule of thumb' *may* be to think of a ' figure 8 or one of these dead space invader critters; ∝ - can't make it bigger', where one end of the '8' is the coil, and the other end is the wires that poke through the brush plate......all flowy like..