Generator output dropping off

[TKelly]

Hi guys,

I'm having a bit of a problem with the generator in my '26 Essex at higher rpm the output drops of dramatically. It is a 6v negative earth car generator is 3rd brush regulated.

 

The generator will charge up to about 2000 engine rpm and then drop of very rapidly from there. I had it said at about 4.5A charge at 2000rpm but by 2500rpm the output has dropped off to the point it is no longer charging but is not discharging.

 

I have checked brush tensions the all feel about the same and have checked the resistance through the brush connections everything seems to check out ok.

 

I have been told this could just be a design flaw of the particular generator (it is an American Bosch unsure of model) but I'm not sure. Also it's worth mentioning it has done this since I've wired it up it is not a new problem that has started occurring.

 

Thanks in advance, Tristan. 

[37_Roadmaster_C]

Tristan,

This sounds right for a third brush generator. The third brush is set for proper charge current at a average driving rpm. Above or below that rpm the output drops. I am not an expert on this, but there are others on the forum that have much more experience. Hopefully, one of them will jump in and explain better!

Edited March 28, 2021 by 37_Roadmaster_C (see edit history)

[Bloo]

I don't know exactly what is normal on a 26 Essex, but third brush generators in general have a curve of output current. there is only one particular RPM where they are at their best, they drop off if you go higher or lower. I am assuming you have a third brush generator and a simple cutout relay (no voltage regulator). If that's wrong, please correct me.

 

Typically you would start out with the third brush set to whatever current the original service information for the car says, and then check the battery with a hydrometer after driving it a week, and adjust a little up or down as necessary. How full the battery gets depends heavily on the owner's driving habits and how much the lights get used.

 

Many of these systems they cannot carry the lights indefinitely, the battery will go dead. If you turn the third brush up enough to carry the lights, it might boil the battery really bad when running a long time without them.

 

Unlike a system where the voltage is regulated, a third brush system will TRY to charge at the current the third brush is set to no matter whether the battery is full or not. It can't really do that because it wont be at it's ideal RPM all the time but it will try. If the battery is already too hot, it does not back off.

 

Back in the day the goal was to get the third brush set so that the battery is just getting full when the owner shuts the car off at the end of the day.

 

I doubt you should be going into high RPM discharge in normal driving if your lights are off, but I am not sure.

 

[TKelly]

Thanks for responses guys, I have found and rectified the issue.

 

After much reading and researching I decided to pull the armature out of the generator and put it in a lathe to check information the commutator was out of round. Sure enough it was a long way out of round so I machined it round put it all back together and away it went.

 

It does peak and drop off but nothing like it was it peaks at 5A charge at about 2000 rpm or just under and by 2500 rpm is starting charging at 4A.

 

Cheers, Tristan. 

[Rusty_OToole]

Did you remember to undercut the mica on the commutator?

[TKelly]

I didn't undercut the mica I didn't deem it necessary the grooves are still quite deep.

Cheers, Tristan. 

[Frank DuVal]

Right, as long as you can stick your fingernail in the grooves between commutator segments, they are deep enough for now.

[Flivverking]

I hope your keeping your high reving  29 Essex to a limit of 50 MpH max.?

 

 

[JFranklin]

On 6/5/2021 at 10:53 AM, Jack Bennett said:

Curious about how turning a armature could affect charging rate on a generator.

Truing the commutator stops it from bouncing the brushes at speed and allows better generator action.

Edited June 9, 2021 by JFranklin (see edit history)