MTFCA Forum

I was adjusting a generator yesterday and was puzzled to find I couldn't bring the charge rate below 8 amp.

This is with a replacement armature in a functional generator.

Segments are polished and vee-d and the armature was successfully growled a few years ago.

I set the null point as best I could following the MTFCA handbook and Ron Patterson's advice to use a 12-volt battery (as opposed to a 12-volt charger or 6 volt battery).

The generator will motor on 6 volts — needs a twist to start and draws about 5 amps.

Installed on the car with a good mechanical cutout.

With the 3rd brush fully CCW viewed from the front, the charge rate was 8 amps. Advancing the 3rd brush CW raised the charge rate as expected; it was up to 12 amps before I dialled it back to the minimum 8 amps.

I'm OK with this as a maximum charge rate... the car is often driven after dark and has some additional lighting. I am also running a FP regulator on the car (re-installed now and working fine) so this maximum rate will only be seen for brief top-ups and occasional high-beam sessions.

But I remain puzzled as to why I couldn't seem to bring the rate down to zero or close to it.

Any ideas?

Assuming the ammeter is correct, I'd guess that, for some reason, the third brush's mechanical adjustment range is limited or the field winding or armature is non-stock. I don't know if the third brush adjustment can bring the rate to zero, but it should allow reducing it to a maximum or 4 or 5 amps. Any output tests related to the third brush setting should be made with the regulator out of the circuit and a load, such as a 6 volt lead acid battery, applied.

Thank you Pat.

I’m assuming the ammeter is reasonably accurate, as the discharge reading with headlights on/engine off is about as expected.

The brush carrier plate was replaced a couple years ago. Given that I did the work, mistakes or misassembly are a possibility

The minimum 8-amp charge rate was with no regulator. After I reinstalled the FP regulator, the charge dropped to 6-amps and soon after to maintenance levels.

I’m OK with things as they are, just curious about why the limited adjustment.

Leave the 3rd brush setting & reset the whole brush ring to a lower charge rate whall running.

What John said, is kinda what I was thinking. I am betting your null is off some. At approx what engine speed are you getting your reading?

If you must give it a twist to start it motoring, the null is absolutely off.

Thanks to all who replied. I did have trouble setting the null point, it seemed rather vague.

I will put the cutout back on and try rotating the brush carrier to a lower charge rate.

As long as the charge rate is correct, say 4-5 amps at full CCW, is there any downside to the true null being off a bit?

Before you do anything, take an analog ammeter test instrument and insert in series with the wire between the generator and the terminal block. Then start the engine up and run at the normal engine rpm and see what your reading is. This will also prove whether the ammeter in the car is accurate. Another test would be look at your ammeter in the car when everything is turned off. It should be at zero. Then run the engine at normal driving rpm and turn on the headlights. If it still shows a charge, something is out of adjustment. If it shows a slight discharge, you are set about right. The after market ammeters are notorious for their inaccuracy.
If both the test ammeter and the car ammeter show approximately the same reading, then you need to adjust the generator.
Norm

— Ammeter checked out as accurate enough (original, not repro),

— Entire brush plate rotated to bring charge rate to zero,

— Third brush adjusted to max 5-amp charge, and

— Lessons learned.

Thanks to all who contributed — much appreciated!