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I usually lurk in the background and suck in all the tips-n-tricks that's available here. So, its time I try to repay the group just a bit... if that's possible.
I bought two Geo 55 amp internally regulated ND alternators for my 20B Lancair ES project, wired per Bob Nuckols Z-14 shcematic. I chose these units because of the amperage rating (55amps) and their light weight (5.5#) and small physical size. The problem is the internal regulator has a possible failure mode that could allow the alternator to go into a run-away mode, smoking your avionics in the process. Also, the internally regulated units aren't adjustable. So, Bob N. strongly recommends using externally regulated alternators only, or modifying the internal regulator models so as to allow it to be shut down if necessary. (Supposedly, a voltage run-away can happen in a few seconds.) Problem is that the articles I read explaining how to convert the internally regulated alternators to external regulation didn't work. (Did I do something wrong?) I think I have worked out a simpler solution. The objective here is to isolate the field windings from the diode bridge so that the alternator can then be externally controlled.
This is how I went about converting my alternators. First I removed the back cover, exposing the diode bridge, regulator assy. and brush holder. Then removed the regulator and brush holder and threw the regulator assy in the spare parts bin because it won't be needed. Next I removed the brush holder and built a little brace from 1/8" phenolic that will support one side of the brush holder off the diode bridge. This piece will be about 7/8" x 1/4" with a #8 hole in each end. Do not make this piece out of metal or it will short out the diode bridge. Also make up a jumper wire about three inches long with a ring terminal on each end. One end will attach to the right terminal on the brush holder and the other will attach to the alternator case (gnd.).
The other terminal on the brush holder will need a spacer under it to keep everything aligned properly. The left brush will need to have the lead wire un-soldered from the brush holder. The wire comes through the back of the brush holder. Unsolder it there and drill out the hole just a bit to allow a #20 wire with shrink tube to fit snugly through the hole. Solder the #20 wire to the wire on the brush. Now we're ready to reassemble the whole thing. Pass the #20 wire through the hole drilled in the brush holder and reinsert the spring and brush. Screw the brush holder back into place, with a spacer washer under the left terminal. The ground wire goes on the opposite terminal. Feed the wire soldered to the brush through a hole in the alternator cover, with a grommet to prevent chaffing. This wire will go to the "F" terminal of the external voltage regulator. I used a generic Ford unit from a mid-seventies model (approx. $10/ea from Auto Zone). The S & A terminals of the regulator tie together and get fed from the alt. switch & 5 amp breaker. The "B" terminal on the alternator goes to the battery contactor. The other terminal on the regulator isn't used.
I modified two alternators and they both work great. It takes about 30 minutes to do the modifications. All I need to do now is add a crowbar over-voltage protector to each regulator and I'm set to go. Or you can use a B&C voltage regulator with built-in OVP and low-voltage warning ($228/ea).
Hope this helps someone.
Mark S.
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