Richard,
The following comments apply toi the Rotax 912/914 INTERNAL alternator and
standard regulator. We've been performing extensive measurements on the
Rotax alternator and regulator.
> Would someone like to explain this ?
>
> The Rotax Installation manual just shows an earth connection for the
> regulator body. No sizes, no critical comment.
>
> There are two alternator wires which go to the regulator. Both are
positive.
Not so. Alternator wires are ALTERNATIVE leads. Both go to the
rectifier/regulator. All those I've seen to date are yellow. On the Rotax
diagram they have the little "tilde" symbol.
> The circuit is completed via the alternator body and thus the engine
earth.
>
Not so : the alternator has NO electrical connection to the engine or
airframe ground/earth.
> The 12v output from the regulator goes to the positive buss and thence to
> the battery plus. From the positive buss all the avionics are connected
and
> they have earths that meet back at the engine body, as does the battery
> earth.
The regulators feeds the whole ship's circuit. Every single charge leaving
the regulator through the POSITIVE wire (B+ and R terminals)must go back to
the regulator via the NEGATIVE ground connection at the regulator. In other
words, the circuit from and to the regulator must be CLOSED for the
regulator to feed any electricity at all to the airplane -pardon me,
aeroplane ;-)
>
> I'm not saying the comment "This earthing wire is the negative RETURN PATH
> for the alternator current" is wrong
Nice of you, mon ami ;-)
Due to the poor service record of the regulator, we've been conducting a
thorough thermal investigation, with an actual Rotax alternator driven by an
electric motor in a test bed at the Grenoble University. I stripped a couple
of failed regulators and we analysed their working. By the way, we found the
installation of the big capacitor is MANDATORY.
but it doesn't accord with my
> understanding of how it works. Can you explain it please ?
The nice thing with direct current circuits is that you can easily follow
the current path on the diagrams. Just trace the current path from the B+
and R terminals, and figure out the way it takes to come back to the
regulator.
Hope this helps,
Regards,
Gilles Thesee
Grenoble, France
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