mprather(at)spro.net
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 Posted: Tue Jan 26, 2010 9:43 am Post subject: OV Protection for Three-Phase PM Alternator |
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My recollection is that these Z diagrams have gone through changes over
time as design goals have evolved. I think I remember seeing some of the
PM diagrams without capacitors even. Some systems aren't very sensitive
to alternator noise.. Components never installed never fail.
I'd guess that the vast majority of PM alternators run for the life of the
airplane so long as the components are held to reasonable temperature and
the wiring is done correctly. Similarly, the output voltage of the
unregulated PM alternator isn't _that_ high - <100V. You can pick a cap
that can stand the full output and not worry about it failing should the
regulator pack it in. I see caps rated to (and above) 100V on the digikey
site for reasonable prices.
Regards,
Matt-
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Bob and Etienne, thanks for your replies. You've shed some light on a
controversy that erupted on the CorvairCraft list (on mylist.net) early
this month: somebody started bad-mouthing PM alternators and as evidence
he cited several instances of blown smoothing capacitors on Rotax
installations. The thread got bogged down in ad hominem attacks and
counter-attacks and eventually was dropped without resolving the question
of why the capacitors had blown, but it appears in light of what you two
have pointed out that those setups must have had the capacitors upstream
of their OV protection -- or no OVP at all...
...Which leaves me puzzled with respect to the Z-figures' treatment of
smoothing capacitors with PM alternators: only Z-9 and Z-16 show them
downstream of the OVP; Z-10/8, -13/8, -17, -20 and -21 all have the
capacitor upstream of the OVP. The guy stirring things up on
CorvairCraft confirmed that failures of these caps are indeed spectacular.
How likely to hear such a bang are users of Z-10/8 et al?
Am I right in concluding that my best bet for minimizing the probability
of an unhappy ending with a three-phase PM alternator is to put a single
disconnect relay in the DC line downstream of the R/R but upstream of the
smoothing capacitor, leaving the R/R to fend for itself?
Rick Nordgarden
Council Bluffs IA
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p 83502#283502
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