nuckolls.bob(at)aeroelect
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 Posted: Fri May 08, 2009 12:56 pm Post subject: Radio Noise Problems - Rotax 914 Alternator Problems |
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At 01:55 PM 5/8/2009, you wrote:
| Quote: | --> AeroElectric-List message posted by: "David E. Nelson" <david.nelson(at)pobox.com>
Hi Darin,
While an o-scope would be preferable, many years ago I was told that I could use a Volt-Ohm-Meter to do a quick check of a diode bridge for a running alternator by measuring the AC voltage. I've never had the opportunity (knock on wood) to verify this, however.
I think our basic alternator is essentially a 3 phase alternator. So if a diode fails, I'd think the number would be in neighborhood of 12V/3phase = 4 VAC/phase (there may be a small multiplier in there somewhere) rather in the sub 1 VAC (ripple voltage).
I'm sure the Bob will chime in here somewhere and either validate or correct. Either way, I'd like to update my notes. |
The rectified DC output from a 3-phase alternator
carries about 5% pk-pk ripple. In a 14v system
this is about 0.7 volts. Problem is that there's
a lot of diode switching noise that can be higher.
Depending on the design of the voltmeter, the AC
readings can be all over the place.
At Electro-Mech about 25 years ago, I looked at
offering an accessory to the aircraft industry
based on ac ripple monitoring. There were a number
of ideas/products in the wild that purported
to do the same thing. One example:
http://tinyurl.com/oummzc
What I discovered was that the "smarts" necessary
to accurately detect diode failure under all
operating conditions and all variations on a theme
for alternators in the field made for a pretty
expensive device . . . that didn't really do much
more than a low-voltage warning light. The claims
offered in the patent cited above cannot be supported
in broad practice.
Having said that, if you have a BENCHMARK ac ripple
voltage measurement with a specific instrument,
you can use that combination to tell that something
has drastically changed . . .
The last time I looked at a Rotax 912, it was still
a single phase machine . . . MUCH higher ripple
voltage that is drastically altered how hard the
rectifier/regulator is taxing the alternator. AC
rippled measurements at the DC side might be
a useful diagnostic tool IF you had benchmark
measurements to compare with . . . but unless you
know what the GOOD measurements look like, it's
not likely that you can identify a BAD measurement.
I gave up the AC ripple monitor in favor of the
rudimentary LV warning light. We sold several versions
to about a half dozen airframe OEM.
Bob . . .
[quote][b]
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