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Bob's response:
(For those who don't know, he's a long time employee of Raytheon, working on both small & big iron.)
-------- Original Message --------
Subject: AeroElectric-List: Re: Alternators]
Date: Sun, 20 Mar 2005 16:05:48 -0600
From: Robert L. Nuckolls, III <b.nuckolls@cox.net>
Reply-To: aeroelectric-list@matronics.com
To: aeroelectric-list@matronics.com
--> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <b.nuckolls@cox.net>
At 12:47 PM 3/20/2005 -0600, you wrote:
--> AeroElectric-List message posted by: Charlie England <ceengland@bellsouth.net>
Bob,
This showed up on another list & some would like to see your comments,
since you are directly involved in certified hardware. I'm pretty sure I
could write more about the inconsistencies here than he's written about
the differences, but I'd also like to see your opinion. I'll post back
to the other list if that's acceptable.
Thanks,
Charlie
Begin quote:
From Pifer's Airmotive, Inc. Pontiac MI
DIFFERENCES BETWEEN AIRCRAFT & AUTO ALTERNATORS USING A FORD BELT DRIVEN 12v
OR 24v ALTERNATOR FOR A COMPARISON
1. Although alternators are bi-rotational, aircraft engines turn opposite of
automotive. This means cooling fans must be canted in the opposite direction.
Also, pulley and belt size vary due to coming-in speed.
An automotive fan looses only 10 percent or so of its cooling
efficiency when spun the wrong direction. Van used to sell or
recommend an alternator that featured an external cooling
fan. He recommended removing it because it turned "the wrong
direction". Bad idea. The alternator went from a slightly
compromised fan to NO fan.
Modern alternators have bi-directional fans and I now believe
most have brushes mounted right over the slip-ring shaft center
so these alternators can be spun either direction without
concerns.
2. The thru bolts are of a higher tensile strength utilizing an
anti-rotation device in the form of a lock tab. The rectifier assembly has a heavy duty
diode with higher voltage and amp. capacity. Also, one excites at 90 PIV (Peak
Inverse Voltage) and the other at 150 PIV. Radio suppression is designed for
108 frequencies and up which is the VHF and 108 and down which is FM band.
The gentleman may indeed be looking at the specifications
for some "certified" alternator. Further, the datasheet may
be some revision printed in 1975 . . . It's difficult to know.
Radio noise from alternators is not a big issue at VHF
comm and nav frequencies. ADF and LORAN are the vulnerable
systems. Many (if not all) of the alternators I've seen torn
down over the last 5 years have hi-quality filters built in.
AM radio receivers are the vulnerable system in cars and it's
not difficult to include such filtering in the alternator's
basic design. I doubt anyone even makes a diode that won't run
at 200 volts plus. This used to be a big deal but it's so easy
to craft a diode that nobody worries much about tailoring one
for a low voltage application. Thru bolts are NOT an issue. The
automotive conversions have run quite well by the thousands
of installations over a decade.
Certified aircraft alternators crap with regularity. The
vast majority of B&C alternators sold over the past 15 years
are still running as original installation parts.
3. The brushes have a higher graphite content and they utilize a tin plate
on the brush leads to prevent corrosion.
Bull hocky. Corroded brush leads are the very LEAST of one's
concerns for brush life . . . don't know about the "graphite content"
and can't imagine why it's an issue on slip rings.
4. The stator is of the Delta wind rather than the Y" wind and it does not
utilize the stator terminal. The aircraft unit also carries H" insulation
which is capable of 200 degrees centigrade temperatures. It also is rated at
60 amp. instead of 55.
Also bull hockey . . . one can wind the stator any way they
wish to take advantage of some feature that gives is preference
over another. I've read the sand-sifting offered by several
folks on this subject but there are alternators wound both
ways that perform just fine. Some older airplanes use the center
tap "stator" terminal to drive an alternator failure warning system
but this is super-antiquated. With a low voltage warning lamp, you
can deduce everything the pilot NEEDS to know about altenrator
performance whether the thing is Y or Delta wound.
5. The rotor has a shorter shaft and a smaller thread size. Because of the
opposite rotation it is wound in the opposite direction.
Yes, the nuts retaining the pulley on an ND alternator running
in an airplane are put on with an impact wrench. Sometimes
the pulley is keyed to a flange on the shaft which makes thread
direction irrelevant for pulley retention. Alternator shaft torques
are so low that clamp up forces on a properly installed nut
totally negate any sensitivity to direction of threads.
It also uses "H insulation and Havel varnish.
Everybody used class "H" insulation. An automotive alternator is
more likely to run at temperature limits than anything on an airplane.
Cars don't get the benefits of lower ambient temps and 100+ MPH
ram air if needed.
6. The front and rear housings are the same as automotive.
With this brief description, I hope I have enlightened you on the
differences between aircraft and automotive alternators. Using automotive units in an
aircraft creates a potential safety hazard as well as a short alternator life
and unreliability.
My sense is that the data being offered is outdated and features
offered as uniquely "aircraft" because the product has been spec'd
onto somebody's type certificate are overblown or irrelevant to the
application.
Given the DEMONSTRATED service life of modern automotive alternators
on OBAM aircraft and exemplar performance in automobiles, I venture
to say that one wouldn't want to put an "aircraft alternator" on a
car. They're expensive, fewer folks stock them, even fewer folks have
parts for them and they don't last as long.
I'd opt for an ND alternator modified to run external regulator
for ANY belt driven certified alternator any day.
Bob . . .
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