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paulmillner
Joined: 09 May 2007
Posts: 21
Location: Berkeley, California
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 Posted: Mon Aug 29, 2011 12:22 pm Post subject: Alternator Wye - what voltage? |
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>> The Wye center point is just like the neutral in your house wiring... it should always be close to ground potential except when something bad happens.
The wye is close to neutral in a three-phase AC generation or motor situation. That's NOT true in our airplanes, where the phases are rectified to DC, since each phase-to-phase pair alternates between being ground on one end and bus voltage on the other end. In that case, the wye floats at about half of bus voltage, but of course is alternating current, not direct.
In fact, Bob N's former employer, Beechcraft, took advantage of that on the 12 volt Bonanzas... they attached a six volt *AC* relay between the wye and aircraft ground. When the alternator was putting out power, that relay would close, turning off the "ALTERNATOR FAIL" light. If for whatever reason the alternator quit (open field circuit, broken belt, whatever) then the wye would float near ground, the AC relay would open, and the FAIL light would light. Very clever!
I've been tempted to add just such cleverness to my Cessna Cardinal someday...
Paul
[quote][b]
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Paul Millner, Berkeley CA [OAK] |
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nuckolls.bob(at)aeroelect
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| Posted: Mon Aug 29, 2011 3:48 pm Post subject: Alternator Wye - what voltage? |
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At 03:15 PM 8/29/2011, you wrote:
>> The Wye center point is just like the neutral in your house wiring... it should always be close to ground potential except when something bad happens.
The wye is close to neutral in a three-phase AC generation or motor situation. That's NOT true in our airplanes, where the phases are rectified to DC, since each phase-to-phase pair alternates between being ground on one end and bus voltage on the other end. In that case, the wye floats at about half of bus voltage, but of course is alternating current, not direct.
Actually, it is DC albeit half-wave, 3-phase
rectified which is pretty 'bumpy'.
In fact, Bob N's former employer, Beechcraft, took advantage of that on the 12 volt Bonanzas... they attached a six volt *AC* relay between the wye and aircraft ground. When the alternator was putting out power, that relay would close, turning off the "ALTERNATOR FAIL" light. If for whatever reason the alternator quit (open field circuit, broken belt, whatever) then the wye would float near ground, the AC relay would open, and the FAIL light would light. Very clever!
That 'relay' was in fact a little two-transistor,
plus jelly-beans dc voltage level sensor. I remember
it well. We supplied about 4 versions to Beech and
Cessna. During my early years at Electro-Mech, I was
cleaning up some drawings and got two capacitors transposed
in the bill of materials for the Cessna Wallace Plant
version. The thing still passed functional test but
behaved badly on the airplanes. Worse yet, the puppies
were potted.
Needless to say this young fellow was embarrassed to
report to the boss that we needed to do a recall on
about $30,000 worth of production and launch a
quick-turn replacement program. That was more money
than I made in a year at that time!
Fred would have been fully within 'normal' supervisory
parameters to have chewed my you-know-what down
to bed-rock. He didn't miss a beat. "Talk to production
and get a reading on the replacements . . . I'll have
sales notify Cessna and start he recall. Get back to
me with a schedule." That's about the only words we
had on the matter. I didn't need him to tell me to
revise the functional test fixture to offer a more
realistic functionality.
I always appreciated this man's focus . . . and his
class. I've had about five supervisors that I truly
revered in my life and Fred Coslett was one of them.
I've been tempted to add just such cleverness to my Cessna Cardinal someday...
I can probably dig up the schematic of the original but
an active LV warning like the AEC9005 is probably
closer to the best-we-know-how-to-do today. Better
yet, the 9024 program has stumbled back to life. The
first 5 or so devices will be offered to individuals
who can put them into flying airplanes right now. We'll
want to get some field testing of the product and the
installation instructions before listing the thing in
the catalog.
Haven't got a schedule but the software IS moving
forward again.
Bob . . . [quote][b]
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