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Sifting design goals and establishing protocols

 
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nuckolls.bob(at)aeroelect
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PostPosted: Sun Jan 06, 2019 6:06 am    Post subject: Sifting design goals and establishing protocols Reply with quote

Quote:

Bob,

What would then be the method to recognize an alternator failure except for a preflight check? As long as the remaining alternator is capable of handling the actual load, there would be no warning to the pilot..

It's pretty easy to get wound up with
worries about failures . . . especially
un-annunciated or hidden failures. Electro-
whizzies top the list . . . mostly due
to a general lack of understanding for
crafting a failure-tolerant system from
reasonably reliable parts.

Check out the chapter on system reliability
in the 'Connection. We almost never
concern ourselves with dual failures.
Your posting properly notes that once
airborne, you'd have no notice for
loss of the #2 alternator . . . which
is sitting there spinning away with
nothing to do.

Failure rates on equipment items must
certainly be in many hundreds if not
a few thousands of hours. Consider
two devices, say alternators, having
MTBF numbers on the order of 1000
hours.

To deduce such numbers, one has to
either test a bunch of alternators
for thousands of hours (time consuming
and expensive) or do an analysis
based on stress levels to critical
features and/or a statistical process
study of field history. One thing
we do know about automotive alternators
is that they tend to run a long time.
Then what is the likelihood that two
alternators with demonstrated service
histories will crap out on the same
airplane in a single 4-hour window
(one tank of fuel)? The answer is
vanishingly small if not zero.

If the #2 alternator does fail in
flight, you won't know about it until
next pre-flight . . . unless you add
#2 alternator testing to your shutdown
checklist.

The bid takeaway here is that the
probability for one alternator failure
is small, dual failures is practically
zero.



Bob . . .


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nuckolls.bob(at)aeroelect
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PostPosted: Sun Jan 06, 2019 9:01 am    Post subject: Sifting design goals and establishing protocols Reply with quote

At 09:38 AM 1/6/2019, you wrote:
Quote:
--> AeroElectric-List message posted by: Alec Myers <alec(at)alecmyers.com>

Twin piston engined aircraft routinely have two alternators online simultaneously and it is widely accepted that due to different set points on their respective regulators that only one with carry a load when both are operational. It is immaterial which.


This was a 'hard sell' for years after
alternators replaced generators. Generator
wear rates are related to ampere-hours of
service due to brush wear while alternator
wear is largely independent of electrical
demand in service. It was prudent to have
dual regulators 'talk' to each other to
balance the load between two generators.

[img]cid:.0[/img]

Equalizer busses are still present on the
starter-generators of some production turbine
aircraft.


Quote:
Preflight procedures are to isolate each alternator in turn and verify that in either case the remaining alternator carries the electrical load demanded of it.

When alternators came along, there were
a number of 'paralleling' schemes implemented
with varying degrees of success. Some designers
more aware of the practical results for accurate
paralleling have adopted the run-em-both-and-let-
the-'good'-one-carry-the-loads approach.

The same idea was carried out with the B&C
standby alternator philosophy . . . leave both
on line but set the s/b alternator deliberately
low while adding an "alternator loaded" warning
feature to annunciate failure of the main
alternator.

This is illustrated in Figure Z-12. The
same philosophy could be incorporated in
Igor's RV by controlling the #2 alternator
with an SB-1 regulator . . . but I perceive
little value to be gained with the increase
in complexity.

It made a lot of sense to let us shoe-horn
the standby systems onto TC aircraft (the
FAA's professional worriers worried less).
Simply adding a second alternator with stock
LR-3 regulator offers the same reliability.

I proposed a paralleling system for the Cessna
303 waaayy back when. Demonstrated it and then
had to price it. Powers-that-be of the era
decided the advantages were not worth the cost.


Bob . . .


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supik



Joined: 22 Aug 2018
Posts: 44

PostPosted: Sun Jan 06, 2019 5:21 pm    Post subject: Re: Sifting design goals and establishing protocols Reply with quote

When utilizing LR-3 regulators for both alternators set at the same volts output will they share the loads equally? 50:50? If yes, wouldn't you adjust regulator #2 (30amp alt) slightly lower to prevent it from running at 100% if the loads get high? Very unlikely that I will routinely get above 60amps, that's more of a hypothetical question.

System monitoring & failure identification:

Would be monitoring the respective bus voltages & amp outputs from the respective alternators adequate?

Main distribution bus fed from alt #1
E-bus fed from alt #2

Thanks,


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Igor

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nuckolls.bob(at)aeroelect
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PostPosted: Sun Jan 06, 2019 5:44 pm    Post subject: Sifting design goals and establishing protocols Reply with quote

At 07:21 PM 1/6/2019, you wrote:
Quote:
--> AeroElectric-List message posted by: "supik" <bionicad(at)hotmail.com>

When utilizing LR-3 regulators for both alternators set at the same volts output will they share the loads equally? 50:50?

No . . . not with reliability

Quote:
If yes, wouldn't you adjust regulator #2 (30amp alt) slightly lower to prevent it from running at 100% if the loads get high? Very unlikely that I will routinely get above 60amps, that's more of a hypothetical question.

Independently controlled alternators/generators
on the same bus will not accurately parallel.


Quote:
System monitoring & failure identification:

Would be monitoring the respective bus voltages & amp outputs from the respective alternators adequate?

Main distribution bus fed from alt #1
E-bus fed from alt #2

Do a Z-14 and make them totally independent
of each other.



Bob . . .


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