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1. 11:13 PM - Re: 2 alternators and 3 questions (Pat Little)
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| Subject: | Re: 2 alternators and 3 questions |
Bob (and Art and Joe),
Many thanks for your feedback. I realize from your replies that I didn't
explain why I am proposing an e-bus which is normally for reducing battery
loads, but since my Stby alternator has a strict current limit I think the
e-bus has a role to play in my system. Here is how I think my design would
work, which I hope will make it clear why I am adding the electrical
complexity of the e-bus to achieve a reduction in pilot workload:
1) cruise flight, main alternator ON and stby alt OFF - ammeter shows
current from main alternator (our EMS only has a single ammeter gauge)
2) main alternator fails - LV warning alerts the pilot, he sees current is
zero and deduces main alternator has failed (or maybe the breaker has
popped which makes it easier to see what has happened)
3) battery carries the loads for a short while
4) pilot turns the e-bus alternate feed ON, and the master OFF - this
reduces the electrical loads below the 17A limit of the stby alternator
5) pilot turns Stby Alt ON. The EMS now shows current from Stby Alt (the
Stby Alt switch is a 3PDT that swaps the ammeter shunt signals as well as
controlling the relay) and pilot can verify loads <17A
6) continue flight to destination
In this sequence the benefit of the e-bus is that it gives the pilot a few
simple actions to perform in order to ensure the Stby alternator is happy,
and the process doesn't require a lot of heads-down work.
NOTE - the above scenario assumes worst-case electrical loads. If the pitot
heat is not being used then it would be simpler to just turn on Stby alt
and not use the e-bus alt feed.
So, how to choose a system architecture to achieve this?
Given that I am proposing to use the Stby alt with the master OFF (e-bus
alternate feed ON) I need the output of the Stby alt to feed into the
system upstream of the battery contactor, and that is what Z-13 shows,
whereas Z-12 has it going in downstream where it won't work for my proposed
design. That is why I want to base my design on Z-13. However, Z-12 shows
the B&C regulator, which is what I have for my main alternator, so i would
be incorporating some elements from Z-12 into my drawing.
I have a loads analysis (I based it on one of the examples from your site
Bob) and I'm attaching it. It is still somewhat incomplete but I'd welcome
you thoughts.
Cheers,
Pat
On Sun, May 19, 2019 at 8:05 AM Robert L. Nuckolls, III <
nuckolls.bob@aeroelectric.com> wrote:
>
> Recommend you consider a Z-12 style installation. Leave
> the 17A alternator OFF unless needed during anticipated
> icing conditions . . . another condition that should
> be vanishingly rare -OR- failure of the SD-20.
> You're not going to experience dual alternator failure
> on ANY mission . . . energy rationing with
> an E-bus adds no value.
>
>
> Have you conducted an electrical load analysis
> by flight condition? There are forms that
> assist in organizing the data available
> at https://tinyurl.com/9rt6ymn
>
> Included are some exemplar spread sheets
> uploaded by various List members over
> the years. I did notice that one of
> the .xls spread sheets shows exterior
> lighting as a running load coincident
> with pitot heat.
>
> My instructors taught that exterior
> lights be turned off while in clouds. The
> combination of variable reflection off passing
> clouds combined with flashing of the
> strobe is distracting and can induce
> vertigo . . . I've experienced it
> first hand.
>
> Your RUNNING loads do not include
> landing lights (unless you have a wig-wag
> system or other recognition assist),
> trims, landing gear or flap motors,
> transmitter draws, etc. Just the steady
> state running loads are used to evaluate
> alternator adequacy and in some cases,
> battery supported E-bus operations.
>
> It may be that a well sorted
> load analysis will mitigate concerns
> for alternator adequacy. It also
> helps you define what switches should
> be ON and which ones OFF in the various
> flight conditions. I suspect that your
> suite of alternators will be shown
> to be entirely adequate to all anticipated
> missions . . . especially if your
> exterior lights are all LED.
>
> 20 years ago the most energy hungry
> system on the airplane was position
> lights . . . 2A per bulb x 3 bulbs x
> duration of flight. Xenon flash
> strobes are way up there too. No longer the
> case.
>
> In your case, it seems likely that
> a Z12 architecture with the PM alternator
> as "standby" is possible . . . crunching
> the numbers will confirm/deny that
> assumption.
>
>
> Bob . . .
>
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