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1. 12:57 PM - Re: 2 alternators and 3 questions (user9253)
2. 04:00 PM - Re: 2 alternators and 3 questions (Robert L. Nuckolls, III)
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| Subject: | Re: 2 alternators and 3 questions |
> is it reasonable to design for average loads well within the alternator's capacity
but transient loads that exceed it?
Yes, momentary loads will not overheat the alternator.
If the voltage drops, which it probably will when fully loaded,
then the battery will help out.
A load will always take current from the source with the highest voltage.
If the main alternator is overloaded and its voltage drops, then when the
voltage drops to the set point of the secondary alternator, both alternators
will supply current. Bob will correct me if wrong.
30 amps seems like a lot for a small plane with modern avionics and lights.
I suspect that the main alternator will easily supply the full aircraft load.
> Am I right in assuming that both OVMs will trip
Yes, you are correct. The solution is to only operate the main alternator.
If it fails, shut it off and then turn on the secondary alternator. The battery
will
supply current during the transition.
I agree with Art that an E-Bus is not a necessity. Keep it simple.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=489284#489284
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| Subject: | Re: 2 alternators and 3 questions |
At 01:49 PM 5/17/2019, you wrote:
>I am in the process of defining the electrical architecture for our
>Zenith STOL CH750. The plane has a Jabiru 3300 engine, and is
>intended for day & night VFR. It has the built-in Jabiru
>permanent-magnet alternator (17A) plus a B&C SD-20 (29A at our
>cruise rpm of 3,000). My starting point for the design is Z-13/8.
>
>With two alternators I am designing the system so that if the main
>(SD-20) alt fails then the secondary (Jabiru) will be able to
>sustain the e-bus loads by itself. The biggest load by far is the
>pitot heater which draws around 9A (an estimate, as discussed in a
>previous conversation) and since we are night VFR we want the pitot
>heat available when running on e-bus.
Then don't have an e-bus. The ENDURANCE bus was
crafted to make best utilization of a BATTERY's
stored energy in an alternator-out situation.
> This brings things close to the capacity of the Jabiru PM
> alternator - its output at our cruise rpm of 3,000 is 17A, and the
> manufacturer states that we must not exceed 17A or the PM stator may overheat.
How to you proposed to observe this limitation?
>My preliminary loads analysis shows that the endurance bus load is
>around 13A average (76% of rated alternator capacity) with
>potentially 19A max if all the transient loads (radio, transponder,
>EFIS, trim motor) were to occur simultaneously.
How often does that happen?
>So this is my first question - is it reasonable to design for average
>loads well within the alternator's capacity but transient loads that
>exceed it?
It's reasonable to presume that when alternator
failure occurs, you've got a battery with a KNOWN
quantity of contained energy. EXPECTING it
to shoulder low duty cycle transients is
part of a rational energy budget.
>For my second question I'm back to having both alternators
>available. When the pitot heat is on, the total system current (not
>just e-bus) is slightly above the rated capacity of the SD-20, so we
>would want to have the pilot turn on the secondary alternator as
>well. With both alternators feeding into the main bus I need to
>understand how they will share the loads between them so as to keep
>the Jabiru below its 17A limit, preferably well below.
Which goes to my original question. Suppose your
proposed system was only powered by the engine
PM alternator . . . how would you manage the
system to comply with manufacturers limits on
altenrator loading?
> Ous system has a single ammeter (Dynon EMS-D120) so I plan on
> displaying the Jabiru alternator current any time it is in use
> (using a 3PDT for the secondary alternator switch that would also
> swap out the + and - feeds from each of the two shunts). This way
> the pilot has the ability to monitor and control the current not to
> exceed whatever threshold we declare for the Jabiru alternator.
> However, I'd like to make it a bit less manual than that. The
> Jabiru regulator is fixed output (factory spec is 14.3V) but the
> B&C LR3C regulator is adjustable. Is it possible to set the B&C's
> output a little higher so that it provides most of the current, and
> the Jabiru alternator will only contribute if and when the SD-20's
> output droops? Or does this just put all the loads on the SD-20 and
> exceed its capacity, as if the secondary alternator weren't there?
You're flogging a baseless worry. The
SD20 is manufactured from a 40A core
which has been de-rated in certain installations
based on drive-pad RPM. As a practical matter,
you cannot 'hurt' it by exceeding its 'rating'
for your particular engine's drive pad rpm.
>And my third question is to do with OV protection on this
>dual-alternator system. The Z-13/8 shows a 2-alternator system
>similar to what I am planning. It has two OVMs, one attached to each
>alternator's circuit. If both alternators are online and OV
>condition occurs then presumably both OVMs will trip and both
>alternators will be disconnected even though only one was causing
>the OV, and the other gets shut off unnecessarily. Am I right in
>assuming that both OVMs will trip, or could it be that the one with
>the lower trip point would act and, if it happened to be the one
>attached to the problem alternator, in so doing cut off the rising
>voltage soon enough that the other OVM doesn't trip? I am trying to
>understand if I have a deterministic system or if it will be
>unpredictable in this regard.
An OV condition is exceedingly rare and should
it occur, the SD-20 regulator ov protection
is SELECTIVE. It is prohibited from tripping
if it senses that the ov condition is coming
from another source. It is exceedingly unlikely
that you'll experience an ov event with the
17A alternator . . . more likely that you'll
experience an over-current event due to failure
or inadvertent shut down of the SD-20 . . . which
goes to question above.
Recommend you do 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.
Should you find yourself in
an icing condition in that airplane, belive me . . .
knowing the IAS numbers are the very least
of your worries.
Pitot heat may be 'required' by various and
sundry regulatory agencies . . . but it's
one tiny step above worthless in
situations where icing is likely to mess with
airspeed accuracy.
Install the pitot heater if you must,
but the idea of NEEDING to turn it on
adds a burden to single-pilot IFR that
has caused many a pilot to 'buy the farm'.
Your best response on detection of first ice is
a 180 . . .
If your predictive weather sources are so poor
that you cannot confidently cancel the mission
when there is risk of ice, then you need to
upgrade . . . may I suggest something certified
for flight into known icing?
Bob . . .
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