Today's Message Index:
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1. 11:49 AM - 2 alternators and 3 questions (Pat Little)
2. 02:45 PM - Re: 2 alternators and 3 questions (Art Zemon)
Message 1
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| Subject: | 2 alternators and 3 questions |
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.
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. 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.
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?
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. 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?
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.
Pat Little
Message 2
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| Subject: | Re: 2 alternators and 3 questions |
Pat,
To the best of my knowledge, only one alternator can feed the bus at a
time.
Why not set the SD-20 voltage regulator at 14 volts and the Jabiru voltage
regulator at 13.5 volts? If you do that, the SD-20 will become the primary
and can provide 30 amps, sufficient for all loads. If the SD-20 isn't
turning fast enough, the bus voltage will drop below 13.5 volts and the
Jabiru alternator will come into play, able to provide up to 17 amps.
BTW, you might not even need an ebus, which could simplify your wiring even
further.
Cheers,
-- Art Z.
On Fri, May 17, 2019 at 2:06 PM Pat Little <roughleg@gmail.com> 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.
> 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. 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.
> 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?
>
> 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. 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?
>
> 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.
>
> Pat Little
>
>
--
https://CheerfulCurmudgeon.com/
*Pray as if everything depends on God. Act as if everything depends on you.*
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