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2 alternators and 3 questions

 
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roughleg(at)gmail.com
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PostPosted: Fri May 17, 2019 10:49 am    Post subject: 2 alternators and 3 questions Reply with quote

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


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PostPosted: Fri May 17, 2019 1:45 pm    Post subject: 2 alternators and 3 questions Reply with quote

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(at)gmail.com (roughleg(at)gmail.com)> wrote:

Quote:
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

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https://CheerfulCurmudgeon.com/Pray as if everything depends on God. Act as if everything depends on you.


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Joined: 28 Mar 2008
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Location: Riley TWP Michigan

PostPosted: Sat May 18, 2019 11:56 am    Post subject: Re: 2 alternators and 3 questions Reply with quote

Quote:
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.
Quote:
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.


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PostPosted: Sat May 18, 2019 3:00 pm    Post subject: 2 alternators and 3 questions Reply with quote

At 01:49 PM 5/17/2019, you wrote:
Quote:
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.

Quote:
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?

Quote:
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?

Quote:
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.

Quote:
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?

Quote:
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.


Quote:
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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PostPosted: Sun May 19, 2019 6:00 am    Post subject: 2 alternators and 3 questions Reply with quote

Quote:

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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roughleg(at)gmail.com
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PostPosted: Tue May 21, 2019 10:13 pm    Post subject: 2 alternators and 3 questions Reply with quote

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(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

Quote:
Quote:

 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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Electrical_System_Load_Analysis_-_N750PF_-_5-21-2019.xlsx
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PostPosted: Wed May 22, 2019 2:47 am    Post subject: 2 alternators and 3 questions Reply with quote

Why not just switch off the pitot heat then?
It seems like a huge amount of extra engineering, and the cockpit actions are no simpler because according to your comment you still have to decide about whether your are using worst case electrical loads or not.

On May 22, 2019, at 02:12, Pat Little <roughleg(at)gmail.com (roughleg(at)gmail.com)> wrote:
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.


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PostPosted: Wed May 22, 2019 3:03 am    Post subject: 2 alternators and 3 questions Reply with quote

That would be reasonable. I've turned mine on exactly once in 13 years
in snow at night and it wasn't needed even then as I pay far more
attention to how the controls feel as well as power and attitude. I've
done circuits with the airspeed covered up to prove this to myself.  
It's comforting to have it available when ice is forming but it still
can't be completely trusted and it's more important to exit the icing.
It would definitely not be used after one of my alternators quit.
Ken

On 22/05/2019 6:46 AM, Alec Myers wrote:
Quote:
Why not just switch off the pitot heat then?
It seems like a huge amount of extra engineering, and the cockpit
actions are no simpler because according to your comment you still
have to decide about whether your are using worst case electrical
loads or not.

On May 22, 2019, at 02:12, Pat Little <roughleg(at)gmail.com
<mailto:roughleg(at)gmail.com>> wrote:

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.


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PostPosted: Wed May 22, 2019 3:58 am    Post subject: 2 alternators and 3 questions Reply with quote

Pat,
I encourage you to reexamine your load analysis. Here are three points from your Main Bus section:
  • Flaps motor - 4 amps. The flaps motor rarely operates. This current can be supplied by the battery if the alternator does not have sufficient capacity.
  • Stater contactor - 4 amps. Once the engine is running, the starter contactor disengages and draws no current.
  • Strobe Lights - 4.5 amps. Like you, I have AeroLEDs Pulsar NSP lights on my wing tips and the pair draws 2.40 amps continuous, not 4.50.
It looks like I just saved you 10.1 amps. 🙂
As a point of comparison is the load analysis for my airplane. It is in the right column of this drawing.

[img]https://drive-thirdparty.googleusercontent.com/16/type/application/pdf[/img] overview.pdf
As for your cockpit procedures, they seem like they will certainly work but it is way more effort than I would want to oblige myself to. I fly with both alternator on 100% of the time. If the primary fails, the backup automatically steps in. No pilot action required. If the load is too high for the standby alternator, I will see a low voltage alert and can shed some load. The only two things that I anticipate needing to turn off would be pitot heat (which is almost certainly off anyway) and autopilot servos.
Just my opinion, of course: Since we are designing our own airplanes, we have the ability to reduce pilot workload as much as possible. Doing so is a Really Good Idea.
    -- Art Z.

On Wed, May 22, 2019 at 1:30 AM Pat Little <roughleg(at)gmail.com (roughleg(at)gmail.com)> wrote:

Quote:
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(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

Quote:
Quote:

 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 . . .



--
https://CheerfulCurmudgeon.com/Pray as if everything depends on God. Act as if everything depends on you.


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PostPosted: Wed May 22, 2019 4:45 am    Post subject: Re: 2 alternators and 3 questions Reply with quote

An E-Bus is vulnerable to pilot error. What if the pilot inadvertently shuts off the master switch before turning on the E-Bus switch? Now he has to wait for the avionics to reboot. Instead of an E-Bus, how about locating not-so-important switches on the right side and important switches on the left side. Switches could also be color coded. When it is necessary to conserve electrical power, shut off switches on the right side. This can be accomplished with one motion of the hand if the switches are mounted close together. If the pilot later wants to turn on one of the unimportant loads, no problem, just flick on that one load. There is no need to remember which loads are on which bus and no need to juggle master and E-Bus switches. Keep it simple.

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PostPosted: Wed May 22, 2019 5:06 am    Post subject: Re: 2 alternators and 3 questions Reply with quote

The radio only uses 0.3A, not 2.5 amps. Do NOT count intermittent loads.
If the main alternator fails, shut off the landing lights during cruise and save 1.5 amps.
The trim motor and relay deck are intermittent loads. Subtract another 1.8 amps.
Eliminate the E-Bus diode and save 1/2 amp of wasted power (heat).


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PostPosted: Wed May 22, 2019 7:20 am    Post subject: 2 alternators and 3 questions Reply with quote

At 01:12 AM 5/22/2019, you wrote:
Quote:
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:

Show show. I'm packing up to
run to Wichita . . . be back
tomorrow p.m.

I'll let you guys thrash the
data/options . . .



Bob . . .


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PostPosted: Thu May 23, 2019 7:18 am    Post subject: 2 alternators and 3 questions Reply with quote

I appreciate your suggestions, Art.

In my loads analysis the left column, which is colored, corresponds to
your *Maximum
*column, and I use this for wire sizing (hence the color codes, just as a
quick visual for me to know which PIDG I'll be using). The other columns to
the right show which circuits will be in use during various phases of
flight (so I only show starter contactor when starting, and flap motor is
present in some phases of flight but reduced somewhat to account for air
loads on the flaps - it only shows the max 4A during descent when
presumably the motor is working against the air loads). My final,
right-most, column of numbers is an attempt to show average currents, which
I think corresponds to your *Typical *column.

Maybe all the numbers in my phase-of-flight columns should be average, and
ignore transient peaks?

I would love to be able to run both alternators all the time but I don't
think I can (at least not without giving the pilot extra work to do)
because my secondary alternator has a thermal restriction and needs to be
kept below 17A. Depending on details of how the the two alternators behave
at high currents, i.e., how their voltages droop as current increases, the
secondary may exceed 17A when pitot heat is on and would need to be
cosseted even though the bus voltage, with both alternators contributing,
is still plenty above LV warn level. So, the pilot would have to monitor
the secondary's current which is extra work. And I agree that is a Bad
Idea.

Pat

On Wed, May 22, 2019 at 6:08 AM Art Zemon <art(at)zemon.name> wrote:

[quote] Pat,

I encourage you to reexamine your load analysis. Here are three points
from your Main Bus section:

- Flaps motor - 4 amps. The flaps motor rarely operates. This current
can be supplied by the battery if the alternator does not have sufficient
capacity.
- Stater contactor - 4 amps. Once the engine is running, the starter
contactor disengages and draws no current.
- Strobe Lights - 4.5 amps. Like you, I have AeroLEDs Pulsar NSP
lights on my wing tips and the pair draws 2.40 amps continuous, not 4.50.

It looks like I just saved you 10.1 amps. 🙂
As a point of comparison is the load analysis for my airplane. It is in
the right column of this drawing.

overview.pdf
<https://drive.google.com/a/zemon.name/file/d/0BzOP2gb9_3RQSU5qbVN1ckJNOUk/view?usp=drive_web>

As for your cockpit procedures, they seem like they will certainly work
but it is way more effort than I would want to oblige myself to. I fly with
both alternator on 100% of the time. If the primary fails, the backup
automatically steps in. No pilot action required. If the load is too high
for the standby alternator, I will see a low voltage alert and can shed
some load. The only two things that I anticipate needing to turn off would
be pitot heat (which is almost certainly off anyway) and autopilot servos


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PostPosted: Thu May 23, 2019 8:02 am    Post subject: 2 alternators and 3 questions Reply with quote

Quote:

Maybe all the numbers in my phase-of-flight columns should be average, and ignore transient peaks?

Yes . . .


Quote:
I would love to be able to run both alternators all the time but I don't think I can (at least not without giving the pilot extra work to do) because my secondary alternator has a thermal restriction and needs to be kept below 17A. Depending on details of how the the two alternators behave at high currents, i.e., how their voltages droop as current increases, the secondary may exceed 17A when pitot heat is on and would need to be cosseted even though the bus voltage, with both alternators contributing, is still plenty above LV warn level. So, the pilot would have to monitor the secondary's current which is extra work. And I agree that is a Bad Idea.

Your gut is right . . . AMMETERS are
bad flight management instrumentation.

Your various "plans" based on flight
conditions are predictable. That's
what the load analysis is all about.
We've built millions of airplanes
with no ammeters in them. It's only
since the glass cockpit guys started
adding them to the list of features
that pilots are beginning to think
that (1) gee, if I can go measure a current,
why not? (2) but which current and for
what operational purpose? (3) now
that I can track that feature in flight,
what are my pilot duties (work load)
to observe and react to what I see?

The answer to all three questions is
zilch, zip, nada . . .

If you've need to observe a manufacture's
limit on an alternator load, then factor
that into which switches are ON and OFF
for the pre planned flight conditions
(those columns in the load analysis).

Ammeters are diagnostic instruments
used to deduce malfunctions and
plan repairs . . . on the ground.
If you need to fiddle with the switches
in flight while watching an ammeter,
you've failed to exploit the value
of the load analysis.



Bob . . .


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PostPosted: Thu May 30, 2019 2:55 pm    Post subject: 2 alternators and 3 questions Reply with quote

Bob, Joe, and Art: I have absorbed your comments and revised my loads analysis (attached).
  1. I now only show continuous/typical loads for the various flight phases (one exception, perhaps, is the starter contactor, and it only appears in one column)
  2. I include pitot heat in the phases where it might get used, but also added a line below the TOTALS line to show totals without pitot heat.
  3. Now the loads in all phases of flight, with pitot heat on, are well below 80% of the main alternator's rated load.  With pitot heat off they fall below 80% of rated load for the standby alternator. 
  4. No e-bus.
  5. If the main alternator fails and pitot heat is needed (very low likelihood of occurrence) then the pilot can turn off the landing lights and/or the strobes and be under the stby alternator's rated load. 
  6. I am going to group the switches so the large non-essential loads are at the right-hand end of the row to make load shedding easy and intuitive. In the analysis spreadsheet I have placed these at the bottom of the main bus.

More comments welcome Smile
I really appreciate the feedback - this listserve is great!
Thanks,
Pat
On Thu, May 23, 2019 at 10:08 AM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

Quote:
Quote:

Maybe all the numbers in my phase-of-flight columns should be average, and ignore transient peaks?

  Yes . . .


Quote:
I would love to be able to run both alternators all the time but I don't think I can (at least not without giving the pilot extra work to do) because my secondary alternator has a thermal restriction and needs to be kept below 17A. Depending on details of how the the two alternators behave at high currents, i.e., how their voltages droop as current increases, the secondary may exceed 17A when pitot heat is on and would need to be cosseted even though the bus voltage, with both alternators contributing, is still plenty above LV warn level. So, the pilot would have to monitor the secondary's current which is extra work. And I agree that is a Bad Idea.

  Your gut is right . . . AMMETERS are
  bad flight management instrumentation.

  Your various "plans" based on flight
  conditions are predictable. That's
  what the load analysis is all about.
  We've built millions of airplanes
  with no ammeters in them. It's only
  since the glass cockpit guys started
  adding them to the list of features
  that pilots are beginning to think
  that (1) gee, if I can go measure a current,
  why not? (2) but which current and for
  what operational purpose? (3) now
  that I can track that feature in flight,
  what are my pilot duties (work load)
  to observe and react to what I see?

  The answer to all three questions is
  zilch, zip, nada . . .

  If you've need to observe a manufacture's
  limit on an alternator load, then factor
  that into which switches are ON and OFF
  for the pre planned flight conditions
  (those columns in the load analysis).

  Ammeters are diagnostic instruments
  used to deduce malfunctions and
  plan repairs . . . on the ground.
  If you need to fiddle with the switches
  in flight while watching an ammeter,
  you've failed to exploit the value
  of the load analysis.



  Bob . . .


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