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2 alternators and 3 questions

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roughleg(at)gmail.com
Guest

Posted: Fri May 17, 2019 10:49 am Post 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

art(at)zemon.name
Guest

Posted: Fri May 17, 2019 1:45 pm Post subject: 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(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

 	


-- 

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

user9253

Joined: 28 Mar 2008
Posts: 1922
Location: Riley TWP Michigan

Posted: Sat May 18, 2019 11:56 am Post subject: Re: 2 alternators and 3 questions

 	  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.

_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Sat May 18, 2019 3:00 pm Post subject: 2 alternators and 3 questions

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 . . .

nuckolls.bob(at)aeroelect
Guest

Posted: Sun May 19, 2019 6:00 am Post subject: 2 alternators and 3 questions

 	  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 . . .

roughleg(at)gmail.com
Guest

Posted: Tue May 21, 2019 10:13 pm Post subject: 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(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 . . .

Electrical_System_Load_Analysis_-_N750PF_-_5-21-2019.xlsx

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alec(at)alecmyers.com
Guest

Posted: Wed May 22, 2019 2:47 am Post subject: 2 alternators and 3 questions

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.

yellowduckduo(at)gmail.co
Guest

Posted: Wed May 22, 2019 3:03 am Post subject: 2 alternators and 3 questions

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.

art(at)zemon.name
Guest

Posted: Wed May 22, 2019 3:58 am Post subject: 2 alternators and 3 questions

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.

user9253

Joined: 28 Mar 2008
Posts: 1922
Location: Riley TWP Michigan

Posted: Wed May 22, 2019 4:45 am Post subject: Re: 2 alternators and 3 questions

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.

_________________
Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1922
Location: Riley TWP Michigan

Posted: Wed May 22, 2019 5:06 am Post subject: Re: 2 alternators and 3 questions

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).

_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Wed May 22, 2019 7:20 am Post subject: 2 alternators and 3 questions

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 . . .

roughleg(at)gmail.com
Guest

Posted: Thu May 23, 2019 7:18 am Post subject: 2 alternators and 3 questions

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

nuckolls.bob(at)aeroelect
Guest

Posted: Thu May 23, 2019 8:02 am Post subject: 2 alternators and 3 questions

 	  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 . . .

roughleg(at)gmail.com
Guest

Posted: Thu May 30, 2019 2:55 pm Post subject: 2 alternators and 3 questions

Bob, Joe, and Art: I have absorbed your comments and revised my loads analysis (attached).

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)

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.
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. 
No e-bus.
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. 
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 

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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