Matronics Email Lists :: View topic - Why have a switch on ALT Field?

Matronics Email Lists
Web Forum Interface to the Matronics Email Lists

Get Email Distribution Too! FAQ Search Memberlist Usergroups Register

Profile Log in to check your private messages Log in

Why have a switch on ALT Field?
Goto page 1, 2 Next

Matronics Email Lists Forum Index -> AeroElectric-List

View previous topic :: View next topic

Author Message

rvg8tor

Joined: 06 Apr 2009
Posts: 50
Location: Olympia, WA

Posted: Fri Sep 25, 2009 7:22 am Post subject: Why have a switch on ALT Field?

I am brain storming here while I work on my electrical system design (Z12). I wonder why there is a switch for the Alt Field. From my reading on the forums internally regulated alternators get their life shortened if one turns them on under load, (they should be on before start until after start to get the most life out of the alternator). The B&C alternator is externally regulated and can handle the switching under load. But operationally there is no reason to switch the alternator on and off, if you have an abnormal situation and need to turn the alternator off you can do that just as easily with a circuit breaker as you can with a switch. So why not leave the switch out of the loop and wire the alternator field directly to circuit breaker so it is always on, in case of alternator problem, pull circuit breaker. On less switch a few ounces and dollars saved. So, am I missing something obvious here?



I have posted this on  VAF, but thought since this forum is more on point I should post it here.

_________________
Mike "Nemo" Elliott
RV-8A QB (Engine)
www.mykitlog.com/rvg8tor

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Sep 25, 2009 11:16 am Post subject: Why have a switch on ALT Field?

At 10:22 AM 9/25/2009, you wrote:




I am brain storming here while I work on my electrical system design 

(Z12). I wonder why there is a switch for the Alt Field. From my 

reading on the forums internally regulated alternators get their life 

shortened if one turns them on under load, (they should be on before 

start until after start to get the most life out of the alternator).



Bob Nuckolls replies:



    An urban (or perhaps hangar) legend. I've been

    working with alternators and generators for 40+

    years.  There is no basis in physics to support

    the legend. In fact, it has always been a design

    goal of every TC aircraft electrical system to

    offer any time, any conditions, positive ON/OFF

    control of alternators and/or generators without

    concern for degradation of performance or damage

    to equipment.



The B&C alternator is externally regulated and can handle the 

switching under load.



    As can ALL other skillfully designed alternators. One

    of the foundations upon which the hangar-myth was

    built had to do with alternators ostensibly

    wired per Z-24 . . .



http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z24-Interim.pdf



    . . . and apparently switched off (B-lead

    broken) while the alternator was under load.



    This started a hysterical kerfuffle about "turning

    alternators on or off under load". Numerous

    authors have posited the same hypothesis before.

    However, the special case presented by a b-lead disconnect

    contactor energized efforts by the in-experienced

    and un-informed to stoke the fires anew and stir

    the pot of misunderstanding.



    I enjoyed a two-day tour of a LARGE scale alternator

    remanufacturing operation about a year ago. Details

    of this visit can be discovered at:



http://aeroelectric.com/R12A/03_Alternator_12A1.pdf



    During that visit I witnessed a demonstration

    of a full load, max rpm, hot-alternator b-lead

    disconnect that liberated kilojoules of energy

    in the ensuing arc . . . 5 times in a row. The

    flash of fire was so great that it was impossible

    to photograph or record on video. Their chief engineer

    asserted that all of their reman products (2800+

    line items) were tested to the same design goals.

    These alternators were demonstrably immune from

    self-induced load dump damage.



But operationally there is no reason to switch the alternator on and 

off, if you have an abnormal situation and need to turn the 

alternator off you can do that just as easily with a circuit breaker 

as you can with a switch.



    Suppose you have two alternators like Z-12 or

    Z-13? Is it not useful to test the two alternators

    independently of each other irrespective of all

    other conditions?



    Suppose you have smoke in the cockpit? How

    about an unruly regulator? Our brothers in

    the TC aircraft world have adopted any-time,

    any-conditions, ON/OFF control as a design

    goal.



  So why not leave the switch out of the loop and wire the alternator 

field directly to circuit breaker so it is always on, in case of 

alternator problem, pull circuit breaker. On less switch a few ounces 

and dollars saved. So, am I missing something obvious here?



    Perhaps not obvious . . . but certainly not

    'secret'. Any student of contact physics for

    things like relays, switches and CIRCUIT BREAKERS

    will understand that depending on a breaker

    like . . .



http://aeroelectric.com/Pictures/Breakers/Klixon_1.jpg



    . . . to open the 100 volts plus, b-lead of a runaway

    alternator is quite likely to experience a hundreds-of-

    kilojoules arc in the gap of relatively slow moving

    contacts that are guaranteed to produce a lot of smoke

    in the cockpit. BREAKERS are not intended to serve

    as commanding manual control devices. Breakers for

    aircraft are designed to break faults of several

    thousand amps at 32 VOLTS or less. See paragraph

    4.7.14 of . . .



http://www.aeroelectric.com/Reference_Docs/Mil-Specs/5809G.pdf



    Since the first generators (and then alternators) went

    aboard an airplane, the pilot has been offered positive

    control of those electrical energy sources by breaking the

    FIELD lead.  At no time did anyone consider the main power

    output feeder protection to be a control device. In fact,

    b-lead protection on TC aircraft isn't even on the panel . . .



http://aeroelectric.com/Pictures/Wiring_Technique/A36_Firewall_A.jpg



I have posted this on  VAF, but thought since this forum is more on 

point I should post it here.



    Please post this reply to VAF. Thanks!



    Bob . . .

rvg8tor

Joined: 06 Apr 2009
Posts: 50
Location: Olympia, WA

Posted: Fri Sep 25, 2009 5:47 pm Post subject: Re: Why have a switch on ALT Field?

Bob,



Thank you for the explanation, this really helps me understand the design better.  I did post your reply on the VAF for all to read.  I think this will add to everyones education.



So if I understand what your post, the gist is that there is potential for excess voltage in some situations as to make a CB fail if used to cut the field to the alternator.  I have heard others say CBs are not meant to be switches, but at my airline we routinely pull CB on certain items while the airplane sits between flights for several hours. Things like the radar, TCAS, VSI indicator.  This use of the CB as a switch happens several times a day for each aircraft.  It this an accepted practice since these are low voltage items.



You have given me the data i was looking for, I now understand why the switch is needed, it is due to potential high loads.

_________________
Mike "Nemo" Elliott
RV-8A QB (Engine)
www.mykitlog.com/rvg8tor

bakerocb

Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA

Posted: Sat Sep 26, 2009 3:57 am Post subject: Why have a switch on ALT Field?

9/26/2009



Hello Mike and Bob, The recent exchange of postings between the two of you 

on the subject of      "Why have a switch on ALT Field?" has left me a bit 

baffled.



Mike, your question dealt with shutting down an alternator by removing the 

electrical source to its FIELD. Figure Z 12 was the starting point for your 

question and presumably dealt with externally regulated alternators, but 

then you threw in the red herring of shutting down internally regulated 

alternators.



Bob, your answer appeared to deal extensively with shutting down an 

internally regulated alternator by opening the B LEAD such as in Figure Z 

24..



Mike, your response below indicated that you believed, as a result of Bob's 

posting, that there was a huge amount of electricity flowing through the 

wire to the alternator FIELD circuit in an externally regulated alternator 

and that therefore a switch rather than a circuit breaker would be needed to 

open that circuit.



"You have given me the data i was looking for, I now understand why the 

switch is

needed, it is due to potential high loads."



It is my understanding that:



1) The alternator FIELD wire in an externally regulated alternator only 

carries a few amps of electricity.



2) Opening this FIELD wire with either a CB or a switch while electricity is 

flowing through this wire generates no huge electrical flow interruption 

issue.



3) The alternator B LEAD (in either an internally regulated or externally 

regulated alternator) can carry a significant amount of electricity and 

interrupting this flow through the B LEAD by opening the B LEAD brings up 

the issues that Bob dealt with extensively in his response.



Mike if you will check some of the Z diagrams (Z 9 as well as Z 12 for 

example) you will see that there is both a CB and a switch that can 

interrupt the electrical flow to the alternator FIELD of an externally 

regulated alternator. And this is good. The CB provides the traditional 

protection to that wiring and the switch makes it possible to turn on / off 

the alternator FIELD as desired with no great concern regarding interrupting 

huge amperage flow.



My airplane has both a CB and a switch (as do the Z figures) and the 

externally regulated alternator FIELD gets turned off by the switch during 

every engine shut down procedure while the engine is still running. Shortly 

after the alternator is turned off by the FIELD switch the low voltage light 

begins to flash telling me that that warning indication is working. I then 

turn on my essential bus** and then turn off the battery master and the 

radio master (yes I have one ##) to ensure that my Garmin 430W and my 

transponder continue to operate off the battery which is now feeding the 

avionics essential bus. The Garmin 430W and the transponder are essential 

for me to return to my home field inside the Washington DC SFRA.



Please let me know if I have confused or clarified the issue raised by 

Mike's original question.



'OC' Says: "The best investment we can make is the effort to gather and 

understand knowledge."

**PS: Yes I am aware that there are means of causing this to happen 

automatically with a diode, but the guy who wired my panel did not do it 

that way and I have been living very happily with what he provided because I 

understand the system.



##PS: Again this was given to me (after I paid big money to have the panel 

built), but I have access to the avionics that I need via an essential 

avionics bus so I have been living very happily with what he provided 

because I understand the system.



=================================================

Time: 06:49:13 PM PST US

Subject: Re: Why have a switch on ALT Field?

From: "rvg8tor" <rvg8tor(at)comcast.net>

Bob,



Thank you for the explanation, this really helps me understand the design 

better.

I did post your reply on the VAF for all to read.  I think this will add to

everyones education.



So if I understand what your post, the gist is that there is potential for 

excess

voltage in some situations as to make a CB fail if used to cut the field to

the alternator.  I have heard others say CBs are not meant to be switches, 

but

at my airline we routinely pull CB on certain items while the airplane sits

between flights for several hours. Things like the radar, TCAS, VSI 

indicator.

This use of the CB as a switch happens several times a day for each 

aircraft.

It this an accepted practice since these are low voltage items.



You have given me the data i was looking for, I now understand why the 

switch is

needed, it is due to potential high loads.



--------

Mike &quot;Nemo&quot; Elliott

RV-8A QB (Fuselage)

nuckolls.bob(at)aeroelect
Guest

Posted: Sat Sep 26, 2009 8:40 am Post subject: Why have a switch on ALT Field?

At 08:47 PM 9/25/2009, you wrote:




Bob,



Thank you for the explanation, this really helps me understand the 

design better.  I did post your reply on the VAF for all to read.  I 

think this will add to everyone's education.



So if I understand what your post, the gist is that there is 

potential for excess voltage in some situations as to make a CB fail 

if used to cut the field to the alternator.



     That's not the bottom line . . .



     We should start with some design goals in mind.

     For all of my career in TC and OBAM aircraft,

     I've embraced and designed systems that meet

     traditional goals for any time, any conditions,

     On-0ff control of the engine driven power source(s).



I have heard others say CBs are not meant to be switches, but at my 

airline we routinely pull CB on certain items while the airplane sits 

between flights for several hours. Things like the radar, TCAS, VSI 

indicator.  This use of the CB as a switch happens several times a 

day for each aircraft.  It this an accepted practice since these are 

low voltage items.



     This isn't about using breakers as switches, it's

     about meeting the design goal cited above. The b-lead

     breaker on an alternator COULD be used to disconnect

     the alternator assuming (A) the alternator is under

     control [producing 14 or so volts] and (B) the

     breaker is on the panel and pullable [many TC aircraft

     have push-only breakers]. The classical control philosophy

     for engine driven power sources has been a SWITCH in

     reach of the crew that offers control over the field

     excitation . . . always a low current, easy-to-manage

     task. This task COULD be managed by flipping a switch

     -OR- pulling the field supply breaker . . . it's your

     choice.



     There are other legacy design goals . . . like automatic

     and absolute control of the alternator in case of

     a failed regulator in an OV runaway condition.

     Figure Z-24 is but one of several recipes for success.

     When Z-24 was combined with alternators NOT designed

     and tested to withstand their own load-dumps that some

     builders experienced alternator failures immediately

     after turning them off while under load.



You have given me the data I was looking for, I now understand

why the switch is needed, it is due to potential high loads.



     No . . . its about dependance on pulling

     of a b-lead breaker for the purpose of taking

     a runaway alternator off line. This is a shaky

     design goal due to the fact that a failed alternator

     can deliver 100-200 volts in a runaway condition.

     Attempting to do this reliably with a 32-volt rated,

     pullable breaker would very likely fail the breaker.



     Further, pulling a b-lead breaker to mitigate an

     ov condition puts human perception, decision, reaction

     delays into the shutdown event. That's never been

     embraced by the TC aircraft world that I'm aware

     of.



     The issues before us are (1) design goals for

     control (2) design goals for automatic millisecond-

     fast mitigation of an OV event. You can pull a

     field breaker for (1) if you wish . . . but NOT

     a b-lead breaker for (2).



     Bob . . .

rvg8tor

Joined: 06 Apr 2009
Posts: 50
Location: Olympia, WA

Posted: Sat Sep 26, 2009 9:17 am Post subject: Re: Why have a switch on ALT Field?

My design goal is a system that is simple as possible but still meets capacity and redundancy goals (glass IFR bird).  Thankfully I found Z12 and this design suits my need.  



My assumption not mentioned by me so far, is that I will have an alternator that will have a controllable filed with OV protection, be that with internal or external regulation.  I never thought of controlling the alternator with the "B" lead.  So just looking at the Z12 diagram I pondered whether I needed the switch for the alternator field, I was looking for way to simplify where I could.  



My CB panel will all be in easy reach by my right leg, the CB layout has 4 breakers at the end of two rows to make then stand out from the rest, I also plan to have them with colored collars, this would make finding them in the dark easy.  Two at the end of the top row are for the primary and standby alternator fields.  Two at the end of the next row control 12V power to the L/R P-mags respectfully.  Also the two rows are not the same length, this also allows for identification in the dark.  Top row is shorter, last two breakers are MAIN ALT then STBY ALT, easy to remember easy to find.



I know a toggle switch will be faster to actuate than finding a breaker but not by much.  So would elimination of the switch change the design goal of Z12, would I not still have a controllable engine driven power source?  I guess from what I have read so far the answer is no.  Even though the switch will be used very little, it is still required?

_________________
Mike "Nemo" Elliott
RV-8A QB (Engine)
www.mykitlog.com/rvg8tor

ianxbrown

Joined: 16 May 2009
Posts: 80

Posted: Sat Sep 26, 2009 9:45 am Post subject: Why have a switch on ALT Field?

OK, now here's one more vote for "confused".   I thought the circuit breaker was supposed to be rated at the output amperage of the alternator because it's that current that one wants to limit.   My 40A circuit breaker would do nothing for me on the alternator field circuit.

 

 As I understand it, the switch is on the alternator field low-current 12V line (called IGN on my alternator) and the circuit breaker protects the output side (B lead).   Mine is wired this way because it made sense.

 

 Ian Brown,

 RV-9A, Bromont, QC

 

 On Sat, 2009-09-26 at 07:53 -0400, bakerocb(at)cox.net wrote:  	  Quote: 	
		    	  Quote: 	
		  

--> AeroElectric-List message posted by: <bakerocb(at)cox.net (bakerocb(at)cox.net)>



9/26/2009



Hello Mike and Bob, The recent exchange of postings between the two of you 

on the subject of      "Why have a switch on ALT Field?" has left me a bit 

baffled.



Mike, your question dealt with shutting down an alternator by removing the 

electrical source to its FIELD. Figure Z 12 was the starting point for your 

question and presumably dealt with externally regulated alternators, but 

then you threw in the red herring of shutting down internally regulated 

alternators.



Bob, your answer appeared to deal extensively with shutting down an 

internally regulated alternator by opening the B LEAD such as in Figure Z 

24..



Mike, your response below indicated that you believed, as a result of Bob's 

posting, that there was a huge amount of electricity flowing through the 

wire to the alternator FIELD circuit in an externally regulated alternator 

and that therefore a switch rather than a circuit breaker would be needed to 

open that circuit.



"You have given me the data i was looking for, I now understand why the 

switch is

needed, it is due to potential high loads."



It is my understanding that:



1) The alternator FIELD wire in an externally regulated alternator only 

carries a few amps of electricity.



2) Opening this FIELD wire with either a CB or a switch while electricity is 

flowing through this wire generates no huge electrical flow interruption 

issue.



3) The alternator B LEAD (in either an internally regulated or externally 

regulated alternator) can carry a significant amount of electricity and 

interrupting this flow through the B LEAD by opening the B LEAD brings up 

the issues that Bob dealt with extensively in his response.



Mike if you will check some of the Z diagrams (Z 9 as well as Z 12 for 

example) you will see that there is both a CB and a switch that can 

interrupt the electrical flow to the alternator FIELD of an externally 

regulated alternator. And this is good. The CB provides the traditional 

protection to that wiring and the switch makes it possible to turn on / off 

the alternator FIELD as desired with no great concern regarding interrupting 

huge amperage flow.



My airplane has both a CB and a switch (as do the Z figures) and the 

externally regulated alternator FIELD gets turned off by the switch during 

every engine shut down procedure while the engine is still running. Shortly 

after the alternator is turned off by the FIELD switch the low voltage light 

begins to flash telling me that that warning indication is working. I then 

turn on my essential bus** and then turn off the battery master and the 

radio master (yes I have one ##) to ensure that my Garmin 430W and my 

transponder continue to operate off the battery which is now feeding the 

avionics essential bus. The Garmin 430W and the transponder are essential 

for me to return to my home field inside the Washington DC SFRA.



Please let me know if I have confused or clarified the issue raised by 

Mike's original question.



'OC' Says: "The best investment we can make is the effort to gather and 

understand knowledge."

**PS: Yes I am aware that there are means of causing this to happen 

automatically with a diode, but the guy who wired my panel did not do it 

that way and I have been living very happily with what he provided because I 

understand the system.



##PS: Again this was given to me (after I paid big money to have the panel 

built), but I have access to the avionics that I need via an essential 

avionics bus so I have been living very happily with what he provided 

because I understand the system.



=================================================

Time: 06:49:13 PM PST US

Subject: Re: Why have a switch on ALT Field?

From: "rvg8tor" <rvg8tor(at)comcast.net (rvg8tor(at)comcast.net)>

Bob,



Thank you for the explanation, this really helps me understand the design 

better.

I did post your reply on the VAF for all to read.  I think this will add to

everyones education.



So if I understand what your post, the gist is that there is potential for 

excess

voltage in some situations as to make a CB fail if used to cut the field to

the alternator.  I have heard others say CBs are not meant to be switches, 

but

at my airline we routinely pull CB on certain items while the airplane sits

between flights for several hours. Things like the radar, TCAS, VSI 

indicator.

This use of the CB as a switch happens several times a day for each 

aircraft.

It this an accepted practice since these are low voltage items.



You have given me the data i was looking for, I now understand why the 

switch is

needed, it is due to potential high loads.



--------

Mike &quot;Nemo&quot; Elliott

RV-8A QB (Fuselage)

nuckolls.bob(at)aeroelect
Guest

Posted: Sat Sep 26, 2009 9:53 am Post subject: Why have a switch on ALT Field?

At 06:53 AM 9/26/2009, you wrote:




   <Snip>



Please let me know if I have confused or clarified the issue raised 

by Mike's original question.



   Your words appear to paint a clear understanding . . .



   The problem with this (and many threads of similar discussion

   in the past) is the stirring of a LOT of simple-ideas (facts)

   into one pot of ill-conceived stew (no definition of design

   goals) and stirred over a hot fire ("don't do that or you're

   gonna die") with the unbridled enthusiasm (mis-interpretation

   of words).



'OC' Says: "The best investment we can make is the effort to gather 

and understand knowledge."



   Yup, you got that right.  I notice that my response to Mike's

   follow-up hasn't passed through the server yet . . . but

   I hope it proves to be a useful clarification of how we . . .



   (1) start with design goals . . .



   (2) then craft an architecture that addresses design

       goals while . . .



   (3) eliminating single points of failure for extra ordinarily

       useful electro-whizzies as revealed by skillfully

       conducted FMEA.



   (4) Study and understand the specs for ingredients that go

       into recipes for success.



   (5) Select style, quality and a minimized quantity of components

       that go into the proposed recipe.



   (6) Verify that the recipe meets design goals.



   (7) Analyze subsequent failures to deduce root cause.

       Wear out? Mis-application? Robustness? Quality?

       Error of stating or conformance to design goals?

       Other? Answers to these questions may prompt a program

       review that goes all the way back to step (1).



    When one makes a career of this process, you eventually

    discover that it's a Mobius Loop which strives for archiving

    a host of lessons-learned while improving upon the best we

    know how to do. This is the essence of good design engineering

    as I've come to understand it.



    Its interesting but frustrating that there are so many

    pitfalls that put the 7-step process at risk. We're

    fortunate that until recently, loss of the entire

    electrical system was unlikely to force an airplane

    down.



    Now that we've embraced performance advantages of modern

    electronics to control engines, there's a pressing

    need to craft an architecture with particular attention

    to step (3). This is relatively easy to do if the

    seven steps are dutifully observed . . . but it's

    easy to be distracted from the process if

    excited rhetoric borne out of misunderstanding

    dominates the tenor of the discussion.



    So while we "gather and understand knowledge" (become

    designers) we  must be ready to calmly defuse

    excitement with lucid explanation of simple-ideas

    and how they become ingredients that go into recipes

    for success (become teachers).



    Bob . . .

rvg8tor

Joined: 06 Apr 2009
Posts: 50
Location: Olympia, WA

Posted: Sat Sep 26, 2009 11:14 pm Post subject: Re: Why have a switch on ALT Field?

Bob,



I know you are trying to help but, this last post does not seem to help me, it actually confuses me.  I  respect your experience but I am not trying to design an electrical system from scratch, much like I did not design my RV-8A from scratch.  I picked a plane from a good designer (Van's).  While building I study the plans and try to understand how the parts relate to be a better builder, if I have a question I call builder assistance and I ask questions on the forum.  



When it came to the electrical system, I went with a design from an expert in the field, in studying the design I asked a simple question, 

'why is the alternator field switch needed". There have been a lot of words written, much of which my little brain doesn'tunderstand how it relates to the very simple question.  The possible answers I was expecting were:



1. It has always been done that way

2. It doesn't matter the non-switch method will work but has these limitations (fill in the blanks)

3.  You need the switch because if you don't have one it will cause XYZ.



Some of what you wrote I get but I really just need the simple short answer, not a run down on design philosophy.  Since you designed the systems I thought of all people you could provide the best explanation.  I thank you for attempting to answer my questions, if nothing else you have convinced me not to deviate from the Z diagram.  I can't say that I understand fully why the alternator field switch is a requirement other than to provide a convenient way to turn off the field.  I look forward to the simple answer if there is one.

_________________
Mike "Nemo" Elliott
RV-8A QB (Engine)
www.mykitlog.com/rvg8tor

jay(at)horriblehyde.com
Guest

Posted: Sun Sep 27, 2009 2:16 am Post subject: Why have a switch on ALT Field?

Hi there Mike,



Perhaps I can shed a bit of light on your questions.  The switch is used as

it switches a smaller current than would be required on the B-lead; thus you

can use a switch and not a contactor.  By switching off the field you have

effectively switched off the alternator in its entirety, not just its

output.

Now its just a spinning mechanical device.  If you switch the output (B

lead) then you have a live spinning device and if the fault is inside the

alternator then you're still feeding it...

Utilising the field to switch the alternator also allows you to fit in

overvoltage protection that will quickly detect and clear any overvoltage

fault that would fry all your avionics leaving you with a damaged alternator

AND no instruments (electrically powered ones in any case); whereas, if the

overvoltage fault is detected and cleared you can still run the instruments

on the battery.

If you have an internally regulated alternator then you need to switch the

output by using a contactor close to the alternator, because you cannot

access the field winding wire.  Protecting the alternator by using a panel

mounted CB means that you lead a fat wire into the cockpit to the breaker in

your panel, which in turn means that there is a long bit of fat wire that is

unprotected- that long bit from the alternator to the CB.  Personally I

don't like that.  

If you use a contactor that is close to the source (alternator or battery)

then the only unprotected bit of fat wire is that bit between the source and

the contactor.  A short bit of fat wire is unlikely to fault, or has a

higher reliability because it is short and mechanically robust.  The closer

a contactor is to the voltage source the better your chances of removing the

power source from a fault.



Jay



--

rckol

Joined: 14 Nov 2008
Posts: 36

Posted: Sun Sep 27, 2009 5:41 am Post subject: Re: Why have a switch on ALT Field?

Mike,



Here is a response without embellishment. 



*************************



snip [You need the alternator field switch because there is no alternator field breaker in Z-12.  A fuse is used instead.  ]



***************************

The statement above is false.  I should have consulted the diagram instead of going from memory.  Items 1 and 2 below are still valid.



You could use the breaker as a switch , but I wouldn't because:



1. You will be switching the field on and off at least once on every flight and a normal breaker is not designed to be a robust switch.



2. You could use a "switch-breaker", but I have heard so many stories about these having short life spans (at least the reasonably priced ones) that I would not put one in my aircraft.



Regards

_________________
rck

Last edited by rckol on Sun Sep 27, 2009 6:23 am; edited 1 time in total

bakerocb

Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA

Posted: Sun Sep 27, 2009 5:50 am Post subject: Why have a switch on ALT Field?

9/27/209



Hello Ian, Thanks for reading my post and responding. Please let me make an 

attempt at reducing your confusion. You wrote:



1) "As I understand it, the switch is on the alternator field low-current 

12V line (called IGN on my alternator) ........"



Can you both turn ON (activate) and turn OFF (deactivate) your alternator's 

electrical output with this alternator field switch?** This is generally 

considered a desireable design goal.



There are some internally regulated alternators that, once activated, draw 

their field current from inside the alternator and it is not possible to 

deactivate those alternator's electrical output by turning OFF an external 

field switch. While this subject has been hotly debated several times on the 

aeroelectric list it is generally considered that the feature of not being 

able to shut down the alternator's electrical output with an external field 

switch is an undesirable design goal.



2) "..........the circuit breaker protects the output side (B lead)."



The primary purpose of a CB or fuse in the B lead is to protect the wiring 

in the B lead. This wiring normally goes from the positive terminal of the 

battery to a main battery contactor and then onto the B output terminal of 

the alternator. If this wiring should encounter a short to ground the 

BATTERY could dump a huge amount of amperage into the wiring and cause a 

fire. Therefore the primary  purpose of the CB or fuse in the B lead wiring 

is to protect the wiring from BATTERY electrical output.



But since there are alternators that could "run away" and not be shut down 

by a switch in the circuit providing electricity to the alternator's field 

some people who have those kinds of alternators look upon the CB in the B 

lead as a means of cutting off the excess electrical flow from the 

alternator.



Depending upon the manual pulling of a circuit breaker in the B lead as the 

only means of terminating an over voltage situation or a "run away" 

alternator is generally not considered a desirable design goal.



Does this help?



'OC' Says: "The best investment we can make is the effort to gather and 

understand knowledge."



**PS: I'd be interested in knowing how your alternator field switch does 

work and if you have an externally or internally regulated alternator?



=======================================================



Time: 10:45:24 AM PST US

Subject: Re: Why have a switch on ALT Field?

From: Ian <ixb(at)videotron.ca>



OK, now here's one more vote for "confused".   I thought the circuit

breaker was supposed to be rated at the output amperage of the

alternator because it's that current that one wants to limit.   My 40A

circuit breaker would do nothing for me on the alternator field circuit.



As I understand it, the switch is on the alternator field low-current

12V line (called IGN on my alternator) and the circuit breaker protects

the output side (B lead).   Mine is wired this way because it made

sense.



Ian Brown,

RV-9A, Bromont, QC

user9253

Joined: 28 Mar 2008
Posts: 1938
Location: Riley TWP Michigan

Posted: Sun Sep 27, 2009 7:07 am Post subject: Why have a switch on ALT Field?

This is the way I understand it.   Please correct me if I am wrong. 

  The  alternator output current does not need to be limited by a circuit breaker  because the alternator output is self-limiting.  If one were to add more loads to a fully  loaded alternator, the voltage would drop, thus limiting the current  automatically.  It is not desired to  unnecessarily run the alternator B lead into the cockpit.  The B-lead circuit breaker is not  necessary, and adds weight, cost and complexity.  The purpose of an ANL fuse on the  alternator output is not to protect the alternator or to prevent it from  producing more than its rated current.   The purpose of the ANL fuse is to protect the wires between the battery  and alternator from 400+ amps of battery current in the event that the  alternator shorts out, most likely from shorted diodes. 

   In  the event that an over-voltage condition occurs (due to a faulty regulator), an  over-voltage module will short out the supply voltage that goes to the  alternator field.  If that happens,  an alternator-field circuit breaker will trip, thus protecting the wires from  excessive current resulting from this short circuit. 

Joe

   [quote][b]

_________________
Joe Gores

Kellym

Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

Posted: Sun Sep 27, 2009 7:57 am Post subject: Why have a switch on ALT Field?

rckol wrote:

 	  Quote: 	
		   

 



 1. You will be switching the field on and off at least once on every flight and a normal breaker is not designed to be a robust switch.

Why?? Is there a need to turn the alternator on/off separate from the 
	


master if it is functioning correctly?



 	  Quote: 	
		   2. You could use a "switch-breaker", but I have heard so many stories about these having short life spans (at least the reasonably priced ones) that I would not put one in my aircraft.

The Klixon switch breakers in my Mooney are almost all OEM from 45 
	


years/6000 hours ago. A couple have been replaced from springs getting weak.

_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

tonybabb(at)alejandra.net
Guest

Posted: Sun Sep 27, 2009 10:27 am Post subject: Why have a switch on ALT Field?

Jay,



Thank you so much. Spelling out what may be obvious or intuitive to

experienced hands is a great assistance to us neophyte electron herders.



Tony

Velocity SEFG 62% done, 78% to go

www.alejandra.net/velocity 



--

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Sep 27, 2009 1:18 pm Post subject: Why have a switch on ALT Field?

At 09:58 AM 9/27/2009, you wrote:



  	  Quote: 	
		   This is the way I understand it.  Please correct me if I am wrong.

 

   The alternator output current does not need to be limited by a circuit breaker because the alternator output is self-limiting. 	


    correct



  	  Quote: 	
		     If one were to add more loads to a fully loaded alternator, the voltage would drop, thus limiting the current automatically. 	


     yes



  	  Quote: 	
		     It is not desired to unnecessarily run the alternator B lead into the cockpit. 	


     yes



  	  Quote: 	
		     The B-lead circuit breaker is not necessary, and adds weight, cost and complexity.

  The purpose of an ANL fuse on the alternator output is not to protect the alternator or to prevent it from producing more than its rated current.  The purpose of the ANL fuse is to protect the wires between the battery and alternator from 400+ amps of battery current in the event that the alternator shorts out, most likely from shorted diodes.

 	
    Yup . . .



 

  	  Quote: 	
		      In the event that an over-voltage condition occurs (due to a faulty regulator), an over-voltage module will short out the supply voltage that goes to the alternator field.  If that happens, an alternator-field circuit breaker will trip, thus protecting the wires from excessive current resulting from this short circuit. 	


    Yes . . . this is a property unique to the

    crowbar philosophy for opening the alternator's

    field supply after an ov condition is detected.

    There are other designs that do not open the

    field supply breaker . . . and can be considered

    as a potential player in your design goals.



    The reason that Z-figures featuring crowbar

    ov protection -AND- fuseblocks have a field

    supply breaker is to accommodate the crowbar

    ov control philosophy. If non-crowbar shutdown

    products are considered, then one could easily

    supply alternator field through one of the

    fuses and leave the circuit breaker out.



 

 

        Bob . . .



         ---------------------------------------

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

         ---------------------------------------



     [quote][b]

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Sep 27, 2009 1:26 pm Post subject: Why have a switch on ALT Field?

Why?? Is there a need to turn the alternator on/off separate from 

the master if it is functioning correctly?



   Depends on your architecture. Does the MASTER control

   the alternator too? Is the MASTER a progressive transfer

   two-pole switch or classic "split rocker". Then indeed,

   the alternator field breaker may never need to be touched.

   But if the MASTER is a simple two-pole switch that controls

   alternator and battery at the same time, then you could

   use the field breaker to shut the alternator down for

   battery-only ground-ops.

2. The Klixon switch breakers in my Mooney are almost all OEM from 45 

years/6000 hours ago. A couple have been replaced from springs getting weak.



   But the Potter-Brumfield W31 style breakers installed in

   many other airplanes for about the same length of time

   have been identified has having a safety of flight

   failure mode that is prompting an AD to replace about

   80,000 of the critters. Not all switch breakers are

   alike. All are much more expensive than the fuse/switch

   architecture featured in the Z-figures.

        Bob . . .



         ---------------------------------------

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

         ---------------------------------------

bakerocb

Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA

Posted: Mon Sep 28, 2009 5:35 am Post subject: Why have a switch on ALT Field?

9/28/2009



Hello Again Mike, You wrote: "I look forward to the simple answer if there 

is one."



Let's try this -- How do you intend to simulate or check the continued 

functioning of essential for flight avionics items when you are airborne and 

have a non functioning alternator because: the field wire broke,**  the belt 

broke, or whatever?



Here is my answer: At the end of each flight while the engine is still 

running I turn off the alternator by means of its field switch and ensure 

that:



A) The essential for flight avionics items (Garmin 430W and transponder) can 

still function off an essential bus. (These items needed to return to my 

home field inside the Washington DC SFRA.)



B) My low voltage warning system (a flashing light) is still functioning.



Discussion:



1) Why not perform check A above at the beginning of each flight before the 

engine is started and the alternator is not yet putting out current? 

Because: a) I do not have my Garmin 430W and transponder turned on prior to 

or during engine start, and b) I don't want to put the electrical burden of 

the Garmin 430W and transponder on the battery before engine start.



2) Why not perform check B above at the beginning of each flight before the 

engine is started and the alternator is not yet putting out current? That 

check is indeed accomplished as soon as the master battery contactor is 

closed at the beginning of the flight. The B check at the end of the flight 

is a duplication of the before flight B check and is just a system design 

fall out in order to perform check A.



3) Why not just use pulling the alternator field CB as a means of turning 

off the alternator at the end of each flight? I have not priced the cost 

differential between replacing my fancy, internally illuminated, rocker 

activated, alternator field  switch and the alternator field CB, but I do 

know this: a) The switch is designed to be switched on and off thousands of 

times, the CB is not, and b) It will be much easier for me to replace the 

switch than the CB.



So please let me know what you decide regarding whether or not you will 

install the alternator field switch and what were the deciding factors in 

that decision. Thanks.



'OC' Says: "The best investment we can make is the effort to gather and 

understand knowledge."



**PS: This happened to a friend of mine in his Beech Sierra just a few 

months ago. He has no low voltage warning system other than looking at an 

aftermarket automotive gadget voltmeter plugged into his cigarette 

lighter -- not very visible. His first real warning was when his VHF comm 

began to not work properly because the battery was becoming depleted. Soon 

he had no electronic nav functions either.



He was on an IFR flight plan, but in VMC. He spotted a little podunk airport 

and landed. A mechanic at the airport quickly diagnosed the problem, 

repaired the wire, and my friend was on his way shortly. But what if he has 

been in IMC? What if he had not spotted podunk airport? What if there was no 

repair person or facility at podunk airport?



Using your word "convenient" do you think that he would have had a 

convenient termination of that flight?



=====================================================

Time: 12:20:59 AM PST US

Subject: Re: Why have a switch on ALT Field?

From: "rvg8tor" <rvg8tor(at)comcast.net>

Bob,



I know you are trying to help but, this last post does not seem to help me, 

it

actually confuses me.  I  respect your experience but I am not trying to 

design

an electrical system from scratch, much like I did not design my RV-8A from

scratch.  I picked a plane from a good designer (Van's).  While building I 

study

the plans and try to understand how the parts relate to be a better builder,

if I have a question I call builder assistance and I ask questions on the 

forum.

When it came to the electrical system, I went with a design from an expert 

in the

field, in studying the design I asked a simple question,

'why is the alternator field switch needed". There have been a lot of words 

written,

much of which my little brain doesn'tunderstand how it relates to the very

simple question.  The possible answers I was expecting were:



1. It has always been done that way

2. It doesn't matter the non-switch method will work but has these 

limitations

(fill in the blanks)

3.  You need the switch because if you don't have one it will cause XYZ.



Some of what you wrote I get but I really just need the simple short answer, 

not

a run down on design philosophy.  Since you designed the systems I thought 

of

all people you could provide the best explanation.  I thank you for 

attempting

to answer my questions, if nothing else you have convinced me not to deviate

from the Z diagram.  I can't say that I understand fully why the alternator

field switch is a requirement other than to provide a convenient way to turn 

off

the field.  I look forward to the simple answer if there is one.



--------

Mike &quot;Nemo&quot; Elliott

RV-8A QB (Fuselage)

nuckolls.bob(at)aeroelect
Guest

Posted: Mon Sep 28, 2009 6:27 am Post subject: Why have a switch on ALT Field?

 	  Quote: 	
		  

When it came to the electrical system, I went with a design from an 

expert in the

field, in studying the design I asked a simple question,

'why is the alternator field switch needed". There have been a lot 

of words written,

much of which my little brain doesn'tunderstand how it relates to the very

simple question.


	


    I'm sorry if my words are too much for you.

    When I wear my teacher-hat, the answers

    I offer are intended to lead to understanding of

    the foundation upon which the answer is based.

    It's my belief that once understanding is

    achieved, the student then becomes a teacher

    on the same discussion. Perhaps I presume too

    much . . . that everyone here on the List is

    as enthusiastic about understanding as I  . . .

 	  Quote: 	
		     The possible answers I was expecting were:



1. It has always been done that way


	


    Okay, with my expert-hat on, that answer works

    for me.



    Fly comfortably my friend . . .

        Bob . . .



         ---------------------------------------

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

         ---------------------------------------

Neal.George(at)hurlburt.a
Guest

Posted: Mon Sep 28, 2009 7:55 am Post subject: Why have a switch on ALT Field?

I'm sorry if my words are too much for you.

    When I wear my teacher-hat, the answers

    I offer are intended to lead to understanding of

    the foundation upon which the answer is based.

    It's my belief that once understanding is

    achieved, the student then becomes a teacher

    on the same discussion. Perhaps I presume too

    much . . . that everyone here on the List is

    as enthusiastic about understanding as I  . . .

Keep at it Bob - 



I generally need to be told more than once.  

As for the presentation style, I learn more than electron herding here.  

Too many of us can barely speak, much less write... 



neal

smime.p7s

Description:

Download

Filename:

smime.p7s

Filesize:

5.04 KB

Downloaded:

463 Time(s)

Display posts from previous:

	Matronics Email Lists Forum Index -> AeroElectric-List	All times are GMT - 8 Hours Goto page 1, 2 Next
Page 1 of 2

Jump to:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum