Matronics Email Lists :: View topic - Z13/8 Battery Contactor Failure

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

Z13/8 Battery Contactor Failure

Matronics Email Lists Forum Index -> AeroElectric-List

View previous topic :: View next topic

Author Message

jpx(at)Qenesis.com
Guest

Posted: Mon Nov 24, 2008 7:36 pm Post subject: Z13/8 Battery Contactor Failure

A friend of mine noticed something when looking at my wiring diagram tonight.



If the battery contactor opens in flight, say due to the internal coil  

failing, or the control wiring open circuiting, then the alternator  

will be alive, but without the battery to stabilize the voltage.



That sounds to me like a bad thing.  I expect I would know about it  

immediately, due to a buzz in the intercom, but would I (or my wife)  

recognize the fault in time to shut of the main switch before damage  

is done ?



Will the overvoltage circuitry react well to this scenareo and open  

the field drive soon enough ?



Obviously many aircraft are wired this way, and presumably, this is an  

unlikely fault, but ...

Is there a way to improve on the design ?



Thanks !



Jeff Page

Dream Aircraft Tundra #10

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Tue Nov 25, 2008 8:20 am Post subject: Re: Z13/8 Battery Contactor Failure

 	  Quote: 	
		  Obviously many aircraft are wired this way, and presumably, this is an 

unlikely fault, but ...Is there a way to improve on the design ? 	




Jeff,



Racecars use a mechanical battery switch. The FAA demands a "one-hand operable battery disconnect and this complies.  Even though this may be difficult to arrange compared to a contactor, it is probably worth the effort. (Google: Flaming River Battery Switch). 



"The problem with the world is that only the intelligent people want to be 

  smarter, and only the good people want to improve." 

             - Eolake Stobblehouse

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

nuckolls.bob(at)cox.net
Guest

Posted: Tue Nov 25, 2008 9:29 am Post subject: Z13/8 Battery Contactor Failure

At 09:35 PM 11/24/2008, you wrote:

 	  Quote: 	
		  



A friend of mine noticed something when looking at my wiring diagram tonight.



If the battery contactor opens in flight, say due to the internal coil

failing, or the control wiring open circuiting, then the alternator

will be alive, but without the battery to stabilize the voltage.


	


    When I revise the chapters on alternators and regulators, I'm

    going to have to revise advice based on to-long and deeply-held

    beliefs that the battery carries a major role in voltage stabilization

    for a battery/alternator power system.



    There IS a role for the battery but it's a more of a firewall

    against (1) system collapse under large transient loads that cause

    the alternator to "stall" and (2) mitigation of surge voltage

    amplitude during large load dumps. This is pretty easy to

    understand:



    Consider that a battery needs 14.0 volts or so to take significant

    charge. It delivers energy at 12.5 volts and below. So if the bus

    is wiggling around between these two values (read alternator

    ripple voltage) how much can the battery participate in smoothing

    this perturbation?  Not much. Battery ability to 'stabilize'

    bus perturbations within the bounds of 12.5 to 14.0 volts is

    nil.

 	  Quote: 	
		  That sounds to me like a bad thing.  I expect I would know about it

immediately, due to a buzz in the intercom, but would I (or my wife)

recognize the fault in time to shut of the main switch before damage

is done ?


	


    Loss of battery is not a recipe for over-stressed electro-

    whizzies . . .



 	  Quote: 	
		  Will the overvoltage circuitry react well to this scenareo and open

the field drive soon enough ?


	


    Any source of 3-phase power rectified to produce DC power

    has an inherent "ripple" or noise component on the order

    of 5% of the DC component. A 14v alternator may be expected

    to have a strong ripple voltage of 0.7 volts with lower energy

    but higher voltage components consisting of harmonics and diode

    switching transients.



    The bible for crafting DC power generation systems for vehicles

    is Mil-Std-704 which allows 3 volts pk-pk noise on 28v systems,

    1.5 volts pk-pk noise on 14v systems. This is allowed because it

    is EXPECTED from a normally operating alternator or generator. See

    Figure 15 of . . .



http://www.aeroelectric.com/Reference_Docs/Mil-Specs/Mil-Std-704_excerpts.pdf



    Okay, take these values a compare them with the battery's ability

    to accept or deliver energy. That 1.5 volt, 'window' of compliance

    described above is almost exactly the same as an expected pk-pk

    noise value delivered by the alternator.



    Here's a couple of 'scope traces taken from my dearly departed

    Safari van a couple of years ago . . .



http://www.aeroelectric.com/Pictures/Curves/Safari_Bus_Noise_1.gif



http://www.aeroelectric.com/Pictures/Curves/Safari_Bus_Noise_2.gif



    The trash on the bus is in keeping with expected performance

    of the alternator as a potential noise source and the battery

    as a wish-it-were-a-better-filter.



 	  Quote: 	
		  Obviously many aircraft are wired this way, and presumably, this is an

unlikely fault, but ...

Is there a way to improve on the design ?


	


    Doesn't need it. Electro-whizzies for aircraft are EXPECTED

    to perform as advertised when powered from systems not unlike

    that measured in the traces above and quantified in mil-std-704.



    There are certified aircraft (including A36 Bonanza and B58

    Baron) that have independent alternator and battery switches.

    The alternators on these aircraft will come on line without

    benefit of external battery. Further, the noise on the bus

    is affected very little by the presence of a battery. Operation

    without battery is not prohibited in the AFM.



    Short answer: Don't loose any sleep over it.



    Bob . . .

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

Display posts from previous:

	Matronics Email Lists Forum Index -> AeroElectric-List	All times are GMT - 8 Hours
Page 1 of 1

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