banner
banner

28 Mar 2024, 10:10 [ UTC - 5; DST ]


Concorde Battery (banner)



Reply to topic  [ 5 posts ] 
Username Protected Message
 Post subject: Dual alternator emergency power connection to battery
PostPosted: 05 May 2022, 07:41 
Offline


 Profile




Joined: 01/10/17
Posts: 1658
Post Likes: +1124
Company: Skyhaven Airport Inc
Aircraft: various mid century
Twin Cessna 421 but would apply to other piston twins. Debate is if alternators are on and battery relay fails. If emergency field power is selected are the alternator outputs still disconnected from the battery. Will this destroy the alternator diodes?

I believe I see where the alternator regulator fuses allow output power through to a capacitor and diode but not sure. Related service kit for reference.
https://support.cessna.com/custsupt/con ... s_id=32787

Where this came from is debate if during avionics refits the avionics shops can replace the original red split Cessna master switch and non pullable output and field breakers with two separate switches one toggle for master relay and the other a breaker switch for field. Not mechanically linked.

I say no because the field breaker and field switch are two different places in the circuit and the master can be turned off but the operating alternator will still supply voltage but with no battery which would pop the diodes in the alternator eventually.

Radio shop pointed out Twin Cessnas and 210s have the emergency power switch and Twin Cessnas have separate battery and field switches.


Top

 Post subject: Re: Dual alternator emergency power connection to battery
PostPosted: 05 May 2022, 18:15 
Offline




User avatar
 Profile




Joined: 12/10/07
Posts: 30418
Post Likes: +10534
Location: Minneapolis, MN (KFCM)
Aircraft: 1970 Baron B55
Username Protected wrote:
Twin Cessna 421 but would apply to other piston twins. Debate is if alternators are on and battery relay fails. If emergency field power is selected are the alternator outputs still disconnected from the battery. Will this destroy the alternator diodes?

I believe I see where the alternator regulator fuses allow output power through to a capacitor and diode but not sure. Related service kit for reference.
https://support.cessna.com/custsupt/con ... s_id=32787

Where this came from is debate if during avionics refits the avionics shops can replace the original red split Cessna master switch and non pullable output and field breakers with two separate switches one toggle for master relay and the other a breaker switch for field. Not mechanically linked.

I say no because the field breaker and field switch are two different places in the circuit and the master can be turned off but the operating alternator will still supply voltage but with no battery which would pop the diodes in the alternator eventually.

Radio shop pointed out Twin Cessnas and 210s have the emergency power switch and Twin Cessnas have separate battery and field switches.


The only issues with operating one or two alternators with the battery disconnected from the bus is the potential for overvoltage and the inability to restart the alternators if they offline for some reason (like an overvoltage event). I see no reason why the diodes in the alternators would be likely to be damaged since they're typically rated at a much higher voltage than you should get on the bus. An overvoltage event could occur when a larg load (e.g. incandescent landing lights) is removed from the bus. It takes a little time for the alternator field(s) to respond to the regulator's command to reduce the output and for a brief time the voltage will climb several volts while the output current exceeds the remaining load. That excess current is normally absorbed by the battery limiting the voltage rise to just a volt or two, without that the peak voltage will be considerably higher, limited only by the other loads on the bus. To make matters worse, many newer avionics employ switching power supplies which actually draw less current when the voltage rises although most also have enough capacitance in their inputs to limit the voltage spike if it's short enough in duration.

_________________
-lance

It's easier to fool people than to convince them that they have been fooled.


Top

 Post subject: Re: Dual alternator emergency power connection to battery
PostPosted: 06 May 2022, 08:13 
Offline


 Profile




Joined: 01/10/17
Posts: 1658
Post Likes: +1124
Company: Skyhaven Airport Inc
Aircraft: various mid century
Searching around I find either that the diodes could fail or regulator concern. or not. Nothing in aviation related articles. Auto articles for essentially the same alternators do mention it.

As I look around airplanes coming into the shop I’m finding where generator to alternator conversions were installed the original double pole master was bypassed for the field.

A separate field switch with connection between the main bus and regulator power replaced it.

The concern is if the battery master is turned off with the alternator operating the main bus will continue to be powered by the operating alternator with already excited field.

Or factory alternator equipped airplanes where the original red plastic split rocker switch ( which mechanically links the battery master and field to the Off position) is replaced with two separate switches.

The STC instructions are pretty vague on this. But the old Inter Av instructions show “Original field switch” on the schematic which in the case of something like an generator equipped Piper is built into the master with a double pole switch. And they specify a non pullable alternator output breaker.

Plane Power says “Typically the original field switch is used” but no reason given for why. Plane power installations for a generator to alternator conversions I have seen had a new 7.5amp field breaker switch installed.

The field breaker and field switch are on opposite sides of the voltage regulator typically.

I guess the simplest way to know is with engine operating momentarily turn off the battery master and see if the bus remains powered.


Top

 Post subject: Re: Dual alternator emergency power connection to battery
PostPosted: 06 May 2022, 08:51 
Online


 Profile




Joined: 11/22/08
Posts: 2913
Post Likes: +921
Company: USAF Propulsion Laboratory
Location: Dayton, OH
Aircraft: PA24, AEST 680, 421
I've jump started numerous older vintage cars and tractors and run them without a battery for short periods of time (less than 30 mins). Never had an issue with the alternators or voltage regulators. Most of those had mechanical regulators, but a couple had electronic regulators. The diodes in alternators are rated for fairly high voltages. Probably not a good idea to run that way.

Looking at the wiring diagram for my Comanche. Originally had a generator, converted to alternator. Sure looks like I could shut the master switch off and the alternator would continue to power the buss. Never tried that though. One thing to do that on a simple car or tractor, quite another when there are expensive avionics involved.


Top

 Post subject: Re: Dual alternator emergency power connection to battery
PostPosted: 06 May 2022, 23:14 
Offline




User avatar
 Profile




Joined: 12/10/07
Posts: 30418
Post Likes: +10534
Location: Minneapolis, MN (KFCM)
Aircraft: 1970 Baron B55
Username Protected wrote:
A separate field switch with connection between the main bus and regulator power replaced it.

The concern is if the battery master is turned off with the alternator operating the main bus will continue to be powered by the operating alternator with already excited field.

I don't understand the concern. Are you thinking about a scenario where the pilot turns the master off in response to an electrical fire or burning smell and the alternator keeps the bus powered? It is true that if the battery master doesn't remove power to the regulator(s) with a separate pole or mechanically linked alternator switch, the bus would remain powered with the master off until the engine stopped or the alternator was overloaded. I think the solution to that would be to simply turn the alternator off along with the battery if you want to depower the bus.

My airplane (B55) doesn't have a mechanical interconnect between the battery master and the alternator switches but it's battery master does remove power to the regulators (via the alternator switches) when off.

I was working on a Cessna 340 today and noticed that there's a bail that the pilot can use to simultaneously move the battery master and both alternator switches off but it's also possible to turn the battery off and leave the alternators on. I suspect that in that case the alternators would power the bus with the battery out of the circuit.

_________________
-lance

It's easier to fool people than to convince them that they have been fooled.


Top

Display posts from previous:  Sort by  
Reply to topic  [ 5 posts ] 




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 post attachments in this forum

Jump to:  

Terms of Service | Forum FAQ | Contact Us

BeechTalk, LLC is the quintessential Beechcraft Owners & Pilots Group providing a forum for the discussion of technical, practical, and entertaining issues relating to all Beech aircraft. These include the Bonanza (both V-tail and straight-tail models), Baron, Debonair, Duke, Twin Bonanza, King Air, Sierra, Skipper, Sport, Sundowner, Musketeer, Travel Air, Starship, Queen Air, BeechJet, and Premier lines of airplanes, turboprops, and turbojets.

BeechTalk, LLC is not affiliated or endorsed by the Beechcraft Corporation, its subsidiaries, or affiliates. Beechcraft™, King Air™, and Travel Air™ are the registered trademarks of the Beechcraft Corporation.

Copyright© BeechTalk, LLC 2007-2024

.aviationdesigndouble.jpg.
.shortnnumbers-85x100.png.
.cav-85x50.jpg.
.AAI.jpg.
.aeroled-85x50-2022-12-06.jpg.
.Foreflight_85x50_color.png.
.ssv-85x50-2023-12-17.jpg.
.temple-85x100-2015-02-23.jpg.
.sierratrax-85x50.png.
.planelogix-85x100-2015-04-15.jpg.
.CiESVer2.jpg.
.chairmanaviation-85x50.jpg.
.traceaviation-85x150.png.
.jetacq-85x50.jpg.
.saint-85x50.jpg.
.concorde.jpg.
.kadex-85x50.jpg.
.blackwell-85x50.png.
.bullardaviation-85x50-2.jpg.
.midwest2.jpg.
.MountainAirframe.jpg.
.tat-85x100.png.
.wilco-85x100.png.
.Wingman 85x50.png.
.stanmusikame-85x50.jpg.
.blackhawk-85x100-2019-09-25.jpg.
.one-mile-up-85x100.png.
.Latitude.jpg.
.Genesys_85x50.jpg.
.pure-medical-85x150.png.
.gallagher_85x50.jpg.
.tempest.jpg.
.aircraftassociates-85x50.png.
.avfab-85x50-2018-12-04.png.
.ei-85x150.jpg.
.centex-85x50.jpg.
.jandsaviation-85x50.jpg.
.kingairacademy-85x100.png.
.geebee-85x50.jpg.
.bpt-85x50-2019-07-27.jpg.
.camguard.jpg.
.daytona.jpg.
.Wentworth_85x100.JPG.
.boomerang-85x50-2023-12-17.png.
.headsetsetc_Small_85x50.jpg.
.SCA.jpg.
.lucysaviation-85x50.png.
.airmart-85x150.png.
.dbm.jpg.
.Marsh.jpg.
.aircraftferry-85x50.jpg.
.Rocky-Mountain-Turbine-85x100.jpg.
.kingairnation-85x50.png.
.wat-85x50.jpg.
.ABS-85x100.jpg.
.pdi-85x50.jpg.
.avionwealth-85x50.png.