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E-bus and Alternator

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jan_de_jong(at)casema.nl
Guest

Posted: Mon Feb 28, 2011 1:23 pm Post subject: E-bus and Alternator

Studying the Z-figures I find (I believe) that in the case of a failed 

contactor it is possible to charge the battery through the E-bus diode 

and Alt Feed switch - if one is so careless as to close that switch 

while leaving the Alternator on. Generally worse and more likely: to 

switch the Alternator back on after a stretch of battery-only operation 

without opening the Alt Feed switch first. Diode and switch are not made 

for that.

I think it is important to reserve the closing of the Alt Feed switch 

for the occasion where the Alternator has failed and is kept off for the 

duration (I wonder if an interlock might be devised that prohibits 

having both Alt Feed on and Alternator on?).



Jan de Jong

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Feb 28, 2011 7:08 pm Post subject: E-bus and Alternator

At 04:18 PM 2/28/2011, you wrote:

 	  Quote: 	
		  



Studying the Z-figures I find (I believe) that in the case of a 

failed contactor it is possible to charge the battery through the 

E-bus diode and Alt Feed switch - if one is so careless as to close 

that switch while leaving the Alternator on. Generally worse and 

more likely: to switch the Alternator back on after a stretch of 

battery-only operation without opening the Alt Feed switch first. 

Diode and switch are not made for that.


	


    Correct . . .



 	  Quote: 	
		  I think it is important to reserve the closing of the Alt Feed 

switch for the occasion where the Alternator has failed and is kept 

off for the duration


	


    . . . yes



 	  Quote: 	
		  (I wonder if an interlock might be devised that prohibits having 

both Alt Feed on and Alternator on?).


	


    Hmmmm . . . sure. You could use a 2-pole switch

    for the alternate feed path control and put one pole

    in series with the alternator control.



    A number of folks on the List have asked about flying

    with the alternate feed closed all the time . . . which

    offers no hazard as long battery contactor is closed

    before and while the alternator is operating. As you've

    noted, battery contactor failure would put this pathway

    at-risk.



    Contactor failure is a low order risk but obviously

    not zero. The simplest action is to craft an alternator-

    out check-list that goes to avoiding the undesired

    configuration.



    Your hypothesis raises another question. How does

    one become aware of contactor failure in flight? The

    AEC9011 offered a two channel low volts warning light

    for both the main bus and any of the batteries. The

    second light was intended to remind a pilot to close

    an aux battery switch for a small battery after the

    engine was started and the alternator came on line.



    If the contactor did fail, the alternator may well

    run happily self excited and things might go as

    anticipated all the way to engine shut-down. Of course,

    you'd know about the failure at the next time you

    came out to fly. That's one of the reasons I like to

    shut the alternator off before killing the engine

    but leaving the battery on. This will get you a low

    volts warning light if the contactor is good, a dark

    panel if not.



   Bob . . .

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