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Using a SPST switch to control both the alternator field

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schu(at)schu.net
Guest

Posted: Mon Feb 01, 2010 9:54 am Post subject: Using a SPST switch to control both the alternator field

Robert L. Nuckolls, III wrote:

 	  Quote: 	
		   I have a switch that I really like that I want to use for the main

 switch.  It's a heavy duty switch that can handle up to 75amp, but it's

 only a SPST switch.

 

 

    What kind of airplane/engine? Where is your battery located

    with respect to the proposed location for the battery master

    switch?


	




The airplane is a bearhawk.  I haven't totally decided where the battery

is going to go, but I think it will end up on the firewall, perhaps even

cockpit side.



 	  Quote: 	
		   If it's wired directly to the main power bus then the second the

 master switch is turned off the contactor isolates the main buss

 anyway causing the field coil to shut down.

 

    No. Alternators can . . . and often do continue to run

    "self-excited" unless you break the field supply lead

    (or open the ON/OFF command lead for internally regulated

    alternators).


	


Ok, so if I'm reading you right, since the battery lead on the alt and

the field lead are both connected to the main power bus, the residual

magnetism in the alternator could cause it to produce some bias

(voltage) that will keep the field coil working, which creates more bias

causing it to continue to work.  Did I get that right?



 	  Quote: 	
		   2.  If it's harmful to have the field coil wired to the main power buss

 without a way to isolate it, then can I wire my main power switch as

 shown in the attached drawing?  Other than a little more wire, is there

 anything wrong with doing it this way?

 

    I didn't see an drawing come through as an attachment.

    If it's your intention to replace a battery contactor with

    a manually operated switch (common to many aircraft back

    in the 40's and 50's) then you need a separate alternator

    control switch. In this configuration, you don't have a

    MASTER switch and both switches need to be opened to kill

    the system.


	


I wonder what happened to my attachment, I got it.  Oh well, you can see

it here:



http://forum.matronics.com/viewtopic.php?t=67973



Anyway, no I'm not looking to replace the battery contactor.



What the drawing shows is a switch connected to the battery + on one

side with it feeding the alternator field and the master contactor on

the other side.



So basically, instead of the coil on the contactor always conntected to

the battery + and the master switch connecting the other side to ground,

 the ground side is always connected, and the SPST master switch

connects the other side to the battery, along with the alternator field.



I think this would work because the switch would bear the load of the

alternator field and master contactor coil which it's certainly capable

of doing, and because there isn't any possibility of 'self-exciting'

since opening the master switch would kill the alt field and the battery

side of the alt is isolated from the battery.



 	  Quote: 	
		   

    The subject of your note does raise a question . . . do

    you plan to use a battery contactor or is the 75A rated

    switch intended to be the replacement for a battery

    contactor?


	


No, I just like the switch (it's a push/pull), and it just happens to be

high current, I'll still use the contactor.



Thanks,

schu

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user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Tue Feb 02, 2010 6:41 am Post subject: Re: Using a SPST switch to control both the alternator fiel

Schu,

In the past, wanting to test the alternator on my car, I disconnected the battery while the engine was running.  The engine kept running, which proved to me that the alternator was working OK.

  As for your proposed circuit, it looks like it will work OK.  The only issue that I see is that a hot wire will be brought into the cockpit with no way to shut it off.  That wire should be double insulated and protected with a fuse near the battery.  You could use a relay near the battery to eliminate the hot wire into the cockpit.  That introduces another failure point, which is OK as long as you have a backup plan.

  Can you manually trip that 5amp breaker?  Is there some failure mode where it is desired to shut off the alternator without shutting off the master contactor?

Joe

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Joe Gores

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