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Using a SPST switch to control both the alternator field

 
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schu(at)schu.net
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PostPosted: Mon Feb 01, 2010 9:54 am    Post subject: Using a SPST switch to control both the alternator field Reply with quote

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: 1907
Location: Riley TWP Michigan

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

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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