Shutting down alternators before engine shutdown

[Ali Ghorashi]

Hello All,
The manufacturer of the alternators in the Cessna 310 that I fly requires
the pilot to turn off the alternators before engine shutdown. Failure to do
this will void the warranty on the alternators. Does anyone know of a reason
for this? I have a suspicion that it has to do with the fact that with one
engine shutdown, the alternator's field on that engine can be energized by
the other engine's alternator. This may cause problems if the battery is low
(i.e reverse bias some diodes/transistors). That's just a WAG. Any ideas?

-Ali

[Dan Thomas]

"Ali Ghorashi" > wrote in message >...
> Hello All,
> The manufacturer of the alternators in the Cessna 310 that I fly requires
> the pilot to turn off the alternators before engine shutdown. Failure to do
> this will void the warranty on the alternators. Does anyone know of a reason
> for this? I have a suspicion that it has to do with the fact that with one
> engine shutdown, the alternator's field on that engine can be energized by
> the other engine's alternator. This may cause problems if the battery is low
> (i.e reverse bias some diodes/transistors). That's just a WAG. Any ideas?
>
> -Ali

The alternator's field current is controlled by the regulator,
and when it's on but not turning the field gets pretty much full
battery voltage. That voltage would be a bit higher than normal if the
other alternator is running, but not dangerously so. The field can
take it.
The danger, as I see it, is either that the field of the
alternator on the dead engine can get hot after a while if it's not
spinning and could burn out (the fan isn't cooling it) or that the
pilot may forget to make sure the alterators are off before starting
the engines next time. Two fields can suck up enough current to make
starting an engine that much harder.

Dan

[G Farris]

In article >,
says...

> The alternator's field current is controlled by the regulator,
>and when it's on but not turning the field gets pretty much full
>battery voltage. That voltage would be a bit higher than normal if the
>other alternator is running, but not dangerously so. The field can
>take it.


Hmmm - maybe it's not the steady-state voltage - somewhat higher than normal,
as you say - that they're concerned about, but the transient due to inductive
field dump. When one is shut down, it could send a high transient down to the
other. If there is no specific transient protection, it could damage it.
That's just a guess too - I'd be interested in hearing more about it.

G Faris

[Ali Ghorashi]

The fields cannot cause a problem during startup since the alternators are
turned on after the engine starts.

-Ali

"Dan Thomas" > wrote in message
m...
> "Ali Ghorashi" > wrote in message
> >...
>> Hello All,
>> The manufacturer of the alternators in the Cessna 310 that I fly requires
>> the pilot to turn off the alternators before engine shutdown. Failure to
>> do
>> this will void the warranty on the alternators. Does anyone know of a
>> reason
>> for this? I have a suspicion that it has to do with the fact that with
>> one
>> engine shutdown, the alternator's field on that engine can be energized
>> by
>> the other engine's alternator. This may cause problems if the battery is
>> low
>> (i.e reverse bias some diodes/transistors). That's just a WAG. Any ideas?
>>
>> -Ali
>
> The alternator's field current is controlled by the regulator,
> and when it's on but not turning the field gets pretty much full
> battery voltage. That voltage would be a bit higher than normal if the
> other alternator is running, but not dangerously so. The field can
> take it.
> The danger, as I see it, is either that the field of the
> alternator on the dead engine can get hot after a while if it's not
> spinning and could burn out (the fan isn't cooling it) or that the
> pilot may forget to make sure the alterators are off before starting
> the engines next time. Two fields can suck up enough current to make
> starting an engine that much harder.
>
> Dan

[Robert M. Gary]

(Dan Thomas) wrote in message >...
> "Ali Ghorashi" > wrote in message >...
> The alternator's field current is controlled by the regulator,
> and when it's on but not turning the field gets pretty much full
> battery voltage. That voltage would be a bit higher than normal if the
> other alternator is running, but not dangerously so. The field can
> take it.
> The danger, as I see it, is either that the field of the
> alternator on the dead engine can get hot after a while if it's not
> spinning and could burn out (the fan isn't cooling it) or that the
> pilot may forget to make sure the alterators are off before starting
> the engines next time. Two fields can suck up enough current to make
> starting an engine that much harder.

In my Mooney we found that the engine starts a little faster with the
alternator off, so that's how we start it.
-Robert