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ELIPPSE wrote:
> GTH wrote:
>
>>I challenge you to
>>
>>>do this test on your plane at cruise rpm with lots of avionics load and
>>>prove me wrong!
>>>
>>
>>That's what overvoltage protection is made for : in the airplane I
>>built, within a few milliseconds, the OV relay opens and disconnects the
>> alternator from the ship's circuit.
>>
>>
>>Regards,
>>Gilles Thesee
>>Grenoble, France
>>http://contrails.free.fr
>
> Hi, Gilles!
> Do you know all the time-constants of the over-voltage protection
> loop? You say your OV has a relay which opens; a relay can take
> 20-60msec to drop out, especially if you have a diode across the coil
> for inductive transients. What is the L/R time-constant of your
> alternator's field when fully charged? Do you know? Most of the OV
> circuits I am familiar with use a solid-state device, such as an SCR,
> which is triggered on in a transient that then shorts out the field
> supply source, causing its circuit breaker to open. What is the
> time-constant of the SCR's trigger circuit to reduce nuisance triggers
> from short transients; do you know? Do you also know how long it takes
> for the field circuit breaker to open? I wrote this posting to make you
> people aware of a possible failure mechanism in your plane's charging
> system that could have disastrous results with your avionics. If you
> wish to dismiss this warning, that is your prerogative. But please, if
> this situation ever happens to you, pull the field breaker before
> closing the alternator through! Paul
>
Paul, have you ever tried to reset a 50, 70, or 100-amp circuit breaker
in flight? In the few cases I have seen when such breakers have tripped,
the crews were unable to reset them. We reset them on the ground, using
a soft mallet.
I remember one airframe manufacture putting the alternator output
circuit breaker in the floor beneath the pilot's feet. That allowed the
pilot to stamp his foot on it to reset it.
How much experience do you have with aircraft electrical systems, Paul?
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Re: Frying your avionics
