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I have a friend who accidently left his
master switch on while at a picnic at a small airport. When he tried to
start the engine - nothing! He got a hand prop, then immediately took off
for the short 10 mile flight back to his home airport. He noticed that his
35A alternator breaker on his O-320's Nissan alternator had popped, so
he pushed it in, and it popped right back out. This happened several times.
After he got back he asked me what I thought happened. It finally dawned on me
that once he took off and the rpm came up that the alternator was putting out so
much current into the almost dead battery that each time he restored the
breaker, the same thing happened, since the battery wasn't getting charged, and
actually was discharging some from his minimal avionics and the field supply.
When he was taxying at his departure airport, there wasn't enough rpm to give
the high current demand of the low battery to pop the breaker. Ive seen CB's
that will just pop with no overload; remember, they are temperature-activated
devices, and their ratings change with ambient temperature as well as the
time-current product of an overload.
Another scenario would be if you were
checking the operation of a back-up alternator in flight. Some people have
installed a second alternator connected to their main buss but with its
regulator adjusted to a lower voltage to pick up the load if the main charging
system fails. So if someone decides to kill the main alternator by pulling its
circuit-breaker in flight to check his secondary system, the lower buss voltage
will still cause the main regulator to pump max voltage into the field to get
its alternator output up. 'Remember the circuit that they used to install, and
maybe still do, to disconnect their truck or car alternator from the battery and
use it to drive power-tools on a job site by putting a higher current on
the field? They would adjust the field supply to give them 120V! A voltage
transient in excess of 100V onto the buss can do a lot of damage in the few
milliseconds that it takes for the filtered OV circuit to short and put a load
on the field breaker, which then takes a few ms to open, which then takes a few
ms of the alternator field and stator windings to discharge. Do you have a
diode or TVS across the field to slow field decay to reduce the field's high
inductance transients back into the wiring. WOW! That will add to the
field's L/R decay time-constant! Then you have the fact that the
battery isn't known for having a low impedance to short-duration transients.
Then, too, there is the wiring inductance between the alternator and the
battery. Are your loads connected in between the alternator and the battery,
where they may experience the inductive transient voltage drops along the
wires?
Guys; if all you want to do is argue
whether it can happen or not, putting forth all kinds of scientific facts and
scenarios, just disregard what I've written as the ravings of a lunatic.
But I still ask you to make sure that if your alternator breaker does pop,
for whatever reason, pull the field breaker or turn it off at a split
master before restoring the alternator breaker. OK?
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