nuckolls.bob(at)aeroelect
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 Posted: Wed Mar 13, 2013 6:50 am Post subject: DISCONNECTING BATTERY WHILE ENGINE IS RUNNING |
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At 07:24 AM 3/13/2013, you wrote:
QUOTE: I've verified this both on cars and airplanes where
disconnection of the battery while the alternator is running produces
a only a small rise in bus noise.
WOW. With an alternator system (as opposed to generator), I would be
very afraid to disconnect the battery while the engine is running.
Alternator performance sans battery is a function
of regulator design. Indeed, there have been
instances of poor regulator design but
I suspect that those are pretty much a thing
of the distant past.
Consider that alternators are manufactured
by the millions per year and intended for
service in very uncontrolled situations.
Situations that must include inadvertent
disconnection of the battery. Folks who
supply such products would be well advised
to consider this possibility in their
product development design goals.
In fact, modern regulator design specs call
for ability to withstand worst case load
dumps (sudden disconnection of battery
and system demands while heavily loaded).
In the current chapter on alternators, I
describe a demonstration I witnessed at
the Motor Car Parts of America development
labs a few years back. I watched a technician
disconnect a heavily loaded alternator's
b-lead 5 times in a row. The flash of light
from the arc precluded taking any video
of the event. I tried two times. I wish
I had 'scope traces for the b-terminal
voltage. The energy release was impressive!
The alternator's regulator took it all in
stride and continued to function after 5
trials by lighting.
The Bonanza has allowed battery-off,
alternator-only ops ever since I can remember.
I did the regulator design for the pad-
driven standby generator about 1979. Beech
liked the design and asked for an alternator version
too. The specification control drawing
required a well behaved alternator without
battery. Those alternators had enough retentivity
in the field-pole material to self excite. So
another feature of the regulator's
specification control drawing called for
the regulator to come-alive and bring
the alternator on line with just a couple
of volts of residual output from an un-excited
alternator.
I don't think the Bonanza (and probably Barons)
ever had the functional equivalent of the
split-rocker master switch designed to preclude
alternator-only operations. The only conversations
I recall at Cessna during the split-rocker
evolution was a concern about making sure
the alternator had battery voltage to come
on line . . . I don't think anyone was concerned
about alternator-only ops being problematic
but the split-rocker thing made it unnecessary
to even consider it.
There's an interesting story about that
self-excitation thing. About 15 years ago,
Beech receiving inspection rejected a batch
of new alternators for failure to self-excite.
This brought a lot of consternated alternator
sales and engineering folks to Beech to see
what all the fuss was about.
It seems that the self-excitation feature
for these machines was never a documented
requirement or design goal. They just happened
to work that way. So for 20+ years, Beech
was writing specs to exploit the feature.
At the same time, the alternator guys were
unaware of Beech's expectations that self-
excitation be consistently demonstrable.
Hence their surprise when purchasing sent them
a 'ding letter' complaining about failure
to demonstrate self-excitation during receiving
inspection.
I don't know what was ultimately done to
renew the flow of alternators to the Bonanza
and Baron lines but I'm sure they worked it
out!
In retrospect, I'm pretty sure the designs we
were producing back then would not withstand the load-dump
test I described above. There was no requirement
to demonstrate it. I wonder if it has been added
since. I'll have to ask.
Bob . . .
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