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I did a test on a Nippondenso
alternator the other day. Its no-load voltage with 13.2V on the field and spun
at 3000rpm was 40.2V. That's average voltage; its 3-phase rectified peaks
are higher. The alternator's pulley OD is 2.75", and the pulley on the starter
ring-gear is about 9.5". This means that its output at 2700 rpm is
about 125V. Have any of you measured the time constant of your OV crowbar?
I would think that it would have a filter in the trigger circuit to prevent
false triggering on transients, so there is probably several milliseconds from
the appearance of an OV event until the crowbar activates. There is probably a
several millisecond L/R field time constant for the decay of the field
current. If one of you has the means to test this total loop, it would be
interesting to see just how long this magnitude of OV would be present at the
alternator output. Keep in mind that there has to be an overvoltage existing on
the main buss before it can be detected. I have queried Exide about the
L-R-C time constants of a typical 25AH lead-acid battery, and its reaction to a
very brief high-voltage transient but have not yet received a reply.
Consider: a lead-acid battery generates voltage by an electrochemical reaction,
and is charged by the reverse reaction. Wouldn't an electrochemical
reaction of ion exchange on the cell walls take a finite amount of time? I
wouldn't think it would be on the order of the light-speed of electricity
flowing in a conductor. I would hazard a guess that any sinking of a
short-duration transient would be more due to battery capacitance than to being
through absorption by the chemical reaction. The inductance in the leads
from the battery to the buss would also add an additional time delay. There is
nothing instaneous in electricity. Even the thought-to-be instantaneity of
doppler radar isn't. The processor counts cycles over a fixed gate period, which
yields the range change, so its apparent effective time, with constant
range-rate, is in the middle of the count period, and the range-rate is range
per count-period. So! How long would a 125 V transient from an
alternator exist on the buss? Long enough to damage some fine
avionics?
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