Today's Message Index:
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1. 10:53 AM - Re: Re: Spark suppression diode on starter MOTOR (Robert L. Nuckolls, III)
2. 10:55 AM - Re: Spark suppression diode on starter MOTOR (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: Spark suppression diode on starter MOTOR |
At 12:48 PM 4/2/2022, you wrote:
>
>Batteries don't absorb the energy from inductive voltage spikes.
Correct. A battery's ability to mitigate
hazardous stimuli in the electrical system
is limited to standing off a runaway power
generation system for the very short period
of time necessary for the o.v. protection
system to operate (hundreds of milliseconds).
This is a HIGH energy event (sometimes exceeding
10,000 watt-seconds).
"Spikes" are short duration (less than 10mS)
excursions generated by release of reactively
stored energy (usually inductive, sometimes
capacitive) and is a very low energy event
measured in millijoules (1/1000th of a watt-
seconds). These events are never a source
of catastrophic system damage. They will
sometimes manifest in transient mis-behavior
of a system that has NOT been qualified to
live in the legacy, aircraft electrical
environment.
>The controlling switch contacts do. As the switch contacts open,
>an arc jumps across the opening contacts. The energy is dissipated as heat.
Some years back, we had some deep-dives into
physics and terminology of electrical transients
here on the List. I produced a number of bench-top
demonstrations illustrating the sources, effects and
mitigation of low energy transients (true spikes).
The attached plot was taken from one of those
experiments.
An S701 battery contactor coil collapse
spike was plotted while being controlled by
a toggle switch and NO coil suppression. Note
that the maximum excursion was on the order of
250 volts impresses across the spreading
contacts gap in the switch. An arc formed
between those contacts which persisted for
about 1.4 mS at which point the arc broke
and remaining energy took the transient to
its maximum excursion.
Energy dissipated in this even was on the order
of tens of millijoules. No single event
being hazardous to the switch . . . but hundreds
of events would eventually erode the switch's
contacts.
I couldn't quickly find examples of how much energy
from such events propagates out onto the system during
a de-energized non-suppressed coil event . . . but
I have measured it. It's TINY . . . 99.9% of the
energy is spent on the controlling device unless
managed with appropriate suppression. Even if the
battery WERE a valuable member of the spike suppression
community, there wouldn't be anything for it to do!
Two other images illustrate a study I did
at Beech on another contact failure investigation.
You can see the difference in burning and metal
transfer of controlling contacts with no
transient suppression on the loading device.
>Starter motors have counter EMF that might limit inductive voltage spikes.
True . . . but they are still quite stressful on
their controlling device. This is why starter contactors
are specially crafted for DOUBLE break contacts
to increase contact spreading velocity for early
break of the inevitable albeit CEMF mitigated
arc.
Perhaps one MIGHT improve service life of a starter contactor
with some purposeful suppression of the arc but
the contactor designers have pretty well slain
that dragon.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
Message 2
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| Subject: | Re: Spark suppression diode on starter MOTOR |
At 08:06 AM 4/2/2022, you wrote:
><Tom.Costanza@gmail.com>
P.S. Heres an article I posted about 22 years
ago on the topic of spike suppression . . .
https://tinyurl.com/y79ht8zm
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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