Today's Message Index:
----------------------
1. 06:25 AM - Re: Overvoltage event (non-lithium battery) (Robert L. Nuckolls, III)
2. 06:29 AM - Re: Overvoltage event (non-lithium battery) (Robert L. Nuckolls, III)
3. 06:36 AM - Re: Overvoltage event (non-lithium battery) (Alec Myers)
4. 11:00 AM - Re: Overvoltage event (non-lithium battery) (Robert L. Nuckolls, III)
Message 1
| INDEX | Back to Main INDEX |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: Overvoltage event (non-lithium battery) |
At 10:23 PM 5/1/2017, you wrote:
>I have met a couple pilots who insist on starting the engine with
>the alternator off. I seem to remember that this was to protect the
>alternator. I'm not sure from what. Has anyone ever seen an
>alternator that has been damaged (or stressed or worn) by being on
>during start?
That can't happen either. Energy input to
the alternator is RPM x Torque. Torque
is absolutely proportional to output
current. Field current rise from a de-energized
state rises on the classic t=l/r time
constant curve over a period of tens
of milliseconds. It's a gentle increase
in alternator effort with no sudden 'peaks'
in mechanical or electrical stress.
Those worries were never given foundation
in physics.
>Also Bob, please forgive what is probably a silly question that I
>should think about rather than asking but if an alternator can't
>raise bus voltage enough to damage avionics why am I building a
>crowbar disconnect over voltage module?
An alternator driven by a failed regulator
has full bus voltage applied to the field.
At cruise rpms, the alternator goes into
a current limited runaway for voltage.
If your airplane needs say 15A from the
alternator in normal flight . . . then
a 60A alternator will produce something on
the order of 50-60 amps of EXCESS battery
charge current.
The voltage will begin to rise as the battery
grunts the excess energy production. In
tens to a couple hundred milliseconds after
the failure event, the bus voltage will rise
above the ov protection trip point . . . generally
32 to 33 volts in a 28v airplane.
were it not for the BATTERY, the bus voltage
could rise much more rapidly with an upper
bound of 100 to 200 volts. In this case,
the ov protection system is still expected
to bring the alternator to heel in a time
frame LESS than 100 milliseconds.
Given that your supplier speaks of a 50 volt
ov protection system, I suspect the appliance
is qualified to DO-160 Category A for abnormal
DC surge voltage. Quoting from DO-160 we find.
Emacs!
These numbers give rise to a design goal for ov
protection systems to LIMIT a runaway alternator's
output voltage to less than those values in the time
frames cited.
>Or is it just this particular avionic that cannot be damaged by an
>alternator but others are vulnerable? Or something to do with it
>being a 28V system instead of 14V?
No. If your battery was on line at the time of the
radio's failure . . . then there was no way that
bus voltage would rise to levels claimed by the
radio technician. If the battery were off line,
then you would have experienced a trip of the
ov protection system in an magnitude/time frame
that was UNDER the envelope cited in DO-160.
You are not the first owner/operator of an aircraft
to be told that "a spike from your airplane killed
your radio" . . . but in fact, if your battery was
on line and you did not experience an ov trip, then
there is no way that your airplane put any stress
on the radio that it was NOT designed to deal with.
14 volt appliances use 1/2 the 28v qualification
levels. I've always designed to Category Z.
Emacs!
Emacs!
Note that category Z considers the extra hazard
for having a very small or no battery at all.
It's not hard to do . . .
Bob . . .
Message 2
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: Overvoltage event (non-lithium battery) |
At 12:36 AM 5/2/2017, you wrote:
>
>
> >>Also Bob, please forgive what is probably a silly question that I
> should think about rather than asking but if an alternator can't
> raise bus voltage enough to damage avionics why am I building a
> crowbar disconnect over voltage module?
>
>A defective regulator can raise the bus voltage high enough to
>damage the battery.
A battery that is well maintained. I.e. periodically
cap and load checked, will be taken out of the airplane
when it no longer meets design goals for battery only
endurance. Such a battery will grunt the alternator's
abusive output during a runaway event for the time
it takes to activate the ov protection system.
In virtually EVERY ov event I have deliberately
generated in 28v systems on the test bench, the
bus voltage has NEVER exceeded 33 volts. The
battery was not damaged and no appliances were
harmed in the making of the video.
Bob . . .
Message 3
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: Overvoltage event (non-lithium battery) |
If the OV protection is absent, a defective regulator regulating the bus voltage
to 18V (for a 12V system) will eventually damage the battery, but not the avionics.
So isn't it fair to characterize the battery as being there to protect the avionics,
and the OV crowbar being there to protect the battery?
On 2May2017, at 9:26 AM, Robert L. Nuckolls, III <nuckolls.bob@aeroelectric.com>
wrote:
At 12:36 AM 5/2/2017, you wrote:
>
>
>
> >>Also Bob, please forgive what is probably a silly question that I should think
about rather than asking but if an alternator can't raise bus voltage enough
to damage avionics why am I building a crowbar disconnect over voltage module?
>
> A defective regulator can raise the bus voltage high enough to damage the battery.
A battery that is well maintained. I.e. periodically
cap and load checked, will be taken out of the airplane
when it no longer meets design goals for battery only
endurance. Such a battery will grunt the alternator's
abusive output during a runaway event for the time
it takes to activate the ov protection system.
In virtually EVERY ov event I have deliberately
generated in 28v systems on the test bench, the
bus voltage has NEVER exceeded 33 volts. The
battery was not damaged and no appliances were
harmed in the making of the video.
Bob . . .
Message 4
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: Overvoltage event (non-lithium battery) |
At 08:33 AM 5/2/2017, you wrote:
>
>
>If the OV protection is absent, a defective regulator regulating the
>bus voltage to 18V (for a 12V system) will eventually damage the
>battery, but not the avionics.
Yyeeaah . . . sort of. A battery that's in good
condition will willingly and ably stand off
most alternators until the ov protection system
can react. If the ov protection is absent, voltage
will rise with a rate dependent on battery condition
and magnitude of the 'overcharge' current.
We've seen cases of sustained ov on a good
battery . . . and yes, it's really hard
on the battery. But this event took a long
time . . . perhaps hours, perhaps more than
one flight to evolve to this condition.
Emacs!
. . . and yes, the battery was trashed. I don't
recall if any other system components were
damaged . . .
>So isn't it fair to characterize the battery as being there to
>protect the avionics, and the OV crowbar being there to protect the battery?
No . . . batteries are there to start engines
and back up alternators. A properly designed
ov protection system will arrest a runaway
alternator before the damages propagate to other
components in the system . . . whether the
battery is on line or not.
It's a happy coincidence that a robust battery
contributes to a graceful recovery from an
ov event . . . but there is no expansion of
a battery's primary role of offering a reversible
chemical energy source.
Bob . . .
Other Matronics Email List Services
These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.
-- Please support this service by making your Contribution today! --
|