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nuckolls.bob(at)aeroelect
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 Posted: Mon Apr 05, 2010 11:28 am Post subject: External vr runaway "B" alternator protection |
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| Quote: | | I had ass-u-me-d that the runaway alternator problem was limited to the internally regulated alternators, because the B (Ford) style external voltage regulators/alternator controls would shut off the alternator field voltage (and shut down the alternator output) if the power to the vr/acu was cut off (either manually such as with a Cessna split master switch/pulling the breaker on the vr/acu power wire, or through an automatic system that would sense high voltage and shut off power to the vr/acu). |
While there ARE certain architecture conventions for
alternator control, both internally and externally regulated
alternators have a single transistor in series with the field
that controls field excitation based on a pulse width modulated
signal from the regulators voltage regulation intelligence
whether a very sophisticated IC like . . .
http://aeroelectric.com/Mfgr_Data/Semiconductors/MC33092A.pdf
or a hand-full of jelly beans like . . .
http://aeroelectric.com/Pictures/Alternators/Internal_Regulator_Circa_1980.pdf
There are a number of failure modes that will
either (1) cause the field control transistor to
be turned on hard or (2) short he the transistor
with the same result. The alternator is fully
fielded from the b-lead and voltage takes off
for the moon.
| Quote: |
I have just read that that some of the vr/acu's are set up that the voltage feed wire (from the split master for example) is only necessary for the initial energization of the alternator field, and from there the field energy is derived from the voltage (sensing) wire. Then, a diode regulates how much voltage goes from the sensing wire to the field wire. |
If the writer of what you read has studied a credible
schematic for the purpose of conducting a failure mode
effects analysis on "some of vr/acu's" then perhaps
the statement is correct.
My problem with integrating automotive hardware into
airplanes is not what I KNOW about them but what I
DO NOT KNOW about them. Folks have suggested that
I was discouraging their use based on some personal
perceptions of un-reliability. Not so. I was only
saying that I cannot recommend them because (1)
I don't have access to the data needed for traditional
FMEA studies and (2) what I do know is that they're
difficult to integrate into legacy aircraft power
systems where (a) the pilot has positive, any time,
any conditions, ON-OFF control of the alternator
at no risk to any part of the system and (b) the
system is fitted with over-voltage protection that
is totally independent of alternator's regulator
regulator whether internal or external.
Bottom line about what you read is . . . who
knows? "Some" devices may indeed function as you've
hypothesized . . . but until we can see a detailed
schematic . . . we just don't know. It's probable
that the writer of the words you read doesn't know
either.
| Quote: |
The issue presented is if the diode inside the vr fails, and shorts out, there will be full voltage from the sensing wire applied to the Field, even if all power to the vr/acu is disconnected.
Question: is the above correct, at least in some cases? is there an issue in an external type B regulator that a failure "inside the box" can result in a runaway alternator, and that simply pulling off power to the vr will not de-energize the alternator? what happens if I simply pull the 60 amp B lead breaker while the alternator is running away? should I have a pullable breaker between the vr and the Field terminal on the alternator, that can be manually switched off? is this the best way to assume control in the unlikely event of a failure to short in the vr switching diode? |
Don't know what is being referred to as a
"VR switching diode". Folks who have successfully
integrated modern alternators into aircraft
have accomplished the design goals I outlined
above. Figure Z-24 in the 'Connection is another
approach for achieving those goals as well.
http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z24-Interim.pdf
But if anyone has data to support the assertions
you're pondering, then there are dozens of folks here
on the List that would be pleased to see it including
yours truly.
In the mean time my best recommendations are to (1) modify
the alternator for external regulation and OV control
like B&C, (3) modify for external control of field power
including OV control like Plane Power, or (3) use Z-24.
Bob . . . [quote][b]
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nuckolls.bob(at)aeroelect
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| Posted: Mon Apr 05, 2010 4:17 pm Post subject: External vr runaway "B" alternator protection |
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Seems I didn't answer your questions specifically.
My apologies and thanks to a couple of List readers
that rattled my cage . . .
| Quote: |
Question: is the above correct, at least in some cases? |
Have no way to know without detailed data on
the specific alternator/regulator being
considered.
| Quote: | | Is there an issue in an external type B regulator that a failure "inside the box" can result in a runaway alternator, and that simply pulling off power to the vr will not de-energize the alternator? |
Matters not whether B-type or A-type. There
are components that can fail in a manner that
full-fields the alternator and produces a
current limited but not very voltage limited
runaway event.
| Quote: | | what happens if I simply pull the 60 amp B lead breaker while the alternator is running away? |
The alternator b-lead terminal in a runaway
condition can reach well over 100 volts DC
and sustain it for some time before the field
coil smokes. The b-lead breaker is generally
designed for operation in systems not exceeding
32vdc. It's quite likely that by the time you
sense that an ov event is in progress and
react by pulling the breaker, the alternator will
be in a current limited attempt to pull the bus
up against the battery's limited ability to
hold it down.
Opening the breaker is guaranteed to start a
fire between contacts designed to break much
lower voltages. Once the arc starts, it would
liberate several thousand watts of concentrated
energy within the confines of the plastic breaker
housing.
| Quote: | | should I have a pullable breaker between the vr and the Field terminal on the alternator, that can be manually switched off? |
An OV event is one of those things we always
managed with a millisecond-fast protection
system. This is not a duty you want to
take on as pilot.
is this the best way to assume control in the unlikely event of a failure to short in the vr switching diode?
Whether it's a bad "vr switching diode" or any other component
of the alternator voltage control system, the legacy technique
calls for fast, automatic OV management techniques.
Bob . . . [quote][b]
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