 |
Matronics Email Lists
Web Forum Interface to the Matronics Email Lists
|
| View previous topic :: View next topic |
| Author |
Message |
nuckolls.bob(at)aeroelect
Guest
|
 Posted: Sun Oct 18, 2020 11:23 am Post subject: "Load Dump" revisited |
 |
|
The term is part of the vernacular of both
automotive and aviation worlds but they
have similar but not equal definitions.
The term is somewhat self explanatory . . .
a big alternator load is suddenly removed.
The resulting event is something akin to
the ice cream social game of tug-o-war.
Two opposing but relatively equal forces
compete with each striving to satisfy
disparate goals in physics. In this case,
the ALTERNATOR, under command of a regulator,
works within its limitations to maintain
the system bus at some pre-set voltage.
The totality of system loads team up to
comprise a total LOAD on the alternator.
Their demand tends to pull bus voltage down
while the alternator/regulator is working
to sustain the bus at the regulator's
set point.
At such time that total LOAD is very high,
sometimes equal to the alternator name-plate
limit, when have the electrical equivalent of
two teams of individuals pulling on a rope
from each end.
It's easy to visualize the prospects for
a sudden change of equilibrium if the LOAD
should suddenly reduce . . . perhaps even go
to zero. The rope breaks, load is zero. Half
of a crew looses traction and fall, the load
is greatly reduced. In either case, the
tension impressed on the 'alternator' side
goes down. The alternator was already balls-to-
the wall before the reduction. Sudden release
of tension allows the bus voltage to rise
suddenly and before the regulator's response
dynamic can react, the voltage will rise quickly
to some limit defined by (1) percentage of
load reduction and (2) time it takes for the
regulator to regain control.
Now, here's where the automotive and aviation
communities diverge a little when considering
the cause/effects of a load-dump.
When one reads the literature for mitigating load
dump in cars, virtually every condition speaks to
disconnection of the battery as worst case root
cause of the voltage transient. For example, suppose
you've had a hard start on the vehicle and the battery
is flagging. The car starts and the alternator rises
to the call and begins the task of recharging the battery.
It's daytime and weather mild . . . no headlights, no
HVAC motors . . . the BATTERY comprises the greatest
load on the system because it's being demanding
replenished from a largely discharged state.
Now, unhook the battery and . . . you got it . . .
the alternator is now lightly loaded by system accessories
and the alternator/regulator overshoot/recovery characteristics
prevail. Further, the system loads are light and the
PRIMARY load-dump mitigator, the battery, is out to lunch.
This is the automotive description of the perfect
storm of load dumps and yes, it's the worst
case scenario for a load dump in a battery-alternator
DC power system.
But the really BIG question is, how does that battery
become unhooked? Pretty rare event on a vehicle-by-vehicle
case . . . but there are tens of millions of them out
there. It's kinda like lightning strikes . . . doesn't
happen often but risk is not zero and the quality of
workmanship for battery installation and maintenance
is not as rigorous as in airplanes.
In airplanes, we have the perfect configuration for
generating the ultimate load dump: the battery contactor(s).
In most piston aircraft, DC power controls are configured
for BATTERY ON BEFORE ALTERNATOR and ALTERNATOR OFF
BEFORE BATTERY. It's the legacy split-rocker on many
production aircraft and the DP3P(progressive) toggle
switch in a lot of OBAM aircraft.
Except for conditions arising from poor craftsmanship
or maintenance, the battery is pretty tightly
wedded to the bus. There ARE still rare disconnects
with root cause in contactor failure . . . but
in the whole constellation of contactor failures,
what proportion occur during a heavy battery recharge
condition?
Aviation's load-dump events have been extensively
studied over the last 100 years. MIL-STD-704 and
DO-160 qualifications combined with sundry TSO
requirements suggest that power generation and
control systems be designed and qualified to
limit load dump excursions to 40/80 Volts (14/28
Volt systems). Accessories are designed and qualified
to stand off those same transients.
Just how the system designers choose to meet those
goals is not dictated. They may use
a sprinkling of transient voltage suppressors
or simply configure limit/withstand those levels
by design.
In any case, the highest risk condition is
set up by an inadvertent disconnect of a badly
discharged battery. Folks who travel on the
ground are many times more likely to experience
this than folks who fly . . . who are supposed
to KNOW better!
Bob . . .
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
|
|
| Back to top |
|
 |
bob.verwey(at)gmail.com
Guest
|
| Posted: Sun Oct 18, 2020 9:45 pm Post subject: "Load Dump" revisited |
|
|
Thank you Sir...illucidating as usual!
On Sun, 18 Oct 2020, 21:32 Robert L. Nuckolls, III, <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
| Quote: | The term is part of the vernacular of both
automotive and aviation worlds but they
have similar but not equal definitions.
The term is somewhat self explanatory . . .
a big alternator load is suddenly removed.
The resulting event is something akin to
the ice cream social game of tug-o-war.
Two opposing but relatively equal forces
compete with each striving to satisfy
disparate goals in physics. In this case,
the ALTERNATOR, under command of a regulator,
works within its limitations to maintain
the system bus at some pre-set voltage.
The totality of system loads team up to
comprise a total LOAD on the alternator.
Their demand tends to pull bus voltage down
while the alternator/regulator is working
to sustain the bus at the regulator's
set point.
At such time that total LOAD is very high,
sometimes equal to the alternator name-plate
limit, when have the electrical equivalent of
two teams of individuals pulling on a rope
from each end.
It's easy to visualize the prospects for
a sudden change of equilibrium if the LOAD
should suddenly reduce . . . perhaps even go
to zero. The rope breaks, load is zero. Half
of a crew looses traction and fall, the load
is greatly reduced. In either case, the
tension impressed on the 'alternator' side
goes down. The alternator was already balls-to-
the wall before the reduction. Sudden release
of tension allows the bus voltage to rise
suddenly and before the regulator's response
dynamic can react, the voltage will rise quickly
to some limit defined by (1) percentage of
load reduction and (2) time it takes for the
regulator to regain control.
Now, here's where the automotive and aviation
communities diverge a little when considering
the cause/effects of a load-dump.
When one reads the literature for mitigating load
dump in cars, virtually every condition speaks to
disconnection of the battery as worst case root
cause of the voltage transient. For example, suppose
you've had a hard start on the vehicle and the battery
is flagging. The car starts and the alternator rises
to the call and begins the task of recharging the battery.
It's daytime and weather mild . . . no headlights, no
HVAC motors . . . the BATTERY comprises the greatest
load on the system because it's being demanding
replenished from a largely discharged state.
Now, unhook the battery and . . . you got it . . .
the alternator is now lightly loaded by system accessories
and the alternator/regulator overshoot/recovery characteristics
prevail. Further, the system loads are light and the
PRIMARY load-dump mitigator, the battery, is out to lunch.
This is the automotive description of the perfect
storm of load dumps and yes, it's the worst
case scenario for a load dump in a battery-alternator
DC power system.
But the really BIG question is, how does that battery
become unhooked? Pretty rare event on a vehicle-by-vehicle
case . . . but there are tens of millions of them out
there. It's kinda like lightning strikes . . . doesn't
happen often but risk is not zero and the quality of
workmanship for battery installation and maintenance
is not as rigorous as in airplanes.
In airplanes, we have the perfect configuration for
generating the ultimate load dump: the battery contactor(s).
In most piston aircraft, DC power controls are configured
for BATTERY ON BEFORE ALTERNATOR and ALTERNATOR OFF
BEFORE BATTERY. It's the legacy split-rocker on many
production aircraft and the DP3P(progressive) toggle
switch in a lot of OBAM aircraft.
Except for conditions arising from poor craftsmanship
or maintenance, the battery is pretty tightly
wedded to the bus. There ARE still rare disconnects
with root cause in contactor failure . . . but
in the whole constellation of contactor failures,
what proportion occur during a heavy battery recharge
condition?
Aviation's load-dump events have been extensively
studied over the last 100 years. MIL-STD-704 and
DO-160 qualifications combined with sundry TSO
requirements suggest that power generation and
control systems be designed and qualified to
limit load dump excursions to 40/80 Volts (14/28
Volt systems). Accessories are designed and qualified
to stand off those same transients.
Just how the system designers choose to meet those
goals is not dictated. They may use
a sprinkling of transient voltage suppressors
or simply configure limit/withstand those levels
by design.
In any case, the highest risk condition is
set up by an inadvertent disconnect of a badly
discharged battery. Folks who travel on the
ground are many times more likely to experience
this than folks who fly . . . who are supposed
to KNOW better!
Bob . . .
|
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
|
|
| Back to top |
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum
|
Powered by phpBB © 2001, 2005 phpBB Group
|
Get Email Distribution Too!
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
